This page shows what was measured, who it was measured in, and what that does not settle.
What Phenytoin does in the body
Generalised and focal epilepsy, seizures around brain surgery, and prolonged seizures in hospital
Phenytoin binds sodium pores in nerve membranes and holds shut the ones that have just been used, so a cell firing repeatedly loses more and more of them and the burst dies out. The complication is not in the brain but in the liver. The enzyme that destroys phenytoin runs out of capacity at ordinary doses. Below that point, doubling the dose doubles the blood level; above it, a small increase can send the level far higher than expected, which is why this drug is one of the few that needs blood testing to use safely.
What happened in people
A disproportionate rise in steady-state serum level from a dose increment of 10% or more, on a saturable CYP2C9 hydroxylation system
✎ Source-linked record
Where this came from
A person wrote this into the record with the study named beside it. No reviewer has signed it off.
The recorded finding that sits closest to something a person would notice. Written into the record, not signed off, and it carries no population or interval.
ESETT: Established Status Epilepticus Treatment Trial (NCT01960075) · a recorded source, not a stored snapshot
The limit that matters most
That a total phenytoin level inside the reference range excludes toxicity, when the label directs unbound-fraction monitoring in renal disease, hepatic disease and hypoalbuminaemia
Where it acts
Axonal membrane of cortical neurons; also the gingival fibroblast and the cerebellar Purkinje layer, where its long-term harms appear
Kind of result
A number that stands in for health
Supervision
Professional supervision is normally required. A regulator classifies this substance in a way that restricts how it is supplied.
What the registries record it as
The substance registry classes this as chemical.
FDA substance registry · 6158TKW0C5 · read 2026-08-29
Where each sentence above came from
No recorded sentence describes the change this makes in a way that stands on its own. The page opens with what it is taken for instead, and the recorded explanation follows below.
Shown as the opening line on this page.
A limit recorded against this substance. Not signed off as a reviewed claim.
The four opening statements run to 132 words.
Words this page uses
Four words worth knowing first
Chosen from what this page shows, with each one explained before the word it depends on.
Comparator
A comparator is whatever the treatment was measured against.
A picture of it, and where the picture fails
It is like the other runner in a race.
Where that stops being true. A race has one winner. A study can show both arms improved.
What people get wrong. Results are read without asking what the other group got. Beating nothing is not beating a treatment.
The control condition against which the experimental intervention is assessed.
Randomisation
Randomisation means chance decides who gets which treatment.
A picture of it, and where the picture fails
It is like a coin toss deciding the groups.
Where that stops being true. A coin toss is fair once. Fairness here comes from doing it for everyone.
What people get wrong. It is thought to make groups identical. It makes them similar on average, including on things nobody measured.
Allocation of participants to arms by a chance process, so that unmeasured factors are balanced in expectation.
Absolute difference
An absolute difference is how many more people in a hundred were affected.
A picture of it, and where the picture fails
It is like counting heads in two rooms of a hundred.
Where that stops being true. Heads are easy to count. Study results carry a margin of error too.
What people get wrong. It is confused with a percentage change, which can look far larger.
The arithmetic difference in event rates between arms.
Confidence interval
A confidence interval is the range the true answer is likely to sit in.
A picture of it, and where the picture fails
It is like a weather forecast giving a range rather than one number.
Where that stops being true. A forecast range covers tomorrow. This one covers what the study could resolve.
What people get wrong. The middle number is read as the answer. A range crossing zero means no change is still possible.
An interval estimate that would contain the true parameter in a stated proportion of repeated studies.
What happened in people◇Read from sources, not yet reviewed
What happened in people
Each card is one study: the exact question it asked, who was in it, and whether it showed what it set out to show.
Overall treatment success combining seizure control and tolerability, across carbamazepine, phenobarbital, phenytoin and primidone
Highest with carbamazepine or phenytoin, lowest with primidone, P<0.002; no significant difference between drugs for control of tonic-clonic seizures
Repeated elsewhere
Replicated
What this does not prove. Meeting one endpoint in one population does not carry to other goals or other people.
The limits of this study, and where it came from
Main limit. Phenytoin caused more dysmorphic effects and hypersensitivity than the other three drugs, an outcome the trial recorded but which no protocol was designed to quantify over two years.
How far it carries. This study is recorded from the curated record, not from the registry match.
Form and route. Oral extended-release capsule, chewable tablet, oral suspension, and intravenous or intramuscular injection
Interval reported. Not recorded in this summary
Written into the record, not signed off as a reviewed claim.
45% on fosphenytoin (95% credible interval 36 to 54) against 47% levetiracetam and 46% valproate; posterior probability of being most effective 0.24
Repeated elsewhere
Unreplicated
What this does not prove. Meeting one endpoint in one population does not carry to other goals or other people.
The limits of this study, and where it came from
Main limit. Numerically more hypotension and intubation occurred with fosphenytoin, neither difference significant. The trial stopped early for futility of separating the three drugs.
How far it carries. This study is recorded from the curated record, not from the registry match.
Form and route. Oral extended-release capsule, chewable tablet, oral suspension, and intravenous or intramuscular injection
Interval reported. Not recorded in this summary
Written into the record, not signed off as a reviewed claim.
Genetic variants associated with phenytoin-related severe cutaneous adverse reactions
✓ The study showed what it set out to show
Who was studied
Phenytoin severe cutaneous adverse reaction GWAS (Chung 2014)
How many people
3968
Study design
Genome-wide association study with three-population replication
Compared against
Not recorded for this study
Kind of result
A number that stands in for health
What was found
CYP2C9*3 (rs1057910) odds ratio 12 (95% CI 6.6 to 20), P=1.1 x 10^-17 in Taiwan; meta-analytic odds ratio across Taiwan, Japan and Malaysia 11 (6.2 to 18), P<0.00001
Repeated elsewhere
Replicated
What this does not prove. Meeting one endpoint in one population does not carry to other goals or other people.
The limits of this study, and where it came from
Main limit. This is a case-control genetic study, not a prevention trial. No prospective study has shown that CYP2C9 genotyping before treatment reduces the incidence of severe cutaneous reactions, as the Taiwanese HLA-B*1502 study did for carbamazepine.
