Phenytoin-Antibiotic Interactions

Critical Interactions Leading to Phenytoin Toxicity

Isoniazid significantly increases phenytoin levels by inhibiting CYP2C19 and CYP2C9 enzymes, preventing phenytoin metabolism and leading to potentially catastrophic toxicity in patients with epilepsy, requiring careful monitoring and dose adjustments 1
Trimethoprim-Sulfamethoxazole (TMP-SMX) increases the risk of phenytoin toxicity through enzyme inhibition, affecting patients on long-term phenytoin therapy, and close monitoring of phenytoin levels is essential when initiating TMP-SMX 2, 3
Azithromycin does not interact with phenytoin and is the preferred macrolide when concurrent use is necessary, providing a safe alternative for patients with bacterial infections 3

Rifampin is a potent inducer of CYP450 enzymes and dramatically reduces phenytoin levels, requiring substantial dose increases to maintain therapeutic concentrations in patients with seizure disorders, and the American Heart Association recommends careful monitoring of phenytoin levels during rifampin therapy 4, 5
When rifampin is discontinued, phenytoin doses must be reduced within 2 weeks to prevent toxicity as the inductive effect resolves, and the American College of Chest Physicians suggests close monitoring of phenytoin levels after rifampin discontinuation 1
Penicillin G may reduce phenytoin absorption when administered with bile acid sequestrants, though this interaction is less clinically significant, and the American Gastroenterological Association recommends separating the administration of these drugs by at least 2 hours 6

The American Geriatrics Society recommends identifying the antibiotic class and assessing the risk of toxicity or subtherapeutic levels when initiating antibiotics in phenytoin-treated patients, and considering alternative antibiotics or anticonvulsants when necessary 3, 5
For toxicity-risk antibiotics, the American College of Clinical Pharmacy suggests obtaining baseline phenytoin levels, monitoring for early signs of toxicity, and rechecking phenytoin levels at 2, 4, and 8 weeks, and considering prophylactic phenytoin dose reduction of 20-30% 6