This page shows what was measured, who it was measured in, and what that does not settle.
What Ondansetron does in the body
Being sick, or feeling sick, after chemotherapy, radiotherapy or an operation
Chemotherapy damages the lining of the small intestine, and the damaged cells release serotonin. The serotonin lands on the vagus nerve, which runs from the gut to the brainstem, and the message the brainstem receives is: vomit. Ondansetron plugs the specific serotonin receptor on that nerve, so the message is never sent. It does nothing to the chemotherapy and nothing to the damage — it cuts one wire in the alarm system.
What happened in people
70% of patients with zero vomiting against 0% on concurrent placebo after cyclophosphamide-based chemotherapy (P = .008)
✎ 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.
That the 66% response rate in cisplatin chemotherapy is a placebo-controlled effect — it was benchmarked against a historical control, with no concurrent placebo arm
Where it acts
Vagal afferent nerve endings in the wall of the small intestine, and the chemoreceptor trigger zone in the area postrema of the brainstem — the one part of the vomiting circuit that sits outside the blood-brain barrier
Kind of result
The kind of result is not recorded
Supervision
RNAWiki has not recorded a supervision or regulatory status for this substance.
What the registries record it as
Its recorded molecular formula is C18H19N3O, weighing 293.4 g/mol.
US prescribing information · e436e361-da0f-7d6d-e053-2995a90a10e4 · read 2026-08-30
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 100 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.
Complete absence of emesis after cyclophosphamide-containing chemotherapy, with nausea visual analogue score as co-primary
70% with no vomiting on ondansetron against 0% on placebo, P = .008; median nausea 8 mm against 65 mm, P < .001
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. A small trial. The arm sizes are not stated in the abstract; the enrolled total is small enough that the confidence interval around 70% is wide, and the result is quoted here as a proportion rather than a precise estimate.
How far it carries. This study is recorded from the curated record, not from the registry match.
Form and route. Oral tablet, orally disintegrating tablet, oral soluble film, oral solution, and intravenous or intramuscular injection
Interval reported. Not recorded in this summary
Written into the record, not signed off as a reviewed claim.
66% on 24 mg once daily, 55% on 8 mg twice daily, 55% on 32 mg once daily; each superior to a historical placebo control, with no concurrent placebo arm
Repeated elsewhere
Partially 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. No concurrent control. Steroids were excluded from the design, so the comparison is not to contemporary practice. Two of the three regimens tested — 8 mg twice daily and 32 mg once daily — are no longer recommended in the label.
How far it carries. This study is recorded from the curated record, not from the registry match.
Form and route. Oral tablet, orally disintegrating tablet, oral soluble film, oral solution, and intravenous or intramuscular injection
Interval reported. Not recorded in this summary
Written into the record, not signed off as a reviewed claim.
Maximum mean placebo-corrected, baseline-adjusted QTcF change after a 15-minute intravenous infusion
✗ The study did not show it
Who was studied
Thorough QT study (reported in the United States label, section 12.2)
How many people
58
Study design
Randomised, double-blind, placebo- and positive-controlled crossover
Compared against
A dummy treatment
Kind of result
A number that stands in for health
What was found
19.5 ms (95% upper bound 21.8) at 32 mg; 5.6 ms (7.4) at 8 mg; significant concentration-QTcF exposure-response relationship
Repeated elsewhere
Unreplicated
What this does not prove. A study that did not show an effect does not show that no effect exists anywhere.
The limits of this study, and where it came from
Main limit. The 32 mg single intravenous dose was subsequently removed from the label. This is the rare case of a registration-era dose being withdrawn on the strength of the sponsor’s own cardiac safety study rather than an epidemiological signal.
How far it carries. This study is recorded from the curated record, not from the registry match.
Form and route. Oral tablet, orally disintegrating tablet, oral soluble film, oral solution, and intravenous or intramuscular injection
Interval reported. Not recorded in this summary
Written into the record, not signed off as a reviewed claim.
Ondansetron -27 mm (95% CI 22 to 33), metoclopramide -28 mm (22 to 34), saline placebo -23 mm (16 to 30); no significant difference
Repeated elsewhere
Replicated
What this does not prove. A study that did not show an effect does not show that no effect exists anywhere.
