Phenytoin and Trimethoprim: Drug Interaction Guide

Quick answer: This combination requires caution. Phenytoin and Trimethoprim can be used together with adjustments and monitoring.

Interaction severity: Moderate — caution required

How Phenytoin interacts with Trimethoprim

Trimethoprim inhibits CYP2C9-mediated phenytoin metabolism and adds antifolate effect, raising phenytoin levels and risk of toxicity.

What you should do

Monitor phenytoin levels during trimethoprim therapy and watch for nystagmus, ataxia and confusion. Consider alternative antibiotic.

Key facts

Severity

Moderate — caution required

Mechanism

Trimethoprim inhibits CYP2C9-mediated phenytoin metabolism and adds antifolate effect, raising phenytoin levels and risk...

Action

Monitor phenytoin levels during trimethoprim therapy and watch for nystagmus, ataxia and confusion. Consider alternative...

Important medical disclaimer: This page provides educational information about drug interactions for general reference. It is not a substitute for professional medical advice, diagnosis, or treatment. Always discuss your specific medications with a qualified healthcare provider before starting, stopping, or changing any treatment. About our editorial team.

Frequently asked questions

Can I take Phenytoin and Trimethoprim together?

With caution. Monitor phenytoin levels during trimethoprim therapy and watch for nystagmus, ataxia and confusion. Consider alternative antibiotic.

What is the severity of this interaction?

The interaction between Phenytoin and Trimethoprim is classified as Moderate — caution required. Trimethoprim inhibits CYP2C9-mediated phenytoin metabolism and adds antifolate effect, raising phenytoin levels and risk of toxicity.

What should I do if I'm prescribed both?

Monitor phenytoin levels during trimethoprim therapy and watch for nystagmus, ataxia and confusion. Consider alternative antibiotic. Always consult your prescribing clinician — do not stop or change medications without medical guidance.

Last reviewed: by iMedic Medical Editorial Team. Our editorial process.