Phenytoin: uses, dosing, side effects & brands
Phenytoin is a medicine sold in the U.S. under one brand. Below: what the FDA label says, every product that contains it, what the pills look like, and its recall record.
Key facts
- Available as
- Capsule · Injectable
- Sold as
- Phenytoin Sodium
- Prescription?
- Prescription only
- Generic available?
- Yes
- Half-life
- About 22 hours on average, ranging from 7 to 42 hours — but phenytoin does NOT have a single fixed half-life. It follows nonlinear, saturable (Michaelis-Menten) kinetics, so the half-life lengthens as the dose and blood level rise; the label warns that "small incremental doses may increase the half-life and produce very substantial increases in serum levels" once levels are in the upper range.
- What the pharmacy pays
- about $0.18 per ml — not your price
Every phenytoin product we track (1)
Same active ingredient — different manufacturer, form, price and FDA recall record. That last one is what our independent score measures.
Only one: Phenytoin Sodium.
What phenytoin pills look like
Imprint codes, colour and shape from the FDA’s labelling data. Match the imprint on your pill — or search any imprint.
| Imprint | Strength | Colour | Shape | Maker |
|---|---|---|---|---|
| X;51 | 100 mg | white | capsule | — |
| IP;212 | 100 mg | white | capsule | — |
| TARO;PHN;100 | 100 mg | orange | capsule | — |
| TARO;PHN;100 | 100 mg | orange | capsule | — |
| TARO;PHN;100 | 100 mg | orange | capsule | — |
Can you crush or split phenytoin?
At least one phenytoinproduct is labelled to be swallowed whole — crushing an extended-release tablet releases the whole day’s dose at once. Which applies depends on the form you were given.
What each phenytoin label says about crushing, splitting and chewingHow long phenytoin keeps
No phenytoin label we read sets a separate limit for after opening, but they do specify how it must be stored — and the stability behind any date assumes those conditions.
Does phenytoin expire? The in-use limits and storage rules from its labelsPhenytoin and breastfeeding
From LactMed, the US National Library of Medicine’s Drugs and Lactation Database — quoted, not rewritten.
Full LactMed record for phenytoin: levels in milk, effects in breastfed infants, and the drugs it would consider insteadBreastfeeding during phenytoin monotherapy does not appear to adversely affect infant growth or development, and breastfed infants had higher IQs and enhanced verbal abilities than nonbreastfed infants at 6 years of age in one study. If phenytoin is required by the mother, it is not a reason to discontinue breastfeeding.
National Institute of Child Health and Human Development, record revised November 15, 2024. LactMed states its information is not a substitute for professional judgement.
What people report to the FDA about phenytoin
The FDA Adverse Event Reporting System (FAERS) collects reports from patients and clinicians. It holds 4,970 reports naming phenytoin, and the FDA flagged 84% of those reports as serious. The effects reported most often — leaving out reports about overdose, misuse or the condition being treated, which dominate the raw list for common medicines:
- convulsion408 reports
- seizure383 reports
- fall179 reports
- dizziness174 reports
- pyrexia157 reports
- epilepsy156 reports
- stevens-johnson syndrome152 reports
- headache140 reports
Read these as a signal, not a rate. A report does not mean phenytoin caused the effect — anyone can file one, and many describe people taking several medicines for several conditions. Crucially there is no denominator: FAERS does not record how many people took the drug, so these counts cannot be turned into “X% of patients” — a bigger number often just means a more widely used or more talked-about drug. Duplicates exist, and publicity drives reporting. For what is actually established, read the FDA label section above.
Source: openFDA drug/event (FAERS), retrieved July 27, 2026.