The limits of this study, and where it came from
Main limit. Rescue medication was needed by 34.5% on ondansetron, 17.9% on metoclopramide and 36.3% on placebo — a spread that goes the wrong way for ondansetron and is not the primary endpoint.
How far it carries. This study is recorded from the curated record, not from the registry match.
Form and route. Oral tablet, orally disintegrating tablet, oral soluble film, oral solution, and intravenous or intramuscular injection
Interval reported. 95% CI 22 to 33), metoclopramide -28 mm (22 to 34), saline placebo -23 mm (16 to 30); no significant difference
Written into the record, not signed off as a reviewed claim.
14% against 35%, relative risk 0.40 (95% CI 0.26 to 0.61); intravenous rehydration 14% against 31%, RR 0.46 (0.26 to 0.79)
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. Hospitalisation (4% against 5%, P = 1.00) and return emergency-department visits (19% against 22%, P = 0.73) did not differ. The drug moved process measures, not disposition.
How far it carries. This study is recorded from the curated record, not from the registry match.
Form and route. Oral tablet, orally disintegrating tablet, oral soluble film, oral solution, and intravenous or intramuscular injection
Interval reported. 95% CI 0
Written into the record, not signed off as a reviewed claim.
RNAWiki holds 6 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.
Where a source records it acting
Intestines: Cytotoxic chemotherapy appears to be associated with release of serotonin from the enterochromaffin cells of the small intestine; the released serotonin may stimulate vagal afferents through 5-HT3 receptors
US prescribing information · 00327696-c496-4c83-a63e-9e29fd6246d4 · read 2026-08-27
Brainstem: Serotonin receptors of the 5-HT3 type are present centrally in the chemoreceptor trigger zone of the area postrema
US prescribing information · 00327696-c496-4c83-a63e-9e29fd6246d4 · read 2026-08-27
Start
Ondansetron
What a person takes: Oral tablet, orally disintegrating tablet, oral soluble film, oral solution, and intravenous or intramuscular injection.
The measurement behind this step
The orally disintegrating tablet and the soluble film exist for the obvious reason: the population being treated is vomiting. Absorption from the oral route is good enough that the intravenous form is reserved for people who cannot swallow, not for people who need a bigger effect.
Getting in
Chemotherapy tears up the lining of the small intestine
The drug does nothing at this stage. What starts the process is the chemotherapy itself damaging the cells that line the gut, and those cells releasing a chemical when they are injured.
──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
Cytotoxic agents damage enterochromaffin cells in the mucosa of the small intestine, which release stored 5-hydroxytryptamine. More than 90% of patients receiving cisplatin at 50 mg/m2 or more vomit in the absence of any antiemetic, which is the figure the historical placebo comparison in the label rests on.
Serotonin lands on the nerve that runs from gut to brainstem
The released chemical hits the vagus nerve, the long nerve connecting the gut to the base of the brain. That is the wire that carries the instruction to vomit.
╌╌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
Released 5-HT activates 5-HT3 receptors on vagal afferent terminals in the gut wall. The 5-HT3 receptor is not a G-protein-coupled receptor: it is a pentameric ligand-gated cation channel, so activation is a direct depolarising current rather than a second-messenger cascade, and it is fast.
The drug sits in the same pocket serotonin would use, and blocks it. The nerve stops firing, and the instruction is never sent.
──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
Ondansetron is a selective competitive antagonist at 5-HT3. The basic imidazole nitrogen occupies the site the protonated primary amine of serotonin would take, holding the channel shut. Selectivity is the whole design: the label reports no effect on plasma prolactin, and no effect on oesophageal motility, gastric motility, lower oesophageal sphincter pressure or small-bowel transit at 0.15 mg/kg intravenously.
The same block happens in the one brain region open to the bloodstream
A small patch of the brainstem sits outside the barrier that keeps most drugs out of the brain. Ondansetron reaches the receptors there too, which is why it works on signals that never came from the gut.
╌╌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 area postrema, the chemoreceptor trigger zone, lies outside the blood-brain barrier and carries 5-HT3 receptors. Antagonism at both the peripheral vagal terminal and the central trigger zone is why 5-HT3 blockade works in postoperative and radiotherapy-induced vomiting as well as chemotherapy-induced vomiting.
