Drug interactions
Efavirenz + Fosamprenavir
Severity: Major
Efavirenz and Fosamprenavir: 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 lowers amprenavir concentrations, the active metabolite of fosamprenavir, which may lead to loss of virologic suppression and resistance. The effect is more pronounced when fosamprenavir is not boosted with ritonavir.
What this interaction means in plain words
Efavirenz and Fosamprenavir 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.
Why it happens
Efavirenz induces CYP3A4, increasing the clearance of amprenavir.
What to do
Use fosamprenavir only with ritonavir, with the dose adjusted per labeling when combined with efavirenz, or choose a different regimen. Monitor HIV viral load.
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
Frequently asked questions
Can I take Efavirenz with Fosamprenavir?
MediHelp rates the interaction as major. Efavirenz and Fosamprenavir 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 lowers amprenavir concentrations, the active metabolite of fosamprenavir, which may lead to loss of virologic suppression and resistance. The effect is more pronounced when fosamprenavir is not boosted with ritonavir.
How serious is the interaction between Efavirenz and Fosamprenavir?
MediHelp rates it major, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Use fosamprenavir only with ritonavir, with the dose adjusted per labeling when combined with efavirenz, or choose a different regimen. Monitor HIV viral load.
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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