How far it carries. This study is recorded from the curated record, not from the registry match.
Form and route. Oral extended-release capsule, chewable tablet, oral suspension, and intravenous or intramuscular injection
Interval reported. 95% CI 6
Written into the record, not signed off as a reviewed claim.
RNAWiki holds 3 written-up human studies for this substance, each with the question it asked.
How we know it
Each card names the study, the number of people and what the study set out to measure.
What this does not prove
These summaries were written by a person and have not been signed off as reviewed claims.
Why it matters
A study that failed is as informative as one that worked, and both are here.
What we still do not know
No reviewed claim exists, so no exact population, effect size or interval is published yet.
What it changes in the body◇Read from sources, not yet reviewed
The path through the body
From what a person takes to what changes. A solid line is a step measured in people. A dashed line is a step nobody has measured that way.
Start
Phenytoin
What a person takes: Oral extended-release capsule, chewable tablet, oral suspension, and intravenous or intramuscular injection.
The measurement behind this step
Formulations are not interchangeable by milligram. The chewable tablet is phenytoin free acid and is absorbed faster than the 100 mg extended phenytoin sodium capsule, and the sodium salt contains about 92% phenytoin by weight. The injection is formulated in propylene glycol at high pH, which is why extravasation causes severe local tissue injury and why the water-soluble prodrug fosphenytoin was developed.
Getting in
Absorbed slowly, and cleared by an enzyme that runs out of capacity
Absorption varies between formulations and takes hours. Removal is the unusual part: the liver enzyme responsible saturates, so the relationship between dose and blood level stops being a straight line.
╌╌Described, with no measurement named. No measurement is named for this step, so it is drawn as an unconfirmed link.
The measurement behind this step
Metabolism is primarily by CYP2C9 with a lesser contribution from CYP2C19, and the hydroxylation system is saturable at therapeutic serum levels. Average half-life is 14 hours with a range of 7 to 29. The label states that a dosage increase of 10% or more can disproportionately increase the steady-state level and cause intoxication when levels are already in the upper range.
ESETT: Established Status Epilepticus Treatment Trial (NCT01960075) · a recorded source, not a stored snapshot
Reaching the cell
Most of it travels bound to protein; only the free fraction acts
Nearly all the drug in the blood is stuck to albumin. Only the small unbound share crosses into the brain and does anything, which is why the standard blood test can mislead when albumin is low.
──Measured in people. Written by a person from the study named beside the step. Not signed off as a reviewed claim.
The measurement behind this step
Phenytoin is extensively bound to serum plasma proteins. The label directs that in renal or hepatic impairment or hypoalbuminaemia, monitoring should be based on the unbound fraction. Most of the drug is excreted in bile as inactive metabolites, reabsorbed from the intestine and eliminated in urine.
ESETT: Established Status Epilepticus Treatment Trial (NCT01960075) · a recorded source, not a stored snapshot
What it acts on
It binds sodium channels that have just been used
Sodium pores spend a moment shut and unavailable right after firing. Phenytoin binds them in that state and holds them there, so pores that have been active recently are the ones taken out of service.
╌╌Described, with no measurement named. No measurement is named for this step, so it is drawn as an unconfirmed link.
The measurement behind this step
The label states the precise mechanism has not been established but is thought to involve voltage-dependent blockade of membrane sodium channels reducing sustained high-frequency neuronal discharges. Apparent potency measured in a cell rises steeply from a depolarised holding potential compared with a hyperpolarised one.
ESETT: Established Status Epilepticus Treatment Trial (NCT01960075) · a recorded source, not a stored snapshot
The change it makes
Sustained high-frequency firing collapses
A cell firing occasionally is barely touched. A cell firing in a rapid train loses more of its pores with every spike, so the train cannot sustain itself or recruit neighbouring tissue.
╌╌Described, with no measurement named. No measurement is named for this step, so it is drawn as an unconfirmed link.
The measurement behind this step
Cumulative use-dependent block reduces sustained high-frequency discharge without abolishing normal single action potentials, which is the property that separated phenytoin from the sedating barbiturates in 1938 and remains the clearest statement of what the drug does at a cellular level.
ESETT: Established Status Epilepticus Treatment Trial (NCT01960075) · a recorded source, not a stored snapshot
What that does for a person
Seizure control equal to the best alternatives, at a long-term price
Tied with carbamazepine for the best result among four drugs over two years, and tied with levetiracetam and valproate for stopping status epilepticus. What separates it now is what happens over years: gums, bones, skin and unsteadiness.
──Measured in people. Written by a person from the study named beside the step. Not signed off as a reviewed claim.
The measurement behind this step
Efficacy is established in the 622-patient Veterans Affairs Cooperative Study and, as fosphenytoin, in the 384-patient ESETT trial. The long-term burden is enzyme induction with vitamin D depletion and bone loss, gingival overgrowth, hypersensitivity reactions with a CYP2C9*3 genetic component, and dysmorphic effects.
ESETT: Established Status Epilepticus Treatment Trial (NCT01960075) · a recorded source, not a stored snapshot
No suggested links are held for this record, so nothing is hidden from this path.
What we know
The record describes 5 steps from taking it to an effect in a person.
How we know it
Each step names the study behind it in the technical detail beside it.
What this does not prove
A change inside the body is a reason to look. It is not a result in a person.
Why it matters
A reader can see exactly where the chain stops being measured in people.
What we still do not know
No reviewed claim binds any of these steps to a named population.
What is missing or unclear◇Read from sources, not yet reviewed
Were people like you studied?
RNAWiki cannot tell whether a study fits you. It can show who was in it, and where the result stops carrying.
Who was studied
People similar to this profile were included.
Fewer people than once did, but still very many worldwide. It is a WHO essential medicine, it is extremely cheap, and its intravenous form remains in status epilepticus protocols. Long-term use has visible costs that newer drugs do not impose.
Who is missing from the studies
Which groups were under-represented in the studies has not been recorded for this substance. Pregnancy, breastfeeding, children, older adults, and people with liver or kidney conditions are commonly excluded from trials; nothing here says whether they were.