This is the part patients are least often told. The drug is very good at stopping the act of vomiting and much less good at stopping the feeling of being sick, and the two are measured separately in every trial.
──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
In the label’s own 357-patient dose-comparison trial, 66% of patients on 24 mg once daily had zero emetic episodes and no rescue, while 56% had no nausea at all; on 8 mg twice daily the figures were 55% and 36%. Nausea is a rating-scale outcome mediated by pathways beyond 5-HT3, which is why the addition of an NK-1 antagonist and a corticosteroid changes the delayed-phase result and 5-HT3 blockade alone does not.
At higher intravenous doses the drug also slows a potassium channel in heart muscle, which stretches the heart’s electrical cycle. That finding is why the highest dose no longer exists.
──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
Concentration-dependent hERG-mediated QTcF prolongation: 19.5 ms (95% upper bound 21.8) at 32 mg and 5.6 ms (7.4) at 8 mg over a 15-minute infusion in 58 healthy subjects, with a significant exposure-response relationship. Postmarketing torsade de pointes is recorded. The label now warns against use in congenital long QT syndrome and advises ECG monitoring with hypokalaemia, hypomagnesaemia, heart failure, bradyarrhythmia or other QT-prolonging drugs.
No suggested links are held for this record, so nothing is hidden from this path.
What we know
The record describes 6 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.
People having chemotherapy or radiotherapy, people recovering from surgery, and — far more often than any of those, and outside the licence in most countries — people who have arrived at an emergency department feeling sick, or who are pregnant and vomiting.
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: “The safety and effectiveness of orally administered ondansetron have not been established in pediatric patients for: prevention of nausea and vomiting associated with highly emetogenic cancer chemotherapy prevention of nausea and vomiting associated with radiotherapy prevention of postoperative nausea and/or vomiting”
US prescribing information · e436e361-da0f-7d6d-e053-2995a90a10e4 · read 2026-08-30
On older people, the label states: “Of the total number of subjects enrolled in cancer chemotherapy-induced and postoperative nausea and vomiting in U.S.- and foreign-controlled clinical trials, for which there were subgroup analyses, 938 (19%) were aged 65 years and older.”
US prescribing information · e436e361-da0f-7d6d-e053-2995a90a10e4 · read 2026-08-30
On people who are pregnant, the label states: “Risk Summary Published epidemiological studies on the association between ondansetron use and major birth defects have reported inconsistent findings and have important methodological limitations that preclude conclusions about the safety of ondansetron use in pregnancy (see Data) .”
US prescribing information · e436e361-da0f-7d6d-e053-2995a90a10e4 · read 2026-08-30
On people who are breastfeeding, the label states: “Risk Summary It is not known whether ondansetron is present in human milk.”
US prescribing information · e436e361-da0f-7d6d-e053-2995a90a10e4 · read 2026-08-30
On people with reduced liver function, the label states: “No dosage adjustment is needed in patients with mild or moderate hepatic impairment.”
US prescribing information · e436e361-da0f-7d6d-e053-2995a90a10e4 · read 2026-08-30
On people with reduced kidney function, the label states: “No dosage adjustment is recommended for patients with any degree of renal impairment (mild, moderate, or severe).”
US prescribing information · e436e361-da0f-7d6d-e053-2995a90a10e4 · read 2026-08-30
Where the result stopped carrying
Two of the three oral regimens studied in the pivotal highly-emetogenic trial, 8 mg twice daily and 32 mg once daily, are no longer recommended
The 32 mg single intravenous dose was withdrawn after the sponsor’s own thorough QT study
No statistically significant benefit over saline placebo at 30 minutes in undifferentiated emergency-department nausea, in a 270-patient trial and then in a Cochrane review of 952 participants
Serotonin syndrome, anaphylaxis with cross-reactivity across the class, and myocardial ischaemia were all postmarketing findings, not trial findings
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 tablet, orally disintegrating tablet, oral soluble film, oral solution, 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
RNAWiki has not recorded a supervision or regulatory status for this substance.
❝ Quoted from a source
Where this came from
Copied from a source RNAWiki stored, with the source named.
No register row and no identity class settled the question.
No source is stored against this line.
What is in the pack
The orally disintegrating tablet and the soluble film exist for the obvious reason: the population being treated is vomiting. Absorption from the oral route is good enough that the intravenous form is reserved for people who cannot swallow, not for people who need a bigger effect.
