This page shows what was measured, who it was measured in, and what that does not settle.
What Cefalexin does in the body
Skin, throat, ear, bone and urinary infections caused by susceptible bacteria
Bacteria hold themselves together with a mesh wall that they constantly cut open and re-stitch as they grow. Cephalexin latches onto the stitching tool and will not let go. The cutting carries on, the stitching does not, and the cell bursts under its own pressure. Its ring is shaped so that the enzyme most staphylococci use to destroy penicillin cannot get at it.
What happened in people
Clinical cure 85.5% with cephalexin plus placebo against 83.5% with added trimethoprim-sulfamethoxazole in 411 per-protocol adults with cellulitis
✎ 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 MRSA coverage is unnecessary in non-purulent cellulitis — the intention-to-treat interval reached +15.5 percentage points and the authors called for further research
Where it acts
The bacterial cell envelope — the cross-linking machinery on the outer face of the cytoplasmic membrane
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
The substance registry classes this as chemical.
FDA substance registry · OBN7UDS42Y · read 2026-08-29
Its recorded molecular formula is C16H17N3O4S•H2O, weighing 365.41.
US prescribing information · 864d2046-b2c8-415c-814b-512bc4901c76 · 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 96 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.
Clinical cure of uncomplicated non-purulent cellulitis in the per-protocol population
Per protocol 83.5% against 85.5%, difference -2.0 percentage points (95% CI -9.7 to 5.7), P=.50; modified intention-to-treat 76.2% against 69.0%, difference 7.3 points (95% CI -1.0 to 15.5), P=.07
Repeated elsewhere
Partially 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. The superiority hypothesis failed, but the intention-to-treat interval reached +15.5 points, above the 10% the investigators had called clinically significant. An 18% protocol-deviation rate separates the two populations, and the trial is routinely cited on the per-protocol result alone.
How far it carries. This study is recorded from the curated record, not from the registry match.
Form and route. Oral capsules, tablets and reconstituted oral suspension
Interval reported. 95% CI -9
Written into the record, not signed off as a reviewed claim.
Moran cellulitis trial: cephalexin plus trimethoprim-sulfamethoxazole or placebo (NCT00729937) · a recorded source, not a stored snapshot
Clinical improvement at 48 to 72 hours
✓ The study showed what it set out to show
Who was studied
Chen 2011 paediatric skin and soft-tissue infection trial
How many people
200
Study design
Randomised, controlled, single-centre
Compared against
Not recorded for this study
Kind of result
A number that stands in for health
What was found
Cephalexin 94% against clindamycin 97% improved at 48 to 72 hours (P=.50); resolution at day 7 97% against 94% (P=.33)
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. Drainage occurred spontaneously or was performed in 97% of children, so the trial cannot separate the antibiotic effect from the effect of drainage. It was powered to detect superiority of clindamycin, not to establish equivalence.
How far it carries. This study is recorded from the curated record, not from the registry match.
Form and route. Oral capsules, tablets and reconstituted oral suspension
Interval reported. Not recorded in this summary
Written into the record, not signed off as a reviewed claim.
Moran cellulitis trial: cephalexin plus trimethoprim-sulfamethoxazole or placebo (NCT00729937) · a recorded source, not a stored snapshot
What we know
RNAWiki holds 2 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
Lungs and airways: Indicated for the treatment of respiratory tract infections caused by susceptible bacteria (as recorded)
US prescribing information · 02b5cd6f-3df8-43c1-879d-217fcf55231a · read 2026-08-27
Skin: Indicated for the treatment of skin and skin structure infections caused by susceptible bacteria (as recorded)
US prescribing information · 02b5cd6f-3df8-43c1-879d-217fcf55231a · read 2026-08-27
Bone and bone marrow: Indicated for the treatment of bone infections caused by susceptible bacteria (as recorded)
US prescribing information · 02b5cd6f-3df8-43c1-879d-217fcf55231a · read 2026-08-27
Bladder and urinary tract: Indicated for the treatment of genitourinary tract infections, including acute prostatitis, caused by susceptible bacteria (as recorded)
US prescribing information · 02b5cd6f-3df8-43c1-879d-217fcf55231a · read 2026-08-27
Start
Cefalexin
What a person takes: Oral capsules, tablets and reconstituted oral suspension.