What the label states about particular groups
On pediatric, the label states: “A loading dose of 15 to 20 mg/kg of Phenytoin Sodium Injection intravenously will usually produce serum concentrations of phenytoin within the generally accepted serum total concentrations between 10 and 20 mcg/mL (unbound phenytoin concentrations of 1 to 2 mcg/mL).”
US prescribing information · 035a8d4e-2063-4240-83cb-d7eebcabe301 · read 2026-08-30
On older people, the label states: “Phenytoin clearance tends to decrease with increasing age [see Clinical Pharmacology (12.3) ] .”
US prescribing information · 035a8d4e-2063-4240-83cb-d7eebcabe301 · read 2026-08-30
On people who are pregnant, the label states: “P regnancy Exposure Registry There is a pregnancy exposure registry that monitors pregnancy outcomes in women exposed to antiepileptic drugs (AEDs), such as Phenytoin Sodium Injection, during pregnancy.”
US prescribing information · 035a8d4e-2063-4240-83cb-d7eebcabe301 · read 2026-08-30
On people who are breastfeeding, the label states: “Risk Summary Phenytoin is secreted in human milk.”
US prescribing information · 035a8d4e-2063-4240-83cb-d7eebcabe301 · read 2026-08-30
Where the result stopped carrying
The molecule was synthesised in 1908 and sat unused for thirty years because nobody had a way to test whether it stopped seizures
Its screening method, which found it, encoded the assumption that a drug inactive in the electroshock and pentylenetetrazol tests cannot work; levetiracetam later falsified that
It lost first-line status in most high-income countries on long-term harms rather than on any failure of seizure control
This is a scope explorer, not a diagnosis engine.
It shows who was studied so you can see whether people similar to you were included.
RNAWiki cannot tell whether a study fits you. It can show who was in it.
What it would be like to take◇Read from sources, not yet reviewed
Why it might seem to do nothing
A real effect and a noticed effect are different things. These are the recorded reasons the two come apart.
It was studied in different people
The people in the studies were not much like the reader.
On this record: Which groups were under-represented in the studies has not been recorded for this substance. Pregnancy, breastfeeding, children, older adults, and people with liver or kidney conditions are commonly excluded from trials; nothing here says whether they were.
The evidence may simply be wrong
Small early studies often do not hold up.
On this record: RNAWiki has not yet published a reviewed conclusion for this use.
Other reasons RNAWiki checked and found nothing for (11)
It was studied for a different goal
The studies measured something else entirely. Nothing in this record points to this reason.
A different form was studied
The studied form is not the form on the shelf. Nothing in this record points to this reason.
There was nothing to correct
Where a level is already normal, topping it up may change nothing. Nothing in this record points to this reason.
Something else had to happen too
In the studies it was paired with training, a diet or another treatment. Nothing in this record points to this reason.
The change is too small to feel
A real change can still sit below what a person notices. Nothing in this record points to this reason.
Day-to-day swing hides it
Sleep, food, stress and time of day move most numbers more than this would. Nothing in this record points to this reason.
Not enough time yet
The studies ran longer than the person has waited. Nothing in this record points to this reason.
It was not taken as studied
Missed days change the result, and studies count them. Nothing in this record points to this reason.
Something else changed at the same time
Two changes at once cannot be told apart afterwards. Nothing in this record points to this reason.
The product may not be what it says
Contents of a sold product are not always what the label states. Nothing in this record points to this reason.
No recorded reason
RNAWiki has nothing stored that would explain it. Nothing in this record points to this reason.
None of these is a reason to take more. Taking more is not a step this page ever suggests.
What it would be like to take◇Read from sources, not yet reviewed
What taking it involves
The recorded facts about getting hold of it and using it. Nothing here is a suggestion about what you should do.
How it is supplied
Given by a clinician
❝ Quoted from a source
Where this came from
Copied from a source RNAWiki stored, with the source named.
Read from the recorded approval status.
No source is stored against this line.
Form and route
Oral extended-release capsule, chewable tablet, oral suspension, and intravenous or intramuscular injection
✎ Source-linked record
Where this came from
A person wrote this into the record with the study named beside it. No reviewer has signed it off.
The form and route recorded on this substance.
No source is stored against this line.
Who oversees it
Professional supervision is normally required. A regulator classifies this substance in a way that restricts how it is supplied.
❝ Quoted from a source
Where this came from
Copied from a source RNAWiki stored, with the source named.
Suppression classes recorded: S3.
No source is stored against this line.
What is in the pack
Formulations are not interchangeable by milligram. The chewable tablet is phenytoin free acid and is absorbed faster than the 100 mg extended phenytoin sodium capsule, and the sodium salt contains about 92% phenytoin by weight. The injection is formulated in propylene glycol at high pH, which is why extravasation causes severe local tissue injury and why the water-soluble prodrug fosphenytoin was developed.
✎ Source-linked record
Where this came from
A person wrote this into the record with the study named beside it. No reviewer has signed it off.
Recorded notes on how this is sold and what that means for what is in the pack.
No source is stored against this line.
Where it is registered
Regulatory records are listed in the technical disclosure at the foot of this page.
# Counted from records
Where this came from
A count of rows RNAWiki holds. It describes our records, not your body.
Register entries are stored per jurisdiction and shown with their dates.
No source is stored against this line.
Why people stop
Not recorded.
∅ Nothing recorded
Where this came from
RNAWiki holds nothing here and says so rather than guessing.
No record of why people stopped taking it is stored for this substance.
No source is stored against this line.
What it would be like to take◇Read from sources, not yet reviewed
What can go wrong
Sorted by where each line came from. A regulator's label and a report somebody sent in are not the same kind of fact.
·Worth asking a clinician aboutWritten into the record from the studies named on this page
The injection carries a boxed warning for cardiovascular risk with rapid infusion: not above 50 mg per minute in adults, with cardiac monitoring during and after, and the warning notes events have also occurred at or below that rate. Across formulations: SJS and TEN, DRESS and multi-organ hypersensitivity, angioedema, acute hepatotoxicity, haematopoietic complications, bradycardia and cardiac arrest. Abrupt withdrawal may precipitate status epilepticus. Chronic use lowers vitamin D and is associated with osteopenia, osteoporosis, osteomalacia and fractures. Gingival overgrowth and dysmorphic facial changes are the visible long-term costs. Prenatal exposure carries a named fetal hydantoin syndrome and a neonatal bleeding disorder from reduced vitamin K-dependent clotting factors. Consider avoiding the drug in HLA-B*1502-positive patients and in CYP2C9*3 carriers. The class-wide suicidality warning applies.