How long phenytoin stays in your system
The elimination half-life of phenytoin is About 22 hours on average, ranging from 7 to 42 hours — but phenytoin does NOT have a single fixed half-life. It follows nonlinear, saturable (Michaelis-Menten) kinetics, so the half-life lengthens as the dose and blood level rise; the label warns that "small incremental doses may increase the half-life and produce very substantial increases in serum levels" once levels are in the upper range.. Because phenytoin is hydroxylated by a liver enzyme system that becomes saturated at higher plasma levels, clearance is dose- and concentration-dependent — this is why one number can't describe it. There is no active metabolite that outlasts the parent: the label states the drug is metabolized in the liver and "most of the drug is excreted in the bile as inactive metabolites." Phenytoin itself is not a prodrug, but fosphenytoin (given IV/IM) is a prodrug the body rapidly converts to phenytoin. Since elimination depends on those saturable liver enzymes, liver disease, low serum albumin, genetically slow metabolizers, and interacting drugs can prolong the half-life substantially. The Clinical Pharmacology section does not give fixed half-life figures for specific age, kidney, or liver groups. This is pharmacokinetic information, not a drug-test detection window or dosing guidance.
DILANTIN (extended phenytoin sodium capsule) — FDA label, Clinical Pharmacology: Pharmacokinetics (DailyMed SPL) ↗Half-life is how long the body takes to clear half a dose. It is not the same as how long a drug test can detect it, and it varies with age, kidney and liver function.
Related calculators
Frequently asked questions
How long does phenytoin stay in your system?
The elimination half-life of phenytoin is About 22 hours on average, ranging from 7 to 42 hours — but phenytoin does NOT have a single fixed half-life. It follows nonlinear, saturable (Michaelis-Menten) kinetics, so the half-life lengthens as the dose and blood level rise; the label warns that "small incremental doses may increase the half-life and produce very substantial increases in serum levels" once levels are in the upper range. — that is how long the body takes to clear half of a dose. Because phenytoin is hydroxylated by a liver enzyme system that becomes saturated at higher plasma levels, clearance is dose- and concentration-dependent — this is why one number can't describe it. There is no active metabolite that outlasts the parent: the label states the drug is metabolized in the liver and "most of the drug is excreted in the bile as inactive metabolites." Phenytoin itself is not a prodrug, but fosphenytoin (given IV/IM) is a prodrug the body rapidly converts to phenytoin. Since elimination depends on those saturable liver enzymes, liver disease, low serum albumin, genetically slow metabolizers, and interacting drugs can prolong the half-life substantially. The Clinical Pharmacology section does not give fixed half-life figures for specific age, kidney, or liver groups. This is pharmacokinetic information, not a drug-test detection window or dosing guidance. Half-life is not the same as how long a drug test can detect the drug, and it varies with age, kidney and liver function.
Can you take phenytoin with other medicines?
It depends on the medicine. We check it against the FDA labels rather than guessing: our interaction checker searches each drug's own label for the other and quotes what it says, naming the section it came from. Run phenytoin against whatever else you take — and remember that a label not naming a drug is not the same as that combination being safe.
What forms does phenytoin come in?
Across the brands we track, phenytoin is currently marketed as capsule and injectable, per the FDA's National Drug Code Directory. Each form is dosed differently — follow the label for the exact product you were prescribed.
Is there a generic phenytoin?
Yes. Our catalog lists 1 generic phenytoin product alongside the brand versions. A generic has the same active ingredient and must meet the FDA's bioequivalence standard; it usually costs less. Ask your pharmacist which one your plan covers.
Cite this page
- APA
- pharmaranks. (2026, July 24). Phenytoin: uses, dosing, side effects & brands. https://pharmaranks.com/drugs/phenytoin
- MLA
- “Phenytoin: uses, dosing, side effects & brands.” pharmaranks, 24 July 2026, https://pharmaranks.com/drugs/phenytoin.
We summarise public FDA and NIH sources — for a clinical claim, cite the primary source we link to as well.
Sources: FDA openFDA drug label, National Drug Code Directory, and Enforcement (recall) database. This page reproduces public FDA data and is not medical advice. Dosing is set by your prescriber.
Read the full FDA label for phenytoin on DailyMed (NIH) ↗.