Phenytoin

Dilantin, Euthasol, Euthaphen, Phenytek

Antiepileptic • Hydantoin

Premium

NTICNS DepressantHepatotoxic

Half-life

7-42 hours (dose-dependent)

Protein Binding

90%

Metabolized By

CYP2C9, CYP2C19

External

Drugs.com

Boxed warning

Cardiovascular risk with rapid IV administration

Clinical alerts

Boxed warning

1 boxed warning

Narrow therapeutic index

Small dose changes can cause toxicity or loss of effect

Hepatotoxic

Monitor liver function

CNS depressant

Additive sedation with other CNS depressants and alcohol

Mechanism of action

Blocks voltage-gated sodium channels; zero-order kinetics at therapeutic levels

Recorded indications

From the drug index — may include conditions studied rather than approved indications

·Generalised tonic-clonic seizures
·Focal (partial) seizures
·Status epilepticus (IV formulation)
·Seizure prophylaxis after neurosurgery

Dosing

Typical adult starting point — always confirm against local guidelines

Typical dose: [object Object] mg every 24 h; Oral
Loading dose: [object Object] mg

Monitoring

Therapeutic range: 10–20 mg/L; Trough
Steady state: 7-10 days (capacity-limited)
Re-check: 2-4 weeks after dose change; then every 3-6 months

Trough: before morning dose
Free phenytoin if albumin < 3.0

Interactions

1 curated · 1 major or contraindicated · select a row for management

Interacting drug	Severity	Effect
rifampicin	major	Sub-therapeutic phenytoin levels; increased seizure risk.

Common adverse effects

Gingival hyperplasia
Nystagmus
Ataxia
Rash

Food, alcohol & herbal

Food

Enteral feeding may reduce absorption; hold feeds 1-2h before/after

Alcohol · major

Acute: increases phenytoin levels. Chronic: decreases levels. Unpredictable.

Herbal

St. John's Wort (major) — Reduces phenytoin levels via enzyme induction

Pharmacokinetics

Published parameters · PMID:3360972

Half-life: 7-42 hours (dose-dependent)
Protein binding: 90%
Metabolised by: CYP2C9, CYP2C19
Induces: CYP3A4, CYP2C9, CYP2C19, CYP1A2
Clearance: 0 L/h; adult epilepsy patients
Volume of distribution: 45 L
Renally eliminated: 5%
Model: Michaelis-Menten (non-linear)

Pregnancy & lactation

Pregnancy category: D
Lactation: caution

Sources

Assembled from: drug index, curated drug database, curated interaction pairs, published PK literature, TDM population profiles.

Not a substitute for clinical judgement

Phenytoin

Dilantin, Euthasol, Euthaphen, Phenytek

Antiepileptic • Hydantoin

Premium

NTICNS DepressantHepatotoxic

Half-life

7-42 hours (dose-dependent)

Protein Binding

90%

Metabolized By

CYP2C9, CYP2C19

External

Drugs.com

Boxed warning

Cardiovascular risk with rapid IV administration

Clinical alerts

Boxed warning

1 boxed warning

Narrow therapeutic index

Small dose changes can cause toxicity or loss of effect

Hepatotoxic

Monitor liver function

CNS depressant

Additive sedation with other CNS depressants and alcohol

Mechanism of action

Blocks voltage-gated sodium channels; zero-order kinetics at therapeutic levels

Recorded indications

From the drug index — may include conditions studied rather than approved indications

·Generalised tonic-clonic seizures
·Focal (partial) seizures
·Status epilepticus (IV formulation)
·Seizure prophylaxis after neurosurgery

Dosing

Typical adult starting point — always confirm against local guidelines

Typical dose: [object Object] mg every 24 h; Oral
Loading dose: [object Object] mg

Monitoring

Therapeutic range: 10–20 mg/L; Trough
Steady state: 7-10 days (capacity-limited)
Re-check: 2-4 weeks after dose change; then every 3-6 months

Trough: before morning dose
Free phenytoin if albumin < 3.0

Interactions

1 curated · 1 major or contraindicated · select a row for management

Interacting drug	Severity	Effect
rifampicin	major	Sub-therapeutic phenytoin levels; increased seizure risk.

Common adverse effects

Gingival hyperplasia
Nystagmus
Ataxia
Rash

Food, alcohol & herbal

Food

Enteral feeding may reduce absorption; hold feeds 1-2h before/after

Alcohol · major

Acute: increases phenytoin levels. Chronic: decreases levels. Unpredictable.

Herbal

St. John's Wort (major) — Reduces phenytoin levels via enzyme induction

Pharmacokinetics

Published parameters · PMID:3360972

Half-life: 7-42 hours (dose-dependent)
Protein binding: 90%
Metabolised by: CYP2C9, CYP2C19
Induces: CYP3A4, CYP2C9, CYP2C19, CYP1A2
Clearance: 0 L/h; adult epilepsy patients
Volume of distribution: 45 L
Renally eliminated: 5%
Model: Michaelis-Menten (non-linear)

Pregnancy & lactation

Pregnancy category: D
Lactation: caution

Sources

Assembled from: drug index, curated drug database, curated interaction pairs, published PK literature, TDM population profiles.

Not a substitute for clinical judgement

Phenytoin Drug Profile | PharmaGraphX