Phenytoin drug interactions

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

List of drug interactions

Drugs which may increase phenytoin serum levels

Drugs which may decrease phenytoin levels

Drugs which may either increase or decrease phenytoin serum levels

Tricyclic antidepressants

Drugs whose efficacy is impaired by phenytoin

Complete List of Drug Interactions



Drugs which may increase phenytoin serum levels

Acute alcohol intake, amiodarone, chloramphenicol, chlordiazepoxide, cimetidine, diazepam, dicumarol, disulfiram, estrogens, ethosuximide, fluoxetine, H2-antagonists, halothane, isoniazid, methylphenidate, phenothiazines, phenylbutazone, salicylates, succinimides, sulfonamides, ticlopidine, tolbutamide, trazodone. Return to top

Drugs which may decrease phenytoin levels

These include carbamazepine, chronic alcohol abuse, reserpine, and sucralfate. Moban® brand of molindone hydrochloride contains calcium ions which interfere with the absorption of phenytoin. Ingestion times of phenytoin and antacid preparations containing calcium should be staggered in patients with low serum phenytoin levels to prevent absorption problems. Return to top

Drugs which may either increase or decrease phenytoin serum levels

Phenobarbital, sodium valproate, and valproic acid. Similarly, the effect of phenytoin on phenobarbital, valproic acid, and sodium valproate serum levels is unpredictable. Return to top

Tricyclic antidepressants

Although not a true drug interaction, tricyclic antidepressants may precipitate seizures in susceptible patients and phenytoin dosage may need to be adjusted. Return to top

Drugs whose efficacy is impaired by phenytoin

Corticosteroids, coumarin anticoagulants, digitoxin, doxycycline, estrogens, furosemide, oral contraceptives, paroxetine, quinidine, rifampin, theophylline, vitamin D. Return to top

Complete List of Drug Interactions

Major Interactions

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Moderate Interactions

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Minor Interactions

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Adapted from the FDA Package Insert.


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