✎ 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
No boxed warning. Contraindicated with apomorphine because of profound hypotension and loss of consciousness, and in known hypersensitivity. Warnings cover hypersensitivity including anaphylaxis and bronchospasm with cross-reactivity to other 5-HT3 antagonists; QT prolongation and torsade de pointes, with avoidance in congenital long QT syndrome and ECG monitoring where electrolytes are abnormal or other QT-prolonging drugs are used; serotonin syndrome, including fatal cases, particularly alongside other serotonergic drugs; myocardial ischaemia after oral administration; and masking of progressive ileus or gastric distension after abdominal surgery. Constipation and headache are the common complaints.
Nobody counted how many people took this and were fine, so this cannot be turned into a rate.
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 tablet, orally disintegrating tablet, oral soluble film, oral solution, 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
Absorption from the oral route is good enough that the intravenous form is reserved for people who cannot swallow, not for people who need a bigger effect.
✎ 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
316 products list this as an active ingredient in the United States drug directory. 316 of them contain it and nothing else.
FDA National Drug Code directory · 71610-119 · read 2026-08-29
They are sold as film, injection, injection, solution, powder, solution and tablet, taken intramuscular, intravenous and oral.
FDA National Drug Code directory · 71610-119 · read 2026-08-29
The regulator's established pharmacologic class for it is serotonin 3 receptor antagonists [moa] and serotonin-3 receptor antagonist [epc].
FDA National Drug Code directory · 71610-119 · read 2026-08-29
222 published labels name it as an active ingredient. 222 of them describe this substance alone, which is where its own label text on this page comes from.
US prescribing information · e436e361-da0f-7d6d-e053-2995a90a10e4 · read 2026-08-29
Those labels are classed as human prescription drug.
US prescribing information · e436e361-da0f-7d6d-e053-2995a90a10e4 · read 2026-08-29
1 marketed supplement label lists this ingredient, classed as other combinations.
Those labels carry all other 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.
ondansetron hydrochloride is film-coated tablets at Tablets: 4 mg, 8 mg, 16 mg and 24 mg, recorded as prescription product; fda label in effect 2024-04-24 in the United States.
US prescribing information · 00327696-c496-4c83-a63e-9e29fd6246d4 · read 2026-08-27
Recorded price in US: 0.05966–0.08122 USD per one unit as the pricing file counts it — a tablet, capsule, patch or single item, across 33 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: 0.16997–0.28036 USD per one millilitre, across 27 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.
RNAWiki could not confidently classify this substance, so no self-experiment plan is offered.
Questions worth asking
Which of the trials of Ondansetron 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 the 66% response rate in cisplatin chemotherapy is a placebo-controlled effect — it was benchmarked against a historical control, with no concurrent placebo arm
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 preventing vomiting is the same as relieving nausea; in the label’s own trial the two rates differ by ten to twenty percentage points at every dose
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 emergency-department use, which is the commonest use, is supported by the chemotherapy evidence — the randomised comparison there is against saline and it is a draw
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 reducing the drip rate in a vomiting child changes whether that child is admitted; admission and return visits did not move
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 Ondansetron 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.
Placebo-controlled from the start: 70% did not vomit against 0% on placebo
In plain words
In the first randomised trial, every single patient given a dummy injection before cyclophosphamide chemotherapy vomited. Seven in ten given ondansetron did not vomit at all.
What was measured
Proportion of patients with zero emetic episodes, and median nausea visual analogue score, against concurrent double-blind placebo
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
Cubeddu and colleagues randomised patients receiving cyclophosphamide 500 to 600 mg/m2, mostly for breast cancer and mostly in combination with doxorubicin or fluorouracil, to three intravenous doses of ondansetron 0.15 mg/kg or to placebo, double-blind. All placebo-treated patients vomited; 70% of ondansetron-treated patients did not (P = .008). Median nausea score was 8 mm on ondansetron against 65 mm on placebo (P < .001). Seventy per cent on ondansetron retained normal appetite against 10% on placebo. Adverse events occurred in six placebo patients and one ondansetron patient, and there were no extrapyramidal reactions — the point of the whole exercise, since the drug it was replacing was high-dose metoclopramide.
