Phenytoin

Hydantoin·Neurology - Anticonvulsants

Status epilepticus, generalized tonic-clonic seizures, focal seizures

PBSANTICONVULSANTDilantin30 mg tabletS4+1

Max Dose

Titrated via TDM

Half-life

22 h (Non-linear)

Toxicity

CRITICAL

Zero-Order

KINETICS

Rapid Summary

Overview

Ancient, highly complex sodium-channel blocker. Exhibits non-linear (zero-order) kinetics, meaning a tiny dose increase can cause a massive, toxic spike in blood levels.

Maximum Dose

Maintenance is usually 300-400 mg/day. Strictly guided by TDM.

Clinical Focus

IV Phenytoin is intensely caustic. Rapid infusion causes fatal arrhythmias and 'Purple Glove Syndrome'.

Bottom Line

Phenytoin is a potent anticonvulsant for status epilepticus and seizure prophylaxis, but has zero-order kinetics making dosing treacherous, and requires therapeutic drug monitoring and causes gingival hyperplasia.

Formulation & Access

Brand Names

Dilantin

Oral Routes

Capsules (30 mg, 100 mg). Chewable tablets (50 mg). Oral liquid.

Parenteral

Ampoules (250 mg/5 mL) for IV use. Do NOT give IM (crystallizes in muscle).

Prescribing & PBS

Unrestricted General Benefit (S4).

Indications & Use

Primary

PBSTGA

Status epilepticus, generalized tonic-clonic seizures, focal seizures.

Clinical Place

Largely superseded by Levetiracetam for ward loading due to extreme toxicity, but remains a vital second-line agent in status epilepticus.

Quick reference

Class & Role

Hydantoin — Ancient, highly complex sodium-channel blocker.

Route / Formulation

Capsules (30 mg, 100 mg). Chewable tablets (50 mg). Oral liquid. (Dilantin)

Usual Dose & Max

IV 15-20 mg/kg. MUST infuse extremely slowly (Max 50 mg/minute) with continuous ECG monitoring. Maintenance is usually 300-400 mg/day. Strictly guided by TDM.

Key Indication Doses

Status Epilepticus (Load): IV 15-20 mg/kg. MUST infuse extremely slowly (Max 50 mg/minute) with continuous ECG monitoring. Maintenance: PO 300 mg OD or divided. Adjust by NO MORE than 25-50 mg increments. Enteral Feeding (NGT): Enteral feeds bind Phenytoin. Feeds must be stopped 2 hours before and 2 hours after the dose, or absorption will be zero.

Best Uses

Avoid / Cautions

2nd/3rd degree heart block, sinus bradycardia. Pregnancy Category D. Causes Fetal Hydantoin Syndrome (cleft lip/palate, microcephaly).

Key Risks

Neurological: Nystagmus (first sign of toxicity), ataxia, slurred speech, confusion, coma. Cardiovascular: Hypotension, severe bradycardia, AV block, asystole (if pushed IV > 50 mg/min). Dermatological: Gingival hyperplasia, hirsutism, coarsening of facial features. SJS/TEN. Purple Glove Syndrome (tissue necrosis from IV extravasation).

Key Interactions

Broad-spectrum enzyme INDUCER (CYP3A4, CYP2C9). Obliterates blood levels of OCPs, DOACs, Warfarin, and Quetiapine. Highly protein bound. Valproate will knock Phenytoin off albumin, causing 'free' (active) Phenytoin levels to spike and cause toxicity, even while total Phenytoin lab levels look normal.

Monitoring

TDM (Target Total: 40-80 umol/L. Target Free: 4-8 umol/L). Check Albumin; low albumin artificially lowers total phenytoin levels, hiding toxicity. LFTs and FBC. Continuous ECG and BP monitoring during IV loading.

Clinical Pearl

IV Phenytoin precipitates instantly if it touches glucose/dextrose. It MUST be flushed and mixed exclusively with Normal Saline.

Access

Dilantin30 mg tablet

Brand: Dilantin
Formulation: Capsules (30 mg, 100 mg)