Phenytoin

Pronunciation: FEN-i-toh-in.

Generic name
phenytoin
Drug class
Anticonvulsant (antiepileptic drug) -- a hydantoin derivative
Form
Extended-release capsule (30 mg, 100 mg), chewable tablet, oral suspension (125 mg/5 mL), and a separate sterile injectable formulation (50 mg/mL) used in hospital for status epilepticus
Route
Oral (capsules, chewable tablets, oral suspension) or, in hospital settings, intravenous/intramuscular injection
Legal status
Prescription-only

Availability and supply rules vary by country

Trade names by country: United States · United Kingdom · Canada · Australia · New Zealand

Australia: Dilantin / Dilantin Infatabs / Dilantin Paediatric Suspension (6) · Full directory

What phenytoin is and how it works

Phenytoin is a long-established anticonvulsant (antiepileptic) medicine used to control certain types of seizures. It is usually taken by mouth, but in hospital it can also be given by injection into a vein to stop a prolonged seizure (status epilepticus) or to prevent seizures around surgery or after a head injury. (1) (2) (3)

Oral phenytoin does not carry a formal boxed warning, but it does carry a real warning (based on what current labels describe as 'limited evidence') about a rare, serious skin reaction risk (Stevens-Johnson syndrome or toxic epidermal necrolysis) that may be somewhat more likely in some people of Asian ancestry -- discuss your family background with your clinician. Phenytoin also has a narrow therapeutic index, meaning the difference between an effective dose and a dose that causes side effects is small, so blood-level monitoring and sticking with the same brand/manufacturer where possible really matter. If phenytoin is given to you by injection in hospital, that specific formulation carries its own separate, genuinely serious FDA boxed warning about giving it too quickly -- this is a hospital-administration safety matter, not something that applies to taking phenytoin capsules at home.

Mechanism of action: how phenytoin works

Phenytoin is a hydantoin-derivative anticonvulsant that stabilises neuronal membranes by blocking voltage-gated sodium channels and limiting the spread of seizure activity, without generally producing overall central nervous system depression at therapeutic doses. It is extensively metabolised via hepatic CYP2C9/CYP2C19 and is itself a potent enzyme inducer, the basis for its extensive drug-interaction profile. Phenytoin has a narrow therapeutic index and non-linear (saturable) pharmacokinetics at therapeutic doses, meaning small dose increases can cause disproportionately large rises in blood level -- the clinical basis for routine therapeutic drug monitoring. (1) (2) (3)

What phenytoin is used for

Epilepsy -- tonic-clonic and complex partial (focal) seizures

Used to control generalised tonic-clonic (grand mal) seizures and complex partial (psychomotor/focal) seizures in adults and children. It is not effective for absence (petit mal) seizures, and combined therapy is needed if both seizure types occur together. (1) (3) (7)

Seizure prevention around neurosurgery or head injury

Used to prevent or treat seizures occurring during or shortly after neurosurgery, or following a head injury. (1) (3)

Status epilepticus (hospital, IV use only)

The injectable form is used in hospital to treat generalised tonic-clonic status epilepticus (a prolonged seizure or repeated seizures without recovery in between), and as a short-term substitute for oral phenytoin when a person temporarily cannot take medicines by mouth. (2)

Trigeminal neuralgia (second-line, UK)

In the UK, phenytoin may be used as a second-line treatment for the severe facial pain of trigeminal neuralgia when carbamazepine has not worked. (3)

Before taking phenytoin

Do not take / not appropriate for

  • Do not take phenytoin if you have a known hypersensitivity to phenytoin or to other hydantoin medicines. (all countries) (1) (3)
  • Phenytoin should not be combined with delavirdine (an HIV medicine). (United States) (1)
  • The injectable form should not be used in people with certain slow heart rhythms (sinus bradycardia, sino-atrial block, second- or third-degree AV block, Adams-Stokes syndrome) or a history of acute liver injury from phenytoin. (United States, injectable form only) (2)