Nobody counted how many people took this and were fine, so this cannot be turned into a rate.
ESETT: Established Status Epilepticus Treatment Trial (NCT01960075) · a recorded source, not a stored snapshot
Over a long time. No completed tested study window is recorded, so long-term effects are not established by the registry record.
Groups the studies covered thinly. Which groups were under-represented in the studies has not been recorded for this substance. Pregnancy, breastfeeding, children, older adults, and people with liver or kidney conditions are commonly excluded from trials; nothing here says whether they were.
What is missing or unclear◇Read from sources, not yet reviewed
Does the exact form matter?
Evidence belongs to the exact thing that was tested. A different salt, a different mixture or a different preparation is a different question.
The form that was studied
Oral extended-release capsule, chewable tablet, oral suspension, and intravenous or intramuscular injection
✎ Source-linked record
Where this came from
A person wrote this into the record with the study named beside it. No reviewer has signed it off.
The form used in the studies cited on this page.
No source is stored against this line.
What is sold
The chewable tablet is phenytoin free acid and is absorbed faster than the 100 mg extended phenytoin sodium capsule, and the sodium salt contains about 92% phenytoin by weight. The injection is formulated in propylene glycol at high pH, which is why extravasation causes severe local tissue injury and why the water-soluble prodrug fosphenytoin was developed.
✎ Source-linked record
Where this came from
A person wrote this into the record with the study named beside it. No reviewer has signed it off.
Recorded notes on which forms are sold and how they compare.
No source is stored against this line.
What is recorded as being sold
74 products list this as an active ingredient in the United States drug directory. 74 of them contain it and nothing else.
FDA National Drug Code directory · 0641-0493 · read 2026-08-29
They are sold as capsule, capsule, extended release, injection, powder, suspension and tablet, chewable, taken intramuscular, intravenous and oral.
FDA National Drug Code directory · 0641-0493 · read 2026-08-29
The regulator's established pharmacologic class for it is anti-epileptic agent [epc], cytochrome p450 1a2 inducers [moa] and cytochrome p450 2b6 inducers [moa].
FDA National Drug Code directory · 0641-0493 · read 2026-08-29
50 published labels name it as an active ingredient. 50 of them describe this substance alone, which is where its own label text on this page comes from.
US prescribing information · 7607ddff-2f1c-272c-e053-2991aa0a1ab3 · read 2026-08-29
Those labels are classed as human prescription drug.
US prescribing information · 7607ddff-2f1c-272c-e053-2991aa0a1ab3 · read 2026-08-29
22 marketed supplement labels list this ingredient, classed as vitamin.
Those labels carry all other, nutrient and structure/function claims. A claim of that kind is written by the manufacturer and is not assessed by any regulator, so its presence says nothing about whether it is true.
Phenytoin Sodium is intramuscular at 3 DOSAGE FORMS AND STRENGTHS Phenytoin Sodium Injection, USP: 50 mg phenytoin sodium per milliliter is available as: 2 mL (100 mg) Single Dose vials 5 mL (250 mg) Single Dose vials Injection: 50 mg phenytoin sodium per…, recorded as fda label in effect 2024-04-09 in the United States.
US prescribing information · 035a8d4e-2063-4240-83cb-d7eebcabe301 · read 2026-08-30
Recorded price in US: 0.07562 USD per one millilitre, across 3 priced products whose strengths and pack forms differ, as of 2026-08-26, paid by retail pharmacies buying the product, as surveyed by the Centers for Medicare & Medicaid Services. It is not a list price, an insurance price, or what anyone pays at a counter..
Recorded source · 2026-08-26 · read 2026-08-28
Recorded price in US: 0.18123 USD per one unit as the pricing file counts it — a tablet, capsule, patch or single item, across 6 priced products whose strengths and pack forms differ, as of 2026-08-26, paid by retail pharmacies buying the product, as surveyed by the Centers for Medicare & Medicaid Services. It is not a list price, an insurance price, or what anyone pays at a counter..
Recorded source · 2026-08-26 · read 2026-08-28
Evidence carries across two names for one substance. It does not carry across a salt, a mirror form or a mixture.
What it would be like to take◇Read from sources, not yet reviewed
What you could measure
This one is decided with a clinician, so this section holds questions to ask rather than a plan to run.
Self-experiment planning is not offered for a prescription or clinician-supervised medicine. The questions below are for a clinician or pharmacist.
Questions worth asking
Which of the trials of Phenytoin studied people like me?
What was measured, and for how long?
Was the result a laboratory value or a health outcome?
What would we watch for, and when would we stop?
Tracking can show whether something changed for you. It cannot show what caused it.
RNAWiki records evidence. It does not say whether this substance is right for you.
What is missing or unclear◇Read from sources, not yet reviewed
Claims that go past the evidence
Things said about this substance that sound reasonable, and the exact step that is missing between the evidence and the claim.
✗Goes past the evidence
That a total phenytoin level inside the reference range excludes toxicity, when the label directs unbound-fraction monitoring in renal disease, hepatic disease and hypoalbuminaemia
Why it sounds right. It follows from something real on this page: a body step, an animal result or a related finding.
The missing step. Nobody measured this in people for this use, or the study that did measure it did not show it.
What would change this
A study in people, for this exact use, measuring this exact thing, that a reviewer signs off.
RNAWiki has not yet published a reviewed conclusion for this use.
✗Goes past the evidence
That the severe cutaneous reactions of anti-seizure drugs share one immune mechanism: for phenytoin the replicated signal is a clearance variant, not an HLA allele
Why it sounds right. It follows from something real on this page: a body step, an animal result or a related finding.
The missing step. Nobody measured this in people for this use, or the study that did measure it did not show it.
What would change this
A study in people, for this exact use, measuring this exact thing, that a reviewer signs off.
RNAWiki has not yet published a reviewed conclusion for this use.