Written into the record, not signed off as a reviewed claim
The cisplatin registration trials had no placebo arm — the comparison was to a historical control
In plain words
For the hardest chemotherapy, the trials that supported approval did not include a group given nothing. The drug was compared against what patients had been recorded doing in earlier studies. That is a weaker design than it sounds, and the label says so.
What was measured
That the 66% response rate in highly emetogenic chemotherapy is a placebo-controlled effect size — it is a single-arm rate benchmarked against historical data, and the two 5-HT3 dosing regimens tested alongside it are no longer recommended
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
The label states that in two randomised double-blind monotherapy trials, a single 24 mg oral dose was superior to a "relevant historical placebo control" for chemotherapy including cisplatin at 50 mg/m2 or more, and that steroids were excluded from those trials. The first compared 24 mg once, 8 mg twice and 32 mg once in 357 adults: 66%, 55% and 55% respectively completed 24 hours with zero emetic episodes and no rescue. The justification offered for the historical comparator is that more than 90% of patients receiving cisplatin at that dose vomit without any antiemetic — which is true, and is an argument that a placebo arm was unnecessary, not evidence that one was run. The concurrent placebo comparison in the same label sits in the moderately emetogenic section: 33 patients on ondansetron against 34 on placebo, 61% against 6% with zero emetic episodes, P < 0.001.
Source
Ondansetron United States prescribing information, section 14.1 Prevention of Chemotherapy-Induced Nausea and Vomiting (openFDA drug/label record, DailyMed set id 00327696-c496-4c83-a63e-9e29fd6246d4)
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
The manufacturer’s own QT study removed the 32 mg intravenous dose
In plain words
The highest intravenous dose had been in the label for two decades. A dedicated heart-rhythm study found it stretched the heart’s electrical cycle by about 20 milliseconds, and that dose was taken off the label. The same study found the 8 mg dose did not.
What was measured
Maximum mean placebo-corrected, baseline-adjusted QTcF change after a 15-minute infusion, 58 healthy subjects
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
A double-blind, single-dose, placebo- and positive-controlled crossover trial in 58 healthy subjects measured QTcF after 15-minute infusions. The maximum mean (95% upper confidence bound) difference from placebo after baseline correction was 19.5 (21.8) milliseconds for 32 mg and 5.6 (7.4) milliseconds for 8 mg. A significant exposure-response relationship between ondansetron concentration and delta-delta QTcF was identified; modelling from it predicted 14 (16.3) milliseconds for 24 mg and 9.1 (11.2) milliseconds for 16 mg. The label now states that the 8 mg dose infused over 15 minutes did not prolong the QT interval to any clinically relevant extent, and the single 32 mg intravenous dose is no longer a recommended regimen anywhere in the label. Postmarketing cases of torsade de pointes are recorded.
Source
Ondansetron United States prescribing information, section 12.2 Pharmacodynamics — Cardiac Electrophysiology, and section 5.2 QT Prolongation (openFDA drug/label record, DailyMed set id 00327696-c496-4c83-a63e-9e29fd6246d4)
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
In the emergency department it did not beat saline
In plain words
For people who arrive at hospital feeling sick for no established reason — which is how the drug is most often used — a randomised trial and then a Cochrane review found it no better than salt water.
What was measured
Mean change in 100 mm nausea visual analogue scale at 30 minutes against saline placebo
Effect estimate
No reviewed effect estimate exists for this record.
Limits
This entry records a failure.
From the source
Egerton-Warburton and colleagues randomised 270 adults with undifferentiated emergency-department nausea and vomiting across two Melbourne hospitals to 4 mg intravenous ondansetron, 20 mg intravenous metoclopramide, or saline placebo; 258 were analysable. Mean fall in the 100 mm nausea visual analogue scale at 30 minutes was 27 mm (95% CI 22 to 33) for ondansetron, 28 mm (22 to 34) for metoclopramide and 23 mm (16 to 30) for placebo. Satisfaction was 54.1%, 61.6% and 59.5% respectively. The Cochrane review that followed pooled eight trials and 952 participants; against placebo at 30 minutes the mean difference was -4.32 mm (95% CI -11.20 to 2.56) for ondansetron and -5.27 mm (-11.33 to 0.80) for metoclopramide. Only droperidol, in one 48-patient trial, was statistically superior to placebo. The reviewers concluded that intravenous fluid alone may be sufficient for most people.