Tell your clinician about

  • Your family background/ancestry -- current labels describe 'limited evidence' that people of Asian ancestry taking phenytoin (and related antiepileptic drugs) who carry the HLA-B*1502 gene variant may have a somewhat higher risk of a serious skin reaction; your clinician can discuss whether genetic testing is appropriate for you.
  • Any past allergic reaction to phenytoin or another hydantoin medicine.
  • Any history of heart rhythm problems, especially if you may need phenytoin by injection.
  • Any history of liver problems.
  • Whether you are pregnant, planning pregnancy, or breastfeeding, since phenytoin has been linked to fetal hydantoin syndrome.
  • All other medicines you take, since phenytoin has many significant drug interactions and a narrow therapeutic index.
  • If your phenytoin brand or manufacturer changes, tell your clinician or pharmacist -- closer monitoring or a blood test may be recommended.
(1) (2) (3) (7)

How to take phenytoin

  • Take phenytoin exactly as prescribed, at the same time(s) each day.
  • Extended-release capsules should be swallowed whole; ask your pharmacist before crushing or opening any capsule.
  • Do not stop taking phenytoin suddenly without medical advice -- seizures can return or worsen if it is stopped abruptly.
  • Any dose change should be made by your clinician, not on your own, because of phenytoin's narrow therapeutic index.
  • Try to keep to the same brand or manufacturer of phenytoin where possible; if a change is unavoidable, your clinician may want to check your phenytoin blood level.
(1) (3)

Procedures and treatment changes

Tell the treating team before surgery, procedures, or any medicine change. Do not alter treatment without the responsible clinician. (1) (2) (3) (4) (5) (6) (7)

Missed dose

If you miss a dose, contact your prescriber or pharmacist for specific guidance -- do not take a double dose to make up for a missed one without checking first, since keeping steady blood levels matters for seizure control and phenytoin has a narrow therapeutic index.

Overdose

Seek emergency medical attention immediately if too much phenytoin has been taken, or if you notice severe unsteadiness, slurred speech, uncontrolled eye movements, confusion, very slow or irregular heartbeat, or loss of consciousness. There is no specific antidote; treatment is supportive and given under emergency medical supervision. (1) (2)

Side effects

Side effects and their urgency vary; report severe, persistent, or concerning symptoms.

Uncontrolled eye movements (nystagmus), unsteadiness (ataxia), and slurred speech, especially at higher blood levels.

What to notice: Uncontrolled eye movements (nystagmus), unsteadiness (ataxia), and slurred speech, especially at higher blood levels.

What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (1) (2)

Drowsiness and mental confusion.

What to notice: Drowsiness and mental confusion.

What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (1)

Nausea.

What to notice: Nausea.

What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (3)

Gum overgrowth (gingival hyperplasia) with long-term use.

What to notice: Gum overgrowth (gingival hyperplasia) with long-term use.

What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (3)

Serious warnings

Signs of a serious skin reaction (Stevens-Johnson syndrome or toxic epidermal necrolysis) — emergency

Symptoms: A spreading rash, blistering or peeling skin, mouth/eye/genital sores, or fever alongside a new rash.

Action: Seek emergency medical help immediately. Current labels describe this as a real but 'limited evidence' risk, possibly somewhat higher in people of Asian ancestry who carry the HLA-B*1502 gene variant. Onset is usually within the first 28 days of treatment, though it can occur later. Any decision about stopping or continuing treatment should be made with a clinician or emergency care team, not on your own. (1) (3)

Purple glove syndrome (IV administration only -- swelling, discolouration, and pain of the limb near an injection site) — emergency

Symptoms: Swelling, purple/blue discolouration, and pain spreading from an IV injection site, sometimes appearing days after the injection.

