Drug interactions
Efavirenz + Phenytoin
Severity: Major
Efavirenz and Phenytoin: major interaction.
Last updated
For healthcare professionals — reference only. This page is a brief summary, not medical advice and not a substitute for official labeling or clinical judgement. Always verify before prescribing or dispensing.
Summary
Phenytoin may lower efavirenz levels via enzyme induction, risking virologic failure, while efavirenz can change phenytoin concentrations in either direction. Phenytoin has a narrow therapeutic index, so toxicity or loss of seizure control is possible.
What this interaction means in plain words
Efavirenz and Phenytoin can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for.
Efavirenz belongs to Anti Virals; Phenytoin belongs to Central Nervous System.
Why it happens
Phenytoin induces CYP3A4 and CYP2B6, lowering efavirenz, while efavirenz affects CYP2C9/2C19-mediated phenytoin metabolism.
What to do
Monitor phenytoin plasma concentrations and HIV RNA, and consider efavirenz levels. Prefer a non-inducing anticonvulsant such as levetiracetam when possible.
What to ask your pharmacist
- Does the benefit of taking both outweigh the risk in my case?
- Do I need any blood test, ECG or other check while on both?
- What early warning signs should I watch for, and what should I do if they appear?
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Other interactions of Efavirenz
- Cobimetinib + Efavirenz Contraindicated
- Dienogest / estradiol valerate + Efavirenz Contraindicated
- Dihydroergotamine intranasal + Efavirenz Contraindicated
- Dihydroergotamine + Efavirenz Contraindicated
- Doravirine + Efavirenz Contraindicated
- Efavirenz + Elbasvir / grazoprevir Contraindicated
Other interactions of Phenytoin
- Artemether / lumefantrine + Phenytoin Contraindicated
- Bictegravir + Phenytoin Contraindicated
- Cabotegravir + Phenytoin Contraindicated
- Doravirine + Phenytoin Contraindicated
- Elbasvir / grazoprevir + Phenytoin Contraindicated
- Elvitegravir / cobicistat / emtricitabine / tenofovir df + Phenytoin Contraindicated
Frequently asked questions
Can I take Efavirenz with Phenytoin?
MediHelp rates the interaction as major. Efavirenz and Phenytoin can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for. Phenytoin may lower efavirenz levels via enzyme induction, risking virologic failure, while efavirenz can change phenytoin concentrations in either direction. Phenytoin has a narrow therapeutic index, so toxicity or loss of seizure control is possible.
How serious is the interaction between Efavirenz and Phenytoin?
MediHelp rates it major, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Monitor phenytoin plasma concentrations and HIV RNA, and consider efavirenz levels. Prefer a non-inducing anticonvulsant such as levetiracetam when possible.
What other medicines interact with Efavirenz?
MediHelp also lists interactions of Efavirenz with, for example, Cobimetinib, Dienogest / estradiol valerate, Dihydroergotamine intranasal. The full list is on the Efavirenz page.
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