The measurement behind this step
Absorbed by the PEPT1 peptide transporter rather than by passive diffusion, which gives near-complete oral bioavailability and makes the drug unusual among beta-lactams in not needing an intravenous route for ordinary infections. Cleared largely unchanged by the kidney, so exposure rises substantially when renal function is reduced.
Getting in
Carried across the gut wall by a transporter, not by diffusion
Most drugs seep through the gut lining. This one is picked up by a pump the body built to absorb fragments of digested protein, which mistakes it for one. That is why an otherwise poorly absorbable molecule ends up almost completely absorbed.
╌╌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
Cephalexin is a substrate for PEPT1 (SLC15A1), the proton-coupled intestinal di- and tripeptide transporter, and for PEPT2 in the renal tubule. Carrier-mediated uptake gives near-complete oral bioavailability for a zwitterionic beta-lactam that would otherwise cross membranes poorly, and it is saturable, which is a different absorption profile from a passively absorbed drug.
The target is not inside the bacterium. The wall-stitching tools sit on the outer face of the bacterial membrane, so the drug never has to get into the cell at all.
╌╌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
Penicillin-binding proteins are anchored in the cytoplasmic membrane with their catalytic domains facing outward into the peptidoglycan layer. In Gram-positive organisms the drug reaches them directly; in Gram-negatives it must first cross the outer membrane through porins, which is one reason cephalexin’s Gram-negative range is narrow.
It impersonates the piece the enzyme is looking for
The stitching enzyme grabs a specific two-unit tail on the wall material. The drug’s ring is shaped like that tail. The enzyme grabs it, opens the ring, and is left permanently attached.
╌╌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 strained four-membered beta-lactam ring mimics the D-alanyl-D-alanine terminus of the peptidoglycan pentapeptide. Nucleophilic attack by the active-site serine opens the ring and produces a covalent acyl-enzyme that deacylates orders of magnitude more slowly than the natural intermediate. The cephem sulfur and the 3-methyl substituent are what make this ring a poor substrate for most staphylococcal penicillinase.
A growing bacterium is constantly cutting its own wall open to expand it. With the repair enzymes jammed, the cutting proceeds unopposed and the wall thins until the cell bursts.
╌╌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
Autolysins continue to hydrolyse peptidoglycan while cross-linking is blocked, producing osmotic lysis. Killing is therefore time-dependent and requires actively dividing cells; a dormant organism inside a biofilm or a walled-off collection is not killed by any concentration.
MRSA carries a replacement stitching tool with a differently shaped grip. Cephalexin cannot get hold of it. No dose changes that — it is a shape mismatch, not a strength problem.
╌╌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 mecA gene encodes PBP2a, a transpeptidase whose active site adopts a closed conformation with very low acylation efficiency for essentially all beta-lactams except the fifth-generation cephalosporins designed against it. Cephalexin also lacks useful activity against Enterococcus, Haemophilus influenzae, Pseudomonas and organisms producing extended-spectrum or AmpC beta-lactamases.
In the two head-to-head trials, adding cover for the organism cephalexin cannot kill did not improve the outcome. In one of them, nearly everyone had the pus drained, and that may be what did the work.
──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
Per-protocol cure was 85.5% with cephalexin plus placebo against 83.5% with added trimethoprim-sulfamethoxazole in 411 adults with non-purulent cellulitis. In 200 children with 69% MRSA prevalence and 97% drainage, day-7 resolution was 97% with cephalexin against 94% with clindamycin. Neither trial measured whether antibiotic choice affected subsequent resistance carriage.
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.
Adults and children with cellulitis, other skin and soft-tissue infection, streptococcal pharyngitis, bone infection or urinary infection due to a susceptible organism.
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 cephalexin in pediatric patients was established in clinical trials for the dosages described in the dosage and administration section [see Dosage and Administration ( 2.2 )].”
US prescribing information · 864d2046-b2c8-415c-814b-512bc4901c76 · read 2026-08-30
On older people, the label states: “Of the 701 subjects in 3 published clinical studies of cephalexin, 433 (62%) were 65 and over.”
