Drug interactions
Efavirenz + Fosphenytoin
Severity: Major
Efavirenz and Fosphenytoin: 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
Efavirenz and phenytoin interact in both directions, with unpredictable effects on concentrations of each drug. This risks either phenytoin toxicity, seizures, or loss of antiretroviral efficacy.
What this interaction means in plain words
Efavirenz and Fosphenytoin 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; Fosphenytoin belongs to Central Nervous System.
Why it happens
Phenytoin, the active metabolite, is metabolized by CYP2C9 and CYP2C19 and induces CYP3A4 and CYP2B6, while efavirenz alters these pathways.
What to do
Monitor phenytoin serum levels and HIV viral load, adjusting doses accordingly. Consider an alternative anticonvulsant without enzyme induction.
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 Fosphenytoin
- Dronedarone + Fosphenytoin Contraindicated
- Fosphenytoin + Ledipasvir-sofosbuvir Contraindicated
- Fosphenytoin + Lorlatinib Contraindicated
- Fosphenytoin + Lurasidone Contraindicated
- Fosphenytoin + Ranolazine Contraindicated
- Fosphenytoin + Sofosbuvir-velpatasvir-voxilaprevir Contraindicated
Frequently asked questions
Can I take Efavirenz with Fosphenytoin?
MediHelp rates the interaction as major. Efavirenz and Fosphenytoin 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 and phenytoin interact in both directions, with unpredictable effects on concentrations of each drug. This risks either phenytoin toxicity, seizures, or loss of antiretroviral efficacy.
How serious is the interaction between Efavirenz and Fosphenytoin?
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 serum levels and HIV viral load, adjusting doses accordingly. Consider an alternative anticonvulsant without enzyme induction.
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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