Written into the record, not signed off as a reviewed claim
In children with gastroenteritis it halved the drip rate
In plain words
One dose given to a vomiting, dehydrated child in the emergency department cut vomiting during oral rehydration from 35% to 14% and roughly halved the proportion who needed a drip. It did not change how many were admitted.
What was measured
Proportion vomiting during oral rehydration, and proportion requiring intravenous rehydration, against placebo
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
Freedman and colleagues randomised 215 children aged 6 months to 10 years with gastroenteritis and dehydration to a single orally disintegrating ondansetron tablet or placebo, double-blind, with standardised oral rehydration. Vomiting during rehydration: 14% against 35% (RR 0.40, 95% CI 0.26 to 0.61). Mean emetic episodes per child 0.18 against 0.65 (P < 0.001). Oral intake 239 mL against 196 mL (P = 0.001). Intravenous rehydration 14% against 31% (RR 0.46, 95% CI 0.26 to 0.79). Emergency-department stay fell 12% (P = 0.02). Hospital admission was 4% against 5% (P = 1.00) and return visits 19% against 22% (P = 0.73) — neither moved.
Written into the record, not signed off as a reviewed claim
The pregnancy signal narrowed from "birth defects" to one small absolute increase in cleft palate
In plain words
Ondansetron is used very widely for sickness in pregnancy, off-licence, and for years the safety question was open. The largest study, of 1.8 million pregnancies, found no increase in heart defects or in birth defects overall, and a small increase in cleft lip and palate: about three extra cases per ten thousand births.
What was measured
That ondansetron in early pregnancy raises the risk of congenital malformations generally — the largest cohort finds no such signal for cardiac or overall malformations, and an increase confined to oral clefts of under three cases per ten thousand births
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
Huybrechts and colleagues ran a retrospective cohort nested in the 2000-2013 Medicaid Analytic eXtract: 1,816,414 pregnancies, of which 88,467 (4.9%) had first-trimester ondansetron dispensing, with propensity-score stratification for indication and confounders. Cardiac malformations: absolute risk 94.4 per 10,000 exposed (95% CI 88.0 to 100.8) against 84.4 unexposed (83.0 to 85.7); adjusted RR 0.99 (0.93 to 1.06), adjusted risk difference -0.8 per 10,000 (-7.3 to 5.7). Any congenital malformation: adjusted RR 1.01 (0.98 to 1.05). Oral clefts: 14.0 per 10,000 exposed (11.6 to 16.5) against 11.1 unexposed (10.6 to 11.6); adjusted RR 1.24 (1.03 to 1.48), adjusted risk difference 2.7 per 10,000 (0.2 to 5.2). A companion analysis of intravenous exposure was published by the same group in JAMA in 2020.
Written into the record, not signed off as a reviewed claim
Serotonin syndrome, and a class of reactions that only showed up after approval
In plain words
A drug that blocks a serotonin receptor can still trigger serotonin syndrome, and some of the reported cases were fatal. None of this was in the registration trials; it came from postmarketing reports.
What was measured
Not recorded
Effect estimate
No reviewed effect estimate exists for this record.
Limits
This entry records a failure.
From the source
The label records serotonin syndrome with 5-HT3 receptor antagonists alone, most reports involving concomitant serotonergic drugs (SSRIs, SNRIs, MAO inhibitors, mirtazapine, fentanyl, lithium, tramadol, intravenous methylene blue), with some fatal cases, and cases on overdose of ondansetron alone. Most reports arose in a post-anaesthesia care unit or an infusion centre. The label further records hypersensitivity reactions including anaphylaxis and bronchospasm with cross-reactivity to other 5-HT3 antagonists; myocardial ischaemia after oral administration; masking of progressive ileus and gastric distension after abdominal surgery; and contraindicates concomitant apomorphine because of profound hypotension and loss of consciousness. All are postmarketing findings.
Source
Ondansetron United States prescribing information, sections 4 Contraindications and 5.1 to 5.5 Warnings and Precautions (openFDA drug/label record, DailyMed set id 00327696-c496-4c83-a63e-9e29fd6246d4)
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
223 documents were read for this substance.
RNAWiki source record
222 of them state the same proteinBinding, and they agree.
