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An anticonvulsant used to treat epilepsy (major and partial seizures) and status epilepticus (repeated seizures with no recovery of consciousness between them) and to control or prevent seizures
following neurosurgery or head injury. It is also used to treat trigeminal neuralgia. Phenytoin is available on prescription only, either as an injection for use under medical supervision or as tablets, chewable tablets, capsules, or a suspension for oral use.

Side effects: include nausea, vomiting, headache, tremor, nervousness, and insomnia; less common effects are unsteadiness, shakiness, slurred speech, acne, increased growth of body hair, overgrowth of the gums, and rashes (if rashes develop the treatment should be discontinued). Rapid involuntary eye movements and blurred vision are signs of overdosage.

Precautions: phenytoin should be used with caution in women who are pregnant or breastfeeding and in people with impaired liver function. Women who are planning to become pregnant should seek specialist advice. When given intravenously, continuous monitoring of the electrocardiogram is essential and resuscitative equipment should be available.

Interactions with other drugs:
Analgesics: the plasma concentration of phenytoin is increased by aspirin, azapropazone (which should not be taken with phenytoin), and phenylbutazone; the effects of methadone are reduced by phenytoin.
Anti-arrhythmic drugs: amiodarone increases the plasma concentration of phenytoin; the plasma concentrations of disopyramide, mexiletine, and quinidine are reduced by phenytoin.
Antibiotics: the plasma concentration of phenytoin is increased by chloramphenicol, cycloserine, isoniazid, metronidazole, co-trimoxazole, and trimethoprim and reduced by rifampicin. The plasma concentration of doxycycline is reduced by phenytoin. Anticoagulants: the metabolism of warfarin and nicoumalone is increased by phenytoin, which may reduce their anticoagulant effect (but in some cases has
enhanced it). Anticonvulsants: taking two or more anticonvulsants together may enhance their adverse effects: phenytoin often reduces plasma concentrations of clonazepam, carbamazepine, lamotrigine, topiramate, and sodium valproate and increases the plasma concentration of phenobarbitone.
Antidepressants: antagonize the anticonvulsant effect of phenytoin. Fluoxetine, fluvoxamine, and viloxazine increase the plasma concentration of phenytoin; the plasma concentrations of mianserin, paroxetine, and tricyclic antidepressants are reduced by phenytoin.
Antifungal drugs: fluconazole and miconazole increase the plasma concentration of phenytoin; the plasma concentrations of itraconazole and ketoconazole are reduced by phenytoin.
Antimalarial drugs: antagonize the anticonvulsant effect of phenytoin; the antifolate effect of pyrimethamine is enhanced by phenytoin.
Antipsychotics: antagonize the anticonvulsant effect of phenytoin; the plasma concentrations of clozapine, quetiapine, and sertindole are
reduced by phenytoin.
Antiviral drugs: the plasma concentrations of indinavir, nelfinavir, and saquinavir may be reduced by phenytoin; zidovudine either increases or reduces plasma concentrations of phenytoin. Calcium antagonists: diitiazem and nifedipine increase the plasma concentration of phenytoin; phenytoin reduces the effects of
felodipine, isradipine, and probably also of nicardipine, nifedipine, diltiazam, and verapamil.
Corticosteroids: their effects are reduced by phenytoin. Cyclosporin: its plasma concentration is reduced by phenytoin.
Disulflram: increases the plasma concentration of phenytoin. Oral contraceptives: their contraceptive effect is reduced by phenytoin.
Sulphinpyrazone: increases the plasma concentration of phenytoin. Ulcer-healing drugs: cimetidine increases the plasma concentration of phenytoin; sucralfate reduces the absorption of phenytoin;
omeprazole increases the effects of phenytoin.
Proprietary preparations: Epanutin; Epanutin Ready-Mixed Parenteral.

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