US prescribing information · 864d2046-b2c8-415c-814b-512bc4901c76 · read 2026-08-30
On people who are pregnant, the label states: “Risk Summary Available data from published epidemiologic studies and pharmacovigilance case reports over several decades with cephalosporin use, including cephalexin use in pregnant women have not established drug-associated risks of major birth defects, miscarriage, or adverse maternal or fetal outcomes (see Data).”
US prescribing information · 864d2046-b2c8-415c-814b-512bc4901c76 · read 2026-08-30
On people who are breastfeeding, the label states: “Risk Summary Data from a published clinical lactation study reports that cephalexin is present in human milk.”
US prescribing information · 864d2046-b2c8-415c-814b-512bc4901c76 · read 2026-08-30
On people with reduced kidney function, the label states: “Cephalexin should be administered with careful monitoring in the presence of renal impairment (creatinine clearance < 30 mL/min, with or without dialysis).”
US prescribing information · 864d2046-b2c8-415c-814b-512bc4901c76 · read 2026-08-30
Where the result stopped carrying
The Moran superiority hypothesis failed: adding MRSA coverage did not raise the per-protocol cure rate
Roughly a quarter to a third of intention-to-treat patients were not cured in either arm under strict failure criteria
Cephalexin has no activity against MRSA, Enterococcus, Pseudomonas or extended-spectrum beta-lactamase producers, and no dose changes that
The class-wide reassurance about cephalosporins after penicillin allergy does not extend to this molecule, because of a shared side chain
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
Prescription only
❝ 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 capsules, tablets and reconstituted oral suspension
✎ 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
Absorbed by the PEPT1 peptide transporter rather than by passive diffusion, which gives near-complete oral bioavailability and makes the drug unusual among beta-lactams in not needing an intravenous route for ordinary infections.
✎ 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. The rest of the recorded wording: Cleared largely unchanged by the kidney, so exposure rises substantially when renal function is reduced.
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
One of the better-tolerated antibiotics in wide use. Gastrointestinal upset and rash are the common events. The serious concerns are hypersensitivity — including reactions in penicillin-allergic patients, where cephalexin’s shared side chain with ampicillin gives a pooled odds ratio of 5.8 — and Clostridioides difficile-associated diarrhoea, reported with essentially all antibacterials. It can produce false-positive urine glucose tests with copper-reduction methods and a positive direct Coombs test.
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 capsules, tablets and reconstituted oral suspension
✎ 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
Cleared largely unchanged by the kidney, so exposure rises substantially when renal function is reduced.
✎ 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
195 products list this as an active ingredient in the United States drug directory. 195 of them contain it and nothing else.
FDA National Drug Code directory · 80425-0121 · read 2026-08-29
They are sold as capsule, for suspension, powder, powder, for suspension, tablet and tablet, film coated, taken oral.
FDA National Drug Code directory · 80425-0121 · read 2026-08-29
The regulator's established pharmacologic class for it is cephalosporin antibacterial [epc] and cephalosporins [cs].
FDA National Drug Code directory · 80425-0121 · read 2026-08-29
156 published labels name it as an active ingredient. 156 of them describe this substance alone, which is where its own label text on this page comes from.
US prescribing information · 864d2046-b2c8-415c-814b-512bc4901c76 · read 2026-08-29
Those labels are classed as human prescription drug.
US prescribing information · 864d2046-b2c8-415c-814b-512bc4901c76 · read 2026-08-29
Cephalexin is capsules at Capsules: 250 mg and 500 mg, recorded as prescription product; fda label in effect 2025-02-11 in the United States.
US prescribing information · 02b5cd6f-3df8-43c1-879d-217fcf55231a · read 2026-08-27
Recorded price in US: 0.04965–0.07503 USD per one millilitre, across 16 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: 0.09205–4.97616 USD per one unit as the pricing file counts it — a tablet, capsule, patch or single item, across 42 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 Cefalexin 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 MRSA coverage is unnecessary in non-purulent cellulitis — the intention-to-treat interval reached +15.5 percentage points and the authors called for further research
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 collapse of the 10% cross-reactivity figure exonerates cephalexin, when cephalexin is the first-generation agent the same analysis flagged
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 paediatric result shows the antibiotic did not matter, when 97% of those children also had the lesion drained
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 a 1971 approval implies the same evidentiary base as a modern one — the label carries no efficacy figures at all
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 Cefalexin 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.
