Phenytoin (Dosage)

Phenytoin is an antiepileptic drug that inhibits the propagation of seizure activity in the cerebral motor cortex: it stabilizes the threshold by promoting sodium diffusion from neurons. It is also used as an antiarrhythmic because it stabilizes myocardial cells.

Phenytoin is contraindicated in patients with hypersensitivity to hydantoins, sinus bradycardia, sinoatrial block, 2nd and 3rd degree atrioventricular block and Adams-Stokes syndrome.

PRECAUTIONS

Precautions should be taken before prescribing phenytoin in the elderly. Also in patients with hepatic insufficiency, hypotension or severe cardiac insufficiency.

Phenytoin can cause osteomalacia, hyperglycemia, hyperplasia, gingival hemorrhage, leukopenia and, used before delivery, maternal or neonatal hemorrhage.

During treatment with phenytoin, avoid alcohol intake. During treatment, monitor serum drug levels for risk of toxicity. There is also a risk of tolerance and dependence, suicidal ideation and behavior, so treatment should be discontinued gradually.

If symptoms or signs of Steve Johnson syndrome and chronic epidermal necrolysis (progressive skin rash with blisters or mucosal lesions) appear, discontinue treatment.

Do not administer intramuscularly, as there is a risk of tissue irritation and inflammation at the injection site. Caution should be taken with the form of administration.

Monitor vital signs and electrocardiogram during phenytoin infusion.

SIDE EFFECTS

Nystagmus, ataxia, speech disturbances, mental confusion, dizziness, insomnia, temporary nervousness. Nausea, vomiting, constipation. Morbilliform or scarlet rash. Thrombocytopenia, leukopenia, granulocytosis, agranulocytosis, pacitopenia. Gingival hyperplasia.

If administered intravenously, it may also have other more severe adverse reactions such as: severe cardiotoxic reactions with atrial and ventricular conduction depression and ventricular fibrillation, periarteritis nodosa; local irritation, inflammation, hypersensitivity, necrosis and eschar at the site of administration.

DRUG INTERACTIONS

Serum levels of phenytoin are increased by: chloramphenicol, dicoumarol, disulfiram, tolbutamide, isoniazid, phenylbutazone, salicylates, chlordiazepoxide, phenothiazines, diazepam, estrogens, ethosuximide, halothane, methylphenidate, sulfonamides, trazodone, amiodarone, fluoxetine and succinimides.

Serum levels of phenytoin can be decreased by: carbamazepine, reserpine, diazoxide, folic acid and sucralfate.

Serum phenytoin levels may be increased or decreased by: phenobarbital, valproic acid and sodium valproate.

There is a risk of seizures if prescribed concomitantly with tricyclic antidepressants.

Excessive cardiac depression may occur if taken concomitantly with lidocaine.

Phenytoin decreases the efficacy of: corticosteroids, coumarin anticoagulants, oral contraceptives, quinidine, vitamin D, digitoxin, rifampicin, doxycycline, estrogens, furosemide and theophylline.

Phenytoin potentiates central nervous system (CNS) depression produced by alcohol and other CNS depressants.

BRAND NAMES

• Epanutin ®
• Synergin ®