Phenytoin (Dosage)

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

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


Precautions should be taken before prescribing phenytoin in the elderly. Also, cautions should be taken in those patients suffering from liver failure, hypotension or severe heart failure.

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

During treatment with phenytoin, avoid alcohol intake. During the treatment, monitor serum levels of the drug due to the risk of toxicity. There is also a risk of tolerance and dependence, and risk of suicidal ideation and behavior, therefore the treatment must be suspended gradually.

If symptoms or signs of Steve Johnson syndrome and chronic epidermal necrolysis (progressive 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. It is mandatory to take precautions with the form of administration.

Control vital signs and electrocardiogram during phenytoin infusion.


Nystagmus, ataxia, speech alteration, mental confusion, dizziness, insomnia, transient nervousness. Nausea, vomiting, constipation. Morbilliform or scarlatiform skin rash. Thrombocytopenia, leukopenia, granulocytosis, agranulocytosis, pacitopenia. Gingival hyperplasia.

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


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

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

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

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

Excessive cardiac depression can occur if taken together 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 caused by alcohol and other CNS depressants.


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