✗Goes past the evidence
That CYP2C9 genotyping before treatment prevents severe reactions, which is plausible and has not been prospectively tested as HLA-B*1502 screening was for carbamazepine
Why it sounds right. It follows from something real on this page: a body step, an animal result or a related finding.
The missing step. Nobody measured this in people for this use, or the study that did measure it did not show it.
What would change this
A study in people, for this exact use, measuring this exact thing, that a reviewer signs off.
RNAWiki has not yet published a reviewed conclusion for this use.
✗Goes past the evidence
That an infusion rate at or below 50 mg per minute is safe, when the boxed warning states the cardiovascular events have also occurred at or below the recommended rate
Why it sounds right. It follows from something real on this page: a body step, an animal result or a related finding.
The missing step. Nobody measured this in people for this use, or the study that did measure it did not show it.
What would change this
A study in people, for this exact use, measuring this exact thing, that a reviewer signs off.
RNAWiki has not yet published a reviewed conclusion for this use.
What is missing or unclear◇Read from sources, not yet reviewed
What nobody knows yet
Open questions, each with why it is open and what would close it.
Missing populations
Which groups were under-represented in the studies has not been recorded.
Why it matters. Recorded by the evidence model as a gap on this record.
What would answer it
A study that measures it, and a reviewer to sign it off.
Last checked 2026-09-04. Searched: The sources listed in the receipts at the foot of this page.
Missing long-term data
No completed tested study window is recorded.
Why it matters. Recorded by the evidence model as a gap on this record.
What would answer it
A study that measures it, and a reviewer to sign it off.
Last checked 2026-09-04. Searched: The sources listed in the receipts at the foot of this page.
No reviewed conclusion
RNAWiki has not yet published a reviewed conclusion for this use.
Why it matters. Recorded by the evidence model as a gap on this record.
What would answer it
A study that measures it, and a reviewer to sign it off.
Last checked 2026-09-04. Searched: The sources listed in the receipts at the foot of this page.
Missing interaction studies
No interaction was found in the registers checked. Not finding one is not the same as showing there is none.
Why it matters. Recorded by the evidence model as a gap on this record.
What would answer it
A study that measures it, and a reviewer to sign it off.
Last checked 2026-09-04. Searched: The sources listed in the receipts at the foot of this page.
Formulation uncertainty
Several salts, forms or products of Phenytoin are recorded. Results from one form may not transfer to another.
Why it matters. Recorded by the evidence model as a gap on this record.
What would answer it
A study that measures it, and a reviewer to sign it off.
Last checked 2026-09-04. Searched: The sources listed in the receipts at the foot of this page.
Mechanism not reviewed
No reviewed mechanism story exists for this substance.
Why it matters. Recorded by the evidence model as a gap on this record.
What would answer it
A study that measures it, and a reviewer to sign it off.
Last checked 2026-09-04. Searched: The sources listed in the receipts at the foot of this page.
What happened in people◇Read from sources, not yet reviewed
Check any of this yourself
Every line above traced back to the study it came from. This is the technical layer, and the vocabulary changes here.
Every line above can be traced to the study named beside it. Follow the link and read it.
Veterans Affairs 1985: tied with carbamazepine for the best overall result
In plain words
Six hundred and twenty-two adults were randomly assigned double-blind to one of four anti-seizure drugs and followed two years. Carbamazepine and phenytoin came out equal at the top; phenobarbital was in the middle and primidone last.
What was measured
Overall treatment success at two years, combining seizure control and tolerability
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
Mattson and colleagues ran a 10-centre double-blind trial in 622 adults with partial and secondarily generalised tonic-clonic seizures, randomised to carbamazepine, phenobarbital, phenytoin or primidone and followed two years or until failure. Overall treatment success was highest with carbamazepine or phenytoin, intermediate with phenobarbital and lowest with primidone (p<0.002), the differences driven mainly by primidone causing more nausea, vomiting, dizziness and sedation. Control of tonic-clonic seizures did not differ significantly between the drugs. Carbamazepine controlled partial seizures completely more often than primidone or phenobarbital (p<0.03). Phenytoin caused more dysmorphic effects and hypersensitivity than the others, and the authors recommended carbamazepine and phenytoin as the drugs of first choice for single-drug therapy in adults.
Written into the record, not signed off as a reviewed claim
ESETT: no better and no worse than levetiracetam or valproate in status epilepticus
In plain words
When a seizure will not stop after a benzodiazepine, three drugs are commonly given next. A blinded trial compared them and found all three stopped the seizure in about half of patients, with no winner.
What was measured
Seizure cessation with improved consciousness at 60 minutes, without additional anticonvulsant
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
ESETT randomised 384 children and adults with benzodiazepine-refractory convulsive status epilepticus to fosphenytoin (118), levetiracetam (145) or valproate (121) in a blinded, response-adaptive design. The primary outcome, absence of clinically evident seizures with improved consciousness at 60 minutes without additional anticonvulsant, occurred in 45% on fosphenytoin (95% credible interval 36 to 54), 47% on levetiracetam (39 to 55) and 46% on valproate (38 to 55), with posterior probabilities of being most effective of 0.24, 0.41 and 0.35. The trial stopped at a planned interim analysis for futility of finding any drug superior or inferior. Numerically more hypotension and intubation occurred with fosphenytoin and more deaths with levetiracetam, neither significantly. Fosphenytoin is the water-soluble prodrug, so this result belongs to phenytoin as the active agent.
Written into the record, not signed off as a reviewed claim
The clearing enzyme saturates: a 10% dose rise can cause intoxication
In plain words
For most drugs, a slightly bigger dose gives a slightly higher blood level. Not this one. The liver enzyme that removes phenytoin runs out of capacity, so once the level is already toward the top of the range, a 10% increase in dose can push it into toxicity.
What was measured
Disproportionate rise in steady-state serum level from a dose increment of 10% or more, and half-life range of 7 to 29 hours
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
Phenytoin is metabolised primarily by CYP2C9 and to a lesser extent CYP2C19. The label states that because phenytoin is hydroxylated in the liver by an enzyme system that is saturable at high serum levels, small incremental doses may increase the half-life and produce very substantial increases in serum levels when these are in the upper range, and that steady-state levels may be disproportionately increased with resultant intoxication from a dosage increase of 10% or more. Average plasma half-life from the chewable tablet studies was 14 hours with a range of 7 to 29. The label also notes wide interpatient variability at equivalent doses, with unusually high levels arising from liver disease, variant CYP2C9 and CYP2C19 alleles, or interacting drugs, and unusually low levels from non-adherence or fast metabolism. Because the drug is extensively protein bound, the label directs that in renal or hepatic impairment or hypoalbuminaemia, monitoring should be based on the unbound fraction rather than the total level.