Action: This is a hospital-recognised risk of intravenous phenytoin injection, not a risk of taking phenytoin by mouth. Clinicians monitor IV injection sites closely because soft-tissue damage can range from mild tenderness to extensive tissue death, occasionally needing surgery (fasciotomy), skin grafting, or, rarely, amputation. (2)

Signs of a dangerously slow heartbeat, low blood pressure, or abnormal heart rhythm (IV administration only) — emergency

Symptoms: Severe dizziness, fainting, chest discomfort, or a very slow or irregular heartbeat during or shortly after an IV phenytoin infusion.

Action: This is the subject of phenytoin sodium injection's real FDA boxed warning -- it is why clinicians give IV phenytoin slowly, with a strict maximum infusion rate, and monitor the heart and blood pressure closely during and after the infusion. (2)

Signs of a serious blood disorder — high caution

Symptoms: Fever, sore throat or mouth ulcers that will not heal, unusual bruising or bleeding, or persistent tiredness.

Action: Contact your clinician urgently for a blood count -- rare but serious blood disorders (thrombocytopenia, leukopenia, agranulocytosis) have been reported with phenytoin. (2)

Signs of pregnancy while taking phenytoin — high caution

Symptoms: Confirmed or suspected pregnancy.

Action: Contact your clinician promptly rather than making any decision about your treatment on your own -- phenytoin has been linked to fetal hydantoin syndrome, but stopping abruptly also carries a real risk of seizures worsening. Your clinician can discuss the safest plan for you. (3) (5)

Interactions

  • Phenytoin reduces the effectiveness of warfarin, hormonal contraceptives, corticosteroids, lamotrigine, and digoxin, among other medicines, because it is a potent inducer of liver enzymes. (3)
  • Chloramphenicol, fluconazole, isoniazid, valproate, and clarithromycin can raise phenytoin blood levels, increasing the risk of side effects. (3)
  • Rifampicin, carbamazepine, St John's Wort, and long-term heavy alcohol use can lower phenytoin blood levels, risking reduced seizure control. (3)
  • St John's Wort is not recommended with phenytoin because it can meaningfully reduce phenytoin's effectiveness. (3)
  • Must not be combined with delavirdine. (1)

Pregnancy, breastfeeding, kidney/liver function, age

Pregnancy

Phenytoin has been linked to fetal hydantoin syndrome, including a higher risk of orofacial clefts, cardiac defects, craniofacial abnormalities, nail/digit hypoplasia, and growth impairment. Untreated seizures also carry real risks to both mother and baby, so treatment should not be stopped abruptly -- discuss the safest plan, folic acid supplementation, and monitoring with your specialist. (1) (2) (3) (4) (5) (6) (7)

Breastfeeding

Phenytoin passes into breast milk; discuss with your clinician, since breastfeeding while taking phenytoin is generally not recommended without specific clinician guidance. (1) (2) (3) (4) (5) (6) (7)

Children

Used in children for epilepsy, with weight-based dosing individualised by blood-level monitoring; not effective for absence seizures. (1) (2) (3) (4) (5) (6) (7)

Elderly

Elderly patients may need a lower or less frequent dose because of reduced clearance, and may be more sensitive to side effects. (1) (2) (3) (4) (5) (6) (7)

Hepatic_impairment

Use with caution; the injectable form is contraindicated after a prior episode of acute liver injury linked to phenytoin. (1) (2) (3) (4) (5) (6) (7)

Renal_impairment

No specific dose adjustment is described in the sources reviewed, though general caution and monitoring are advised. (1) (2) (3) (4) (5) (6) (7)

Source_ids

UK-EMC-PHENYTOIN-FLYNN-2026,CA-HEALTHCANADA-PM-DILANTIN-2026,US-DAILYMED-PHENYTOIN-INJECTION-2026 (1) (2) (3) (4) (5) (6) (7)

Monitoring

Before starting

  • Review of any past hypersensitivity reaction to phenytoin or another hydantoin
  • Discussion of ancestry/family background where the HLA-B*1502 signal may be relevant
  • Baseline blood pressure/heart rhythm assessment if IV administration is anticipated
  • Review of all current medicines because of phenytoin's extensive interaction profile

