Phenytoin and Sulfamethoxazole: Drug Interaction Guide

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

Interaction severity: Moderate — caution required

How Phenytoin interacts with Sulfamethoxazole

Sulfamethoxazole inhibits CYP2C9 metabolism of phenytoin and displaces it from plasma protein binding, raising free phenytoin concentrations.

What you should do

Monitor phenytoin levels (preferably free fraction) and signs of toxicity (nystagmus, ataxia) during co-treatment and adjust dose accordingly.

Key facts

Severity

Moderate — caution required

Mechanism

Sulfamethoxazole inhibits CYP2C9 metabolism of phenytoin and displaces it from plasma protein binding, raising free phen...

Action

Monitor phenytoin levels (preferably free fraction) and signs of toxicity (nystagmus, ataxia) during co-treatment and ad...

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 together?

With caution. Monitor phenytoin levels (preferably free fraction) and signs of toxicity (nystagmus, ataxia) during co-treatment and adjust dose accordingly.

What is the severity of this interaction?

The interaction between Phenytoin and Sulfamethoxazole is classified as Moderate — caution required. Sulfamethoxazole inhibits CYP2C9 metabolism of phenytoin and displaces it from plasma protein binding, raising free phenytoin concentrations.

What should I do if I'm prescribed both?

Monitor phenytoin levels (preferably free fraction) and signs of toxicity (nystagmus, ataxia) during co-treatment and adjust dose accordingly. Always consult your prescribing clinician — do not stop or change medications without medical guidance.

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