Five American emergency departments randomised patients with plain cellulitis — no pus, no abscess — to cephalexin plus an antibiotic that kills MRSA, or cephalexin plus a placebo. The two groups came out the same. If anything the cephalexin-alone group did slightly better.
What was measured
Clinical cure at 14 to 21 days, cephalexin plus trimethoprim-sulfamethoxazole against cephalexin plus placebo
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
Moran and colleagues randomised 500 outpatients older than 12 with cellulitis and no wound, drainage or abscess, with ultrasound at enrolment to exclude an occult abscess. In the pre-specified per-protocol population, clinical cure occurred in 182 of 218 (83.5%) on cephalexin plus trimethoprim-sulfamethoxazole against 165 of 193 (85.5%) on cephalexin plus placebo — a difference of -2.0 percentage points (95% CI -9.7 to 5.7, P=.50). Adverse events, overnight hospitalisation, recurrent skin infection and similar infection in household contacts did not differ through 7 to 9 weeks.
Written into the record, not signed off as a reviewed claim
In children where 69% grew MRSA, a drug with no MRSA activity matched one with it
In plain words
Two hundred children with skin infections were given either cephalexin, which cannot kill MRSA, or clindamycin, which can. Most of them turned out to have MRSA. It made no measurable difference which antibiotic they got.
What was measured
Clinical improvement at 48 to 72 hours and resolution at 7 days, cephalexin against clindamycin
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
Chen and colleagues randomised 200 patients aged 6 months to 18 years with uncomplicated skin and soft-tissue infection to 7 days of cephalexin or clindamycin. MRSA was cultured from 69% of wounds, mostly USA300, Panton-Valentine leukocidin-positive and clindamycin-susceptible. By 48 to 72 hours, 94% of the cephalexin arm and 97% of the clindamycin arm had improved (P=.50); by day 7 every patient had improved, with complete resolution in 97% and 94% respectively (P=.33). Spontaneous drainage occurred or a drainage procedure was performed in 97% of subjects. Fever and age under one year predicted early failure; initial erythema greater than 5 cm did not.
Written into the record, not signed off as a reviewed claim
Roughly a quarter of cellulitis patients were not cured by either arm
In plain words
The headline of the cellulitis trial is that the two arms matched. The number underneath it is that in the population as randomised, only about seven in ten were cured — by a week of oral antibiotics, in an infection generally treated as straightforward.
What was measured
Clinical cure in the modified intention-to-treat population under pre-specified failure criteria
Effect estimate
No reviewed effect estimate exists for this record.
Limits
This entry records a failure.
From the source
In the modified intention-to-treat population of the Moran trial, clinical cure occurred in 189 of 248 (76.2%) on cephalexin plus trimethoprim-sulfamethoxazole and 171 of 248 (69.0%) on cephalexin plus placebo. Failure was defined against explicit criteria at scheduled visits — fever, more than 25% increase in erythema at days 3 to 4, no decrease at days 8 to 10, more than minimal residual signs at days 14 to 21 — so the figure reflects a strict definition rather than clinical judgement. Median erythema at enrolment was 13.0 cm by 10.0 cm; 10.9% of participants had diabetes. Roughly 18% of randomised patients did not complete per protocol, and the gap between the two populations is where most of the trial’s ambiguity lives.
Written into the record, not signed off as a reviewed claim
The trial did not rule out a real benefit from adding MRSA cover
In plain words
This trial is widely quoted as showing MRSA coverage is unnecessary in plain cellulitis. In the population as randomised, the confidence interval reached as high as a fifteen-point advantage for adding it. The authors said so themselves.
What was measured
That covering MRSA adds nothing in non-purulent cellulitis — supported in the per-protocol analysis, not excluded by the intention-to-treat analysis, and quoted far more confidently than either supports
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
In the per-protocol analysis the difference was -2.0 percentage points (95% CI -9.7 to 5.7, P=.50), consistent with no benefit. In the modified intention-to-treat analysis the difference was +7.3 percentage points (95% CI -1.0 to 15.5, P=.07), an interval whose upper bound exceeds the 10% the investigators had pre-specified as clinically significant. The published conclusion is explicit: because imprecision around the intention-to-treat finding included a clinically important difference favouring the combination, further research may be needed. A negative per-protocol result in a trial with 18% protocol deviation is a weaker statement than "it does not help".