During treatment

  • Periodic phenytoin blood-level monitoring, particularly after any dose or brand/manufacturer change, because of the narrow therapeutic index.
  • Prompt skin assessment if any new rash develops.
  • Cardiac (ECG) and blood-pressure monitoring during and after any IV administration.
  • Close observation of IV injection sites for swelling, discolouration, or pain (purple glove syndrome).
  • Periodic blood counts if any signs of a blood disorder develop.
  • Dental/gum checks with long-term use, given the known risk of gum overgrowth.
(1) (2) (3)

Storage

Store at room temperature, protected from moisture, out of reach of children, following the specific product's packaging instructions.

Study the drug class: Antiepileptics

Professional information

Clinician-facing context only, sourced from the current US FDA and UK SmPC labels; verify against the current full label before prescribing.

United States / United Kingdom / European Union / Canada / Australia / New Zealand

Indication: Epilepsy, adults (oral)

Dose summary: Initial approximately 3-4 mg/kg/day; maintenance commonly 200-500 mg/day (US label also describes a rapid oral loading approach of about 1 g divided over 24 hours where clinically indicated), individualised by serum phenytoin level (typical target range approximately 10-20 mcg/mL).

Renal context: No specific dose adjustment described in the sources reviewed; general caution advised. (1) (3)

United States

Indication: Status epilepticus (IV, hospital)

Dose summary: Adult loading dose 10-15 mg/kg IV, at a rate not exceeding 50 mg/minute; paediatric loading dose 15-20 mg/kg IV, at a rate not exceeding 1-3 mg/kg/min (or 50 mg/min, whichever is slower). Maintenance approximately 100 mg every 6-8 hours (oral or IV).

Renal context: Not separately established in the sources reviewed. (2)

Mechanism of action

Hepatic guidance

The injectable form is contraindicated after prior acute hepatotoxicity linked to phenytoin; use oral phenytoin with caution in hepatic impairment.

Overdose note

No specific antidote; manage with supportive care, cardiac/neurological monitoring, and specialist toxicology input as needed. (1) (2) (3)

Frequently asked questions

Is phenytoin available in the US, UK, Europe, Canada, Australia, and New Zealand?

Yes. It is authorised in all of these regions, marketed as Dilantin (US, Canada, Australia, New Zealand) or Phenytoin Sodium Flynn/Epanutin (UK), in both oral and injectable forms. (1) (3) (4) (5) (6) (7)

Does phenytoin have a boxed warning like carbamazepine?

Not in the same way. Oral phenytoin does not carry a formal FDA boxed warning -- its skin-reaction (SJS/TEN) risk is described in current labels as 'limited evidence,' a weaker evidentiary tier than carbamazepine's own boxed-warning-level HLA-B*1502 association. However, the separate injectable form of phenytoin does carry a real, different boxed warning: giving it too quickly into a vein can cause severe low blood pressure and dangerous heart rhythm problems, which is why it must be given slowly under monitoring in hospital. (1) (2)

What is purple glove syndrome?

It is a real, recognised complication of giving phenytoin by injection into a vein (never a risk of taking phenytoin capsules by mouth) -- swelling, discolouration, and pain spreading from the injection site, which can occasionally cause serious tissue damage. Hospital staff monitor IV injection sites closely for this reason. (2)

Why does my phenytoin brand matter?

Phenytoin has a narrow therapeutic index, meaning small differences in how much active drug reaches your bloodstream can matter. Regulators advise avoiding unnecessary brand or manufacturer switches, and recommend closer monitoring (including a blood test) if a switch is unavoidable. (7) (1)

What is the mechanism of action of phenytoin?

Phenytoin is a hydantoin-derivative anticonvulsant that stabilises neuronal membranes by blocking voltage-gated sodium channels and limiting the spread of seizure activity, without generally producing overall central nervous system depression at therapeutic doses. It is extensively metabolised via hepatic CYP2C9/CYP2C19 and is itself a potent enzyme inducer, the basis for its extensive drug-interaction profile. Phenytoin has a narrow therapeutic index and non-linear (saturable) pharmacokinetics at therapeutic doses, meaning small dose increases can cause disproportionately large rises in blood level -- the clinical basis for routine therapeutic drug monitoring. (1) (2) (3)

What is the generic name of Phenytoin?