Source
Phenytoin United States prescribing information, Clinical Pharmacology 12.3 and Warnings and Precautions 5.11 (openFDA drug label endpoint)
Role in the trial
Not matched to a registered study
Identity check
no open identity hold
Audit mark
verified
Review state
Written into the record, not signed off as a reviewed claim
Its severe rash turned out to be a clearance problem, not the immune story carbamazepine had
In plain words
Carbamazepine severe skin reactions were traced to an immune gene. Phenytoin looked like it should follow the same pattern, and partly does. But the strongest genetic signal for phenytoin is a variant that simply slows the drug removal, so more drug hangs around.
What was measured
That severe cutaneous reactions to anti-seizure drugs share one immunogenetic mechanism. For phenytoin the dominant signal is a pharmacokinetic one: reduced clearance, more drug, more reaction.
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
Chung and colleagues ran a genome-wide association study in Taiwanese patients with phenytoin-related severe cutaneous adverse reactions, with validation in further Taiwanese, Japanese and Malaysian samples, comparing 105 cases of phenytoin-related severe cutaneous adverse reactions (61 Stevens-Johnson syndrome or toxic epidermal necrolysis, 44 DRESS) and 78 with maculopapular exanthema against 130 phenytoin-tolerant controls and 3,655 population controls. A cluster of 16 SNPs in the CYP2C genes at 10q23.33 reached genome-wide significance, and direct sequencing identified the missense variant rs1057910, CYP2C9*3, with an odds ratio of 12 (95% CI 6.6 to 20, p=1.1 x 10 to the -17), replicated across all three populations with a meta-analytic odds ratio of 11 (6.2 to 18). Delayed clearance of plasma phenytoin was observed in patients with severe reactions, especially CYP2C9*3 carriers, supplying the functional link. The current label carries both stories: it advises considering avoidance of phenytoin in HLA-B*1502-positive patients on limited evidence borrowed from carbamazepine, and separately in CYP2C9*3 carriers on this evidence. It also states plainly that genotyping has important limitations and must never substitute for clinical vigilance.
Written into the record, not signed off as a reviewed claim
Rapid intravenous infusion causes hypotension and arrhythmia, and the boxed warning is a rate limit
In plain words
The whole boxed warning on the injectable form is about speed. Above 50 mg per minute in an adult, blood pressure can collapse and the heart can lose its rhythm, and the label notes this has also happened at or below the recommended rate.
What was measured
Maximum safe infusion rate, and reported cardiovascular events at and below that rate
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
The boxed warning on phenytoin sodium injection states that the rate of intravenous administration should not exceed 50 mg per minute in adults and 1 to 3 mg/kg/min or 50 mg per minute, whichever is slower, in paediatric patients, because of the risk of severe hypotension and cardiac arrhythmias, and that careful cardiac monitoring is needed during and after administration. It adds that although risk increases with rates above the recommendation, these events have also been reported at or below it. The oral label separately records bradycardia and cardiac arrest under Cardiac Effects. Part of the injection hazard belongs to the vehicle rather than the drug: the formulation is highly alkaline and contains propylene glycol, which is why the water-soluble prodrug fosphenytoin exists and why extravasation can cause the tissue injury known as purple glove syndrome.
Source
Phenytoin Sodium Injection United States prescribing information, boxed warning; phenytoin oral prescribing information, Warnings and Precautions 5.6 (openFDA drug label endpoint)
Role in the trial
Not matched to a registered study
Identity check
no open identity hold
Audit mark
caution
Review state
Written into the record, not signed off as a reviewed claim
EURAP: 6.4% major malformation rate, and a named fetal syndrome
In plain words
Eight of 125 pregnancies exposed to phenytoin alone ended in a major birth defect, a rate of 6.4%. The label separately describes a recognised pattern of facial, finger and growth abnormalities named after the drug.
What was measured
Prevalence of major congenital malformations at 1 year, by drug
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
In the EURAP prospective registry of pregnancies on anti-epileptic monotherapy at conception across 42 countries, major congenital malformation prevalence at one year was 8 of 125 (6.4%) for phenytoin, against 17 of 599 (2.8%) for levetiracetam, 74 of 2,514 (2.9%) for lamotrigine, 10 of 333 (3.0%) for oxcarbazepine, 6 of 152 (3.9%) for topiramate, 107 of 1,957 (5.5%) for carbamazepine, 19 of 294 (6.5%) for phenobarbital and 142 of 1,381 (10.3%) for valproate. The phenytoin denominator is the smallest of the eight, so the estimate is imprecise. The label describes increased frequencies of orofacial clefts and cardiac defects and of abnormalities characteristic of fetal hydantoin syndrome, including dysmorphic skull and facial features, nail and digit hypoplasia, growth abnormalities including microcephaly, and cognitive deficits. It also notes several reported cases of malignancy including neuroblastoma, and a potentially life-threatening neonatal bleeding disorder related to decreased vitamin K-dependent clotting factors.
Written into the record, not signed off as a reviewed claim
Chronic use thins bone, through an interaction with vitamin D
In plain words
Phenytoin switches on liver enzymes that also destroy vitamin D. Over years this lowers vitamin D, calcium and phosphate, and the label links long-term use to osteopenia, osteoporosis, osteomalacia and fractures.
What was measured
Association of chronic phenytoin use with reduced bone mineral density and fracture, via vitamin D metabolism
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
Warnings and Precautions 5.10 states that chronic use of phenytoin in patients with epilepsy has been associated with decreased bone mineral density, specifically osteopenia, osteoporosis and osteomalacia, and with bone fractures, and attributes the mechanism to hepatic enzyme induction enhancing vitamin D metabolism and lowering vitamin D levels, leading to deficiency, hypocalcaemia and hypophosphataemia. The label advises considering screening with bone-related laboratory and radiological tests as appropriate. This is a harm of the same enzyme induction that makes phenytoin interact with almost everything else a person takes, so it cannot be separated from the drug pharmacology or dosed around.