RNAWiki source record
Where else this substance is registered
FDA substance identifier (UNII)
NMH84OZK2B
CAS registry number
99614-02-5
PubChem compound
4595
RxNorm concept
203148
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What is missing or unclear◇Read from sources, not yet reviewed
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.
What the approval register records
85 approved applications cover products containing this substance. The earliest was NDA020007, approved 19910104 to SANDOZ.
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What is not here
7 questions this page could not answer
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Felt, measured, or meaningful — found nothing in the sources checked.
How long anything takes — found nothing in the sources checked.
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Everything above is built from what is below. This layer keeps the technical vocabulary, record identifiers and every stored row, so a reader who wants to check the page can.
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.
A selective blocker of the serotonin-gated 5-HT3 channel on the vagus nerve that stops the gut telling the brainstem to vomit — 70% of patients on cyclophosphamide-based chemotherapy did not vomit at all against 0% on placebo in the first randomised trial, and 61% against 6% in the label’s placebo-controlled study, but the highly-emetogenic registration trials had no concurrent placebo arm, and its own cardiac study later cost it its highest intravenous dose.
Recorded evidence blocks (9)
Q1
On the Ondansetron label: indicated for what?
"Ondansetron is indicated for the prevention of nausea and vomiting associated with: highly emetogenic cancer chemotherapy, including cisplatin greater than or equal to 50 mg/m 2 initial and repeat courses of moderately emetogenic cancer chemotherapy radiotherapy in patients receiving either total body irradiation,…": indications and usage on Ondansetron's label. DailyMed label · f175a336-433b-7ab2-e053-2995a90ad5ca · 2026-08-26
Q2
335 registered trials of Ondansetron — at which phases?
Registered studies posting no result
238 of 335
335 registered studies of Ondansetron: 81 phase4, 76 phase2, 71 na, 61 phase3, 50 phase1, 6 na or unstated, 5 early phase1. CLINICALTRIALS_SNAPSHOT · 2026-09-01
1422 with a PubMed record
Show the evidence
phase4
81
phase2
76
na
71
phase3
61
phase1
50
na or unstated
6
9 more recorded rows
early phase1
5
completed
233
unknown
35
terminated
26
withdrawn
13
not yet recruiting
11
recruiting
11
active not recruiting
4
enrolling by invitation
2
recorded 2026-09-01 · last checked 2026-09-04
Q3
33 of Ondansetron's trials stopped: safety, futility/efficacy, accrual/recruitment, funding/business, other?
safety (2), futility/efficacy (3), accrual/recruitment (14), funding/business (2) and other (12): Ondansetron's stop wording, clustered. CLINICALTRIALS_SNAPSHOT · 2026-09-01
"Closed due to poor accrual and lack of feasibility to finish study per DSMB"; 33 of 335 registered studies
Show the evidence
Trial
NCT00429702
terminated; "Closed due to poor accrual and lack of feasibility to finish study per DSMB"
NCT00499668
withdrawn; "slow accrual"
NCT00609765
terminated; "Development of new chemotherapy standard of care for treatment rendered the trial obsolete."
NCT00655642
terminated; "Conditional analysis showed observed differences were significantly less than power calculations"
NCT00699894
withdrawn; "Logistical difficulties running the study"
NCT00818259
terminated; "Study terminated early prior to completing targeted enrollment of participants \<6 months of age due to recruitment challenges."
14 further recorded trials
NCT00890149
terminated; "Difficulty recruiting target sample"
NCT00891761
withdrawn; "This study has been cancelled prior to enrollment."
NCT01067677
withdrawn; "We do not have the funding or resources to complete the study at this time"
NCT01275248
terminated; "lack of efficacy"
NCT01591291
withdrawn; "Grant was transferred to University of Maryland and results were reported under NCT02354703."
NCT01719042
withdrawn; "No recruitment was achievable"
NCT01843868
withdrawn; "Study halted prematurely, prior to enrollment of first participant due to protracted logistical and subsequent funding matters."
NCT01896440
withdrawn; "FDA did not approve"
NCT01952886
withdrawn; "Lack of approval and funding from company"
NCT02460055
withdrawn; "no subject met enrollment criteria"
NCT02519842
terminated; "Further data are no longer required to support an application for use in pediatric patients. The decision to terminate was not based on any new safety findings"
NCT02592707
terminated; "Terminated (Due to small number of ongoing patients. Patients ongoing at time of termination could choose to join study D-FR-01072-004 for long term follow-up."