The generic name is phenytoin. (1) (3)

What class of drug is phenytoin?

Phenytoin is classed as: Anticonvulsant (antiepileptic drug) -- a hydantoin derivative. (1) (3)

References

  1. DILANTIN / Phenytoin Sodium Extended-Release Capsules, USP -- full prescribing informationU.S. Food and Drug Administration / DailyMed (NIH), full prescribing information (extended-release oral capsules) · United States · accessed 2026-07-31 · source 1
  2. PHENYTOIN SODIUM INJECTION -- full prescribing informationU.S. Food and Drug Administration / DailyMed (NIH), full prescribing information (phenytoin sodium injection) · United States · accessed 2026-07-31 · source 2
  3. Phenytoin Sodium Flynn Hard Capsules 100mg -- Summary of Product Characteristicselectronic medicines compendium (emc), United Kingdom -- Flynn Pharma Ltd · United Kingdom · accessed 2026-07-31 · source 3
  4. Phenytoin -- list of nationally authorised medicinal products (PSUSA/00002392/202308, document EMA/144701/2024)European Medicines Agency (EMA) -- PSUSA (periodic safety update report single assessment), list of nationally authorised medicinal products · European Union · accessed 2026-07-31 · source 4
  5. PrDILANTIN (phenytoin sodium) and PrDILANTIN INFATABS -- Health Canada product monographsHealth Canada -- approved product monograph (Viatris / generic manufacturers) · Canada · accessed 2026-07-31 · source 5
  6. DILANTIN (phenytoin sodium) capsules, Infatabs, and paediatric suspension -- ARTG entries 295264/295265/297268/14309Therapeutic Goods Administration (TGA), Australia · Australia · accessed 2026-07-31 · source 6
  7. DILANTIN (phenytoin) New Zealand Data Sheet (oral and injection), Consumer Medicine InformationMedsafe (New Zealand regulator) · New Zealand · accessed 2026-07-31 · source 7

Brand names by country

Single-ingredient phenytoin names are shown separately by country. Product availability, strengths, and supply rules can change; combination products are not included here.

Priority countries

United States

  • DILANTIN30 mg, 125 mg/5mL, 100 mg · viatris specialty llc
  • Phenytek200 mg, 300 mg · mylan pharmaceuticals inc.
  • Phenytoin Infatabs50 mg · ncs healthcare of ky, llc dba vangard labs
  • Dilantin Infatabs50 mg · parke-davis div of pfizer inc

United Kingdom

  • Phenytoin Sodium Flynn (capsules); Epanutin (oral suspension) (3)MHRA licensed emc smpc confirmed · 25 mg, 50 mg, 100 mg capsules, 30 mg/5 mL oral suspension · Flynn Pharma Ltd (capsules)

Canada

  • DILANTIN -
  • DILANTIN INFATABS
  • PHENYTOIN SODIUM INJ
  • TREMYTOINE INJ

Australia

  • Dilantin / Dilantin Infatabs / Dilantin Paediatric Suspension (6)TGA ARTG registered confirmed · 30 mg, 100 mg capsules, 50 mg Infatabs, 30 mg/5 mL paediatric suspension · Viatris Pty Ltd

New Zealand

  • Dilantin (oral capsules/suspension/tablets); Dilantin injection (7)Medsafe data sheet and CMI confirmed · oral: capsules/suspension/tablets (multiple strengths), injection: 50 mg/mL · Viatris Ltd
View brands in additional countries.

European Union

Various (nationally authorised products vary by member state) (4)

This page provides general reference information about phenytoin and is not a substitute for advice from the prescriber managing your treatment. All dosing and treatment decisions, including any decision to change or stop treatment, should be made with your prescriber.

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