Source
Phenytoin United States prescribing information, Warnings and Precautions 5.10 (openFDA drug label endpoint)
Role in the trial
Not matched to a registered study
Identity check
no open identity hold
Audit mark
verified
Review state
Written into the record, not signed off as a reviewed claim
The discovery method mattered more than the drug, and it encoded a blind spot
In plain words
Merritt and Putnam found phenytoin in 1938 by testing compounds in a cat electroshock model rather than by trying them on patients. Every anti-seizure drug for decades afterwards was found the same way, which meant drugs that fail that particular test were never developed.
What was measured
That an animal screening model which finds one effective drug defines what an effective drug looks like. It defines what that model can find.
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
Merritt and Putnam reported sodium diphenylhydantoinate for convulsive disorders in JAMA in September 1938 after screening compounds in an electrically induced seizure model, and phenytoin became the first anti-seizure drug that suppressed seizures without sedation. The method, systematic animal screening, became the standard discovery route for the next fifty years, chiefly through the maximal electroshock and maximal pentylenetetrazol tests. That pipeline encoded an assumption: that a compound inactive in those two screens cannot work in people. Levetiracetam falsified it, showing no activity in either test up to 540 mg/kg while protecting potently in kindling models, and would have been discarded had those screens been the only filter. Phenytoin is therefore both the founding success of the screening era and the reason the era limits were invisible for so long.
Written into the record, not signed off as a reviewed claim
A total serum level is the number people act on, and it is the wrong number in three common situations
In plain words
Phenytoin is monitored by a blood test, but the standard test measures total drug and most of it is stuck to protein. In kidney disease, liver disease or low albumin, the total number can look fine while the active free drug is toxic.
What was measured
That a total phenytoin level inside the reference range means the patient is not toxic. In renal disease, hepatic disease or hypoalbuminaemia the label says to measure the unbound fraction instead.
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
Phenytoin is extensively bound to serum plasma proteins, and only the unbound fraction is pharmacologically active. Warnings and Precautions 5.11 states that because the unbound fraction is increased in patients with renal or hepatic disease or hypoalbuminaemia, monitoring in those patients should be based on the unbound fraction. In routine practice the assay ordered is usually total phenytoin, and the free level is a separate, less available and more expensive test. The result is a specific and predictable failure mode: a patient with low albumin whose total level reads within the therapeutic range while the free level is toxic. Nothing about this is contested, and the label states it directly; the gap is between what the label directs and what is routinely measured.
Source
Phenytoin United States prescribing information, Warnings and Precautions 5.11 and Clinical Pharmacology 12.3 (openFDA drug label endpoint)
Role in the trial
Not matched to a registered study
Identity check
no open identity hold
Audit mark
caution
Review state
Written into the record, not signed off as a reviewed claim
How many documents were read
47 documents were read for this substance.
RNAWiki source record
Where else this substance is registered
FDA substance identifier (UNII)
6158TKW0C5
RxNorm concept
1313112
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How this medicine reached us
Kept near the foot of the page. A historical event moves up only when it changes something about this substance today.
Withdrawn in United States, 2020, for "Resuspension Problems: Two lots of Phenytoin Oral Suspension USP 125mg/5mL may coagulate and may not resuspend as per the label copy instructions." (openFDA drug enforcement Class I recall)
What the approval register records
43 approved applications cover products containing this substance. The earliest was NDA008762, approved 19530106 to VIATRIS.
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What is not here
7 questions this page could not answer
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The older medicine-wide conclusion held in this record
Written for a clinical reader, and about the medicine as a whole rather than about one indication, population and set of trials. RNAWiki does not treat it as a programme conclusion and no reviewer has signed it off in that form. It is here word for word because it is what the record holds.
The first anti-seizure drug discovered by systematic animal screening, which tied with carbamazepine for the best overall result among four drugs in a 622-patient blinded trial and matched levetiracetam and valproate in status epilepticus, and whose defining hazard is arithmetic: its clearing enzyme saturates, so the label states that a dose increase of 10% or more can push a serum level into intoxication.
Recorded evidence blocks (10)
Q2
On the Phenytoin label: indicated for what?
"Extended phenytoin sodium capsules are indicated for the treatment of tonic-clonic (grand mal) and psychomotor (temporal lobe) seizures and prevention and treatment of seizures occurring during or following neurosurgery. Extended phenytoin sodium capsules are indicated for the treatment of tonic-clonic (grand mal) and…": indications and usage on Phenytoin's label. DailyMed label · ef4e97a7-cd18-47a9-a016-2eca5481a87e · 2026-07-24
Q3
66 registered trials of Phenytoin — at which phases?
Registered studies posting no result
51 of 66
66 registered studies of Phenytoin: 27 phase1, 13 phase4, 12 phase2, 8 phase3, 7 na, 2 na or unstated, 1 early phase1. CLINICALTRIALS_SNAPSHOT · 2026-09-01
747 with a PubMed record
Show the evidence
phase1
27
phase4
13
phase2
12
phase3
8
na
7
na or unstated
2
7 more recorded rows
early phase1
1
completed
41
terminated
11
unknown
6
recruiting
4
not yet recruiting
3
withdrawn
1
recorded 2026-09-01 · last checked 2026-09-04
Q4
11 of Phenytoin's trials stopped: safety, accrual/recruitment, other?
"accrual was slow and sporadic so the study was closed"; 11 of 66 registered studies
Show the evidence
Trial
NCT00040469
terminated; "accrual was slow and sporadic so the study was closed"
NCT00598923
terminated; "End of funding and low enrollment"
NCT00610532
terminated; "Investigators decided not to continue"
NCT00774306
terminated; "study no longer consistent with current clinical practice"
NCT00801931
terminated; "Poor accrual"
NCT01110187
terminated; "Lack of enrollement"
5 further recorded trials
NCT01478035
terminated; "Low recruitment"
NCT01730313
withdrawn; "Did not get approval from the collaborating partners in-country"
NCT01878578
terminated; "prematurely terminated due to impossibility of recruiting the planned number of patients by the study centre."
