Isoniazid and Phenytoin: Drug Interaction Guide
Quick answer: This combination requires caution. Isoniazid and Phenytoin can be used together with adjustments and monitoring.
How Isoniazid interacts with Phenytoin
Isoniazid inhibits CYP2C19 and CYP2C9, slowing phenytoin metabolism, with the effect more pronounced in slow acetylators producing toxic phenytoin concentrations.
What you should do
Monitor phenytoin levels and clinical signs of toxicity within 1-2 weeks of starting isoniazid; reduce phenytoin dose as required.
Key facts
Severity
Moderate — caution required
Mechanism
Isoniazid inhibits CYP2C19 and CYP2C9, slowing phenytoin metabolism, with the effect more pronounced in slow acetylators...
Action
Monitor phenytoin levels and clinical signs of toxicity within 1-2 weeks of starting isoniazid; reduce phenytoin dose as...
Frequently asked questions
Can I take Isoniazid and Phenytoin together?
With caution. Monitor phenytoin levels and clinical signs of toxicity within 1-2 weeks of starting isoniazid; reduce phenytoin dose as required.
What is the severity of this interaction?
The interaction between Isoniazid and Phenytoin is classified as Moderate — caution required. Isoniazid inhibits CYP2C19 and CYP2C9, slowing phenytoin metabolism, with the effect more pronounced in slow acetylators producing toxic phenytoin concentrations.
What should I do if I'm prescribed both?
Monitor phenytoin levels and clinical signs of toxicity within 1-2 weeks of starting isoniazid; reduce phenytoin dose as required. Always consult your prescribing clinician — do not stop or change medications without medical guidance.
Last reviewed: by iMedic Medical Editorial Team. Our editorial process.