Phenytoin and Sulfamethoxazole Trimethoprim: Drug Interaction Guide

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

Interaction severity: Moderate — caution required

How Phenytoin interacts with Sulfamethoxazole Trimethoprim

Sulfamethoxazole inhibits CYP2C9 and displaces phenytoin from plasma protein binding, increasing free phenytoin levels and risk of toxicity.

What you should do

Monitor phenytoin levels (free if available) and watch for nystagmus, ataxia, confusion. Adjust phenytoin dose downward as needed.

Key facts

Severity

Moderate — caution required

Mechanism

Sulfamethoxazole inhibits CYP2C9 and displaces phenytoin from plasma protein binding, increasing free phenytoin levels a...

Action

Monitor phenytoin levels (free if available) and watch for nystagmus, ataxia, confusion. Adjust phenytoin dose downward ...

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 Sulfamethoxazole Trimethoprim together?

With caution. Monitor phenytoin levels (free if available) and watch for nystagmus, ataxia, confusion. Adjust phenytoin dose downward as needed.

What is the severity of this interaction?

The interaction between Phenytoin and Sulfamethoxazole Trimethoprim is classified as Moderate — caution required. Sulfamethoxazole inhibits CYP2C9 and displaces phenytoin from plasma protein binding, increasing free phenytoin levels and risk of toxicity.

What should I do if I'm prescribed both?

Monitor phenytoin levels (free if available) and watch for nystagmus, ataxia, confusion. Adjust phenytoin dose downward as needed. Always consult your prescribing clinician — do not stop or change medications without medical guidance.

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