NCT02088957
terminated; "Termination of study due to low enrollment. There were no safety issues."
NCT02409433
terminated; "due to slow recruitment and budgetary restraints study was prematurely terminated"
recorded 2026-09-01 · last checked 2026-09-04
Q5
Human studies of Phenytoin used Extended Phenytoin Sodium Capsules 100 mg — over how long?
studies of Phenytoin used the recorded amount. ClinicalTrials.gov · 2026-09-01
4 recorded entries; human; capsule; also "Extended Phenytoin Sodium Capsules 100 mg", "Dilantin® Kapseals® 100 mg", "Phenytoin 200 mg capsule"
Show the evidence
human
NCT00647621
Extended Phenytoin Sodium Capsules 100 mg
NCT00647621
Dilantin® Kapseals® 100 mg
NCT02595723
capsule; Phenytoin 200 mg capsule
NCT06379958
Phenytoin 100 Mg Oral Capsule
recorded 2026-09-01 · last checked 2026-09-04
Q6
Phenytoin's half-life is 7 to 10 days — which schedules were studied?
7 to 10 days, the half-life Phenytoin's label states: "Steady-state therapeutic levels are achieved at least 7 to 10 days (5 to 7 half-lives) after initiation of therapy with recommended doses of 300 mg/day." DailyMed label · ef4e97a7-cd18-47a9-a016-2eca5481a87e · 2026-07-24
Show the evidence
half lifepharmacokinetics
7 to 10 days; Steady-state therapeutic levels are achieved at least 7 to 10 days (5 to 7 half-lives) after initiation of therapy with recommended doses of 300 mg/day.
metabolismpharmacokinetics
Metabolism Phenytoin is primarily metabolized by the hepatic cytochrome P450 enzyme CYP2C9 and to a lesser extent by CYP2C19.
recorded 2026-07-24 · last checked 2026-09-04
Q7
Which 23 trials of Phenytoin posted no result?
Posted no result
23 of 23 completed trials
Registrations
NCT00000285, NCT00004403, NCT00146237, NCT00647621, NCT00366067 and NCT00627575, and 17 more
Completion dates
oldest 2001-12; newest 2024-07-30
Show the evidence
Trial
NCT00000285
2001-12
NCT00004403
2002-03
NCT00146237
2005-01
NCT00647621
2005-12
NCT00366067
2007-02
NCT00627575
2008-09-12
14 further recorded trials
NCT01122953
2010-06
NCT01187719
2012-09
NCT01635829
2012-09
NCT01586208
2012-10
NCT01451593
2015-03
NCT03650270
2018-03-01
NCT03632915
2019-12-31
NCT04845646
2021-06-10
NCT04981704
2021-08-17
NCT04956627
2022-04-27
NCT06633185
2023-01-01
NCT06067412
2023-01-31
NCT03196466
2023-06-15
NCT04647877
2023-06-15
Q8
At the median, Phenytoin's trials enrolled 36 people — anything larger?
Median enrolment
36
Largest enrolment
1649
Registered trials counted
66
Q9
What do 5057 spontaneous reports say about Phenytoin — and not say?
These are reports people sent to a regulator. They do not show the medicine caused the reaction. Nobody counted how many people took the medicine and reported nothing. The same event can be reported more than once, and many reports are incomplete. News coverage, lawsuits and new warnings change how often people report. A count is not a rate and not a risk.
Phenytoin appears in spontaneous reports to regulators. Across the 10 most-reported reaction terms, 5057 reaction mentions were counted: convulsion 1004; drug hypersensitivity 765; toxicity to various agents 747; seizure 577. FAERS via Open Targets · CHEMBL16 · 2026-06-24
Show the evidence
convulsion
1004
drug hypersensitivity
765
toxicity to various agents
747
seizure
577
drug interaction
518
drug reaction with eosinophilia and systemic symptoms
347
4 more recorded rows
ataxia
316
confusional state
279
pharmaceutical product complaint
254
nystagmus
250
recorded 2026-06-24 · last checked 2026-09-04
Q10
Which 10 reactions does Phenytoin's label not list?
Phenytoin is primarily metabolized by the hepatic cytochrome P450 enzyme CYP2C9 and to a lesser extent by CYP2C19, and is particularly susceptible to inhibitory drug interactions because it is subject to saturable metabolism.
pharmacokinetics
Metabolism Phenytoin is primarily metabolized by the hepatic cytochrome P450 enzyme CYP2C9 and to a lesser extent by CYP2C19.
pharmacokinetics
Unusually high levels result from liver disease, variant CYP2C9 and CYP2C19 alleles, or drug interactions which result in metabolic interference.
pharmacokinetics
Drug Interaction Studies Phenytoin is primarily metabolized by the hepatic cytochrome P450 enzyme CYP2C9 and to a lesser extent by CYP2C19.
clinical_pharmacology
Metabolism Phenytoin is primarily metabolized by the hepatic cytochrome P450 enzyme CYP2C9 and to a lesser extent by CYP2C19.
clinical_pharmacology
Unusually high levels result from liver disease, variant CYP2C9 and CYP2C19 alleles, or drug interactions which result in metabolic interference.
2 more recorded rows
Interaction statementclinical_pharmacology
Drug Interaction Studies Phenytoin is primarily metabolized by the hepatic cytochrome P450 enzyme CYP2C9 and to a lesser extent by CYP2C19.
Interaction statementclinical_pharmacology
12.5 Pharmacogenomics CYP2C9 activity is decreased in individuals with genetic variants such as the CYP2C9*2 and CYP2C9*3 alleles.
Withdrawn in United States, 2020, for "Resuspension Problems: Two lots of Phenytoin Oral Suspension USP 125mg/5mL may coagulate and may not resuspend as per the label copy instructions." (openFDA drug enforcement Class I recall)
ChEMBL 37 — CC BY-SA 3.0 Unported · ChEMBL ATC CC BY-SA · ClinicalTrials.gov — US Government work · Open Targets 26.06 — CC0 · mixed register set; per-register licences in docs/specs/corpus-20k-sources.md · openFDA / DailyMed — US Government work · openFDA enforcement — US Government work
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