Written into the record, not signed off as a reviewed claim
The 10% penicillin cross-reactivity rule collapsed — and cephalexin is the exception
In plain words
For decades doctors were taught that anyone allergic to penicillin had about a one-in-ten chance of reacting to any cephalosporin. That figure came from a period when cephalosporins were contaminated with traces of penicillin. The modern answer is that the risk depends on whether the two drugs share the same side chain — and cephalexin shares one with ampicillin.
What was measured
That the collapse of the class-wide 10% figure means cephalosporins are broadly safe after penicillin allergy — true for the second and third generations in this analysis, and specifically not shown for cephalexin, the most-prescribed member of the generation it exonerates least
Effect estimate
No reviewed effect estimate exists for this record.
Limits
See the technical detail.
From the source
Pichichero and Casey pooled nine studies from a 1960 to 2005 literature search. Penicillin-allergic patients showed significantly increased reactions to cephalothin (OR 2.5, 95% CI 1.1 to 5.5), cephaloridine (OR 8.7, 95% CI 5.9 to 12.8) and cephalexin (OR 5.8, 95% CI 3.6 to 9.2), and to first-generation cephalosporins plus cefamandole taken together (OR 4.8, 95% CI 3.7 to 6.2). No increase was seen with second-generation agents (OR 1.1, 95% CI 0.6 to 2.1) or third-generation agents (OR 0.5, 95% CI 0.2 to 1.1). Clinical challenge, skin testing and monoclonal antibody work all point to R1 side-chain similarity rather than to the shared beta-lactam ring as the determinant. Cephalexin carries a D-phenylglycine side chain identical to ampicillin’s, and cefadroxil carries one identical to amoxicillin’s.
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What is not here
7 questions this page could not answer
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What was measured, goal by goal — found nothing in the sources checked.
How close this is to real life — found nothing in the sources checked.
Felt, measured, or meaningful — found nothing in the sources checked.
How long anything takes — found nothing in the sources checked.
What it may clash with — found nothing in the sources checked.
Other ways to the same goal — found nothing in the sources checked.
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The record as stored
The full record, for auditing
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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.
A first-generation cephalosporin that jams the same wall-building enzyme a penicillin does, with a ring the common staphylococcal penicillinase cannot open — in the largest randomised trial it cured 85.5% of per-protocol patients with uncomplicated cellulitis on its own, and adding an antibiotic active against MRSA changed the cure rate by -2.0 percentage points (95% CI -9.7 to 5.7).
Recorded evidence blocks (8)
Q1
On the Cefalexin label: indicated for what?
"Cephalexin capsules USP is a cephalosporin antibacterial drug indicated for the treatment of the following infections caused by susceptible isolates of designated bacteria Respiratory tract infection ( 1.1 ) Otitis media ( 1.2 ) Skin and skin structure infections ( 1.3 ) Bone infections ( 1.4 ) Genitourinary tract…": indications and usage on Cefalexin's label. DailyMed label · 1d3e6d33-5dc2-498e-8efa-e3c4fca15dff · 2026-08-24
Q2
56 registered trials of Cefalexin — at which phases?
NCT00300092, NCT00187759, NCT01767584, NCT01073540, NCT01073553 and NCT01248078, and 8 more
Completion dates
oldest 2004-07; newest 2023-07-21
Show the evidence
Trial
NCT00300092
2004-07
NCT00187759
2005-03
NCT01767584
2009-04-14
NCT01073540
2009-09
NCT01073553
2009-10
NCT01248078
2010-01
8 further recorded trials
NCT01105208
2010-03
NCT01767532
2011-02-08
NCT01767571
2011-02-08
NCT01899781
2015-11
NCT02569866
2016-08
NCT01628666
2017-09-08
NCT03802552
2021-04-30
NCT03187106
2023-07-21
Q6
At the median, Cefalexin's trials enrolled 79.5 people — anything larger?
Median enrolment
79.5
Largest enrolment
12814
Registered trials counted
56
Q7
What do 24 spontaneous reports say about Cefalexin — 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.
Cefalexin appears in spontaneous reports to regulators. Across the 1 most-reported reaction term, 24 reaction mentions were counted: drug hypersensitivity 24. FAERS via Open Targets · CHEMBL1200366 · 2026-06-24
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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