NCT02724033
terminated; "PI has left Riley Anesthesia. Lack of staff for continued recruitment"
NCT02732015
terminated; "Terminated per PI's request"
recorded 2026-09-01 · last checked 2026-09-04
Q4
Human studies of Ondansetron used Ondansetron Tablets 24 mg — over how long?
Human studies of Ondansetron used "Ondansetron Tablets 24 mg". ClinicalTrials.gov · 2026-09-01
NCT00000443, NCT00003817, NCT00005994, NCT00006348, NCT00000289 and NCT00653458, and 132 more
Completion dates
oldest 2000-04; newest 2024-07-06
Show the evidence
Trial
NCT00000443
2000-04
NCT00003817
2001-07
NCT00005994
2001-07
NCT00006348
2001-09
NCT00000289
2001-12
NCT00653458
2002-09
14 further recorded trials
NCT00654277
2002-09
NCT00033085
2002-12
NCT00018850
2003-06
NCT00648583
2003-07
NCT00649532
2003-07
NCT00040053
2004-02
NCT00659074
2004-02
NCT00642512
2004-07
NCT00027079
2004-08
NCT00946387
2004-09
NCT00020657
2004-10
NCT00947128
2004-10
NCT00648804
2005-07
NCT00649363
2005-07
Q6
At the median, Ondansetron's trials enrolled 90 people — anything larger?
Median enrolment
90
Largest enrolment
27034
Registered trials counted
331
Q7
What do 4963 spontaneous reports say about Ondansetron — 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.
Ondansetron appears in spontaneous reports to regulators. Across the 10 most-reported reaction terms, 4963 reaction mentions were counted: nausea 913; vomiting 639; pyrexia 535; electrocardiogram qt prolonged 484. FAERS via Open Targets · CHEMBL3186492 · 2026-06-24
Show the evidence
nausea
913
vomiting
639
pyrexia
535
electrocardiogram qt prolonged
484
serotonin syndrome
464
hypotension
437
4 more recorded rows
constipation
388
erythema
377
anxiety
367
abdominal pain
359
recorded 2026-06-24 · last checked 2026-09-04
Q8
Which 10 reactions does Ondansetron's label not list?
7.2 Drugs Affecting Cytochrome P-450 Enzymes Ondansetron does not itself appear to induce or inhibit the cytochrome P‑450 drug‑metabolizing enzyme system of the liver [see Clinical Pharmacology ( 12.3 )] .
drug_interactions
Because ondansetron is metabolized by hepatic cytochrome P‑450 drug‑metabolizing enzymes (CYP3A4, CYP2D6, CYP1A2), inducers or inhibitors of these enzymes may change the clearance and, hence, the half‑life of ondansetron.
drug_interactions
In patients treated with potent inducers of CYP3A4 (i.e., phenytoin, carbamazepine, and rifampin), the clearance of ondansetron was significantly increased and ondansetron blood concentrations were decreased.
pharmacokinetics
In vitro metabolism studies have shown that ondansetron is a substrate for human hepatic cytochrome P‑450 enzymes, including CYP1A2, CYP2D6, and CYP3A4.
pharmacokinetics
In terms of overall ondansetron turnover, CYP3A4 played the predominant role.
pharmacokinetics
Because of the multiplicity of metabolic enzymes capable of metabolizing ondansetron, it is likely that inhibition or loss of one enzyme (e.g., CYP2D6 genetic deficiency) will be compensated by others and may result in little change in overall rates of ondansetron elimination.
2 more recorded rows
Interaction statementpharmacokinetics
Drug Interaction Studies CYP 3A4 Inducers: Ondansetron elimination may be affected by cytochrome P-450 inducers.
Interaction statementpharmacokinetics
In a pharmacokinetic trial of 16 epileptic patients maintained chronically on CYP3A4 inducers, carbamazepine, or phenytoin, a reduction in AUC, C max , and t ½ of ondansetron was observed.
ChEMBL 37 — CC BY-SA 3.0 Unported · 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
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✓ source coverage passed: 6 source rows
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