Efavirenz and Fosamprenavir interaction
Efavirenz and Fosamprenavir: both prescribing labels flag this combination with the strongest language, such as a contraindication, a boxed warning, or an instruction to avoid it.
Do not combine without a prescriber's guidance. The label language below is the strongest FDA uses.
What the FDA labels say
From the Efavirenz label · effective 2024-03-31
Protease inhibitor: Lopinavir/ritonavir ↓ lopinavir * Lopinavir/ritonavir once daily dosing is not recommended when coadministered with efavirenz.
…Alteration in Dose or Regimen May Be Recommended Based on Drug Interaction Studies or Predicted Interaction Concomitant Drug Class: Drug Name Effect Clinical Comment HIV antiviral agents Protease inhibitor: Fosamprenavir Calcium ↓ amprenavir Fosamprenavir (unboosted): Appropriate doses of the combinations with respect to safety and efficacy have not been established.
No change in the ritonavir dose is required when efavirenz is administered with fosamprenavir plus ritonavir twice daily.
Protease inhibitor: Atazanavir ↓ atazanavir * Treatment-naive patients: When coadministered with efavirenz, the recommended dose of atazanavir is 400 mg with ritonavir 100 mg (together once daily with food) and efavirenz 600 mg (once daily on an empty stomach, preferably at bedtime).
Protease inhibitor: Indinavir ↓ indinavir * The optimal dose of indinavir, when given in combination with efavirenz, is not known.
From the Fosamprenavir (Fosamprenavir Calcium) label · effective 2026-06-30
Fosamprenavir calcium/ritonavir: ↑ Amprenavir ↓Delavirdine Non-nucleoside reverse transcriptase inhibitor: Efavirenz a Fosamprenavir calcium: ↓ Amprenavir Fosamprenavir calcium/ritonavir: ↓ Amprenavir Appropriate doses of the combinations with respect to safety and efficacy have not been established.
No change in the ritonavir dose is required when efavirenz is administered with Fosamprenavir calcium plus ritonavir twice daily.
Initiation of medications that inhibit or induce CYP3A may increase or decrease concentrations of fosamprenavir calcium/ritonavir, respectively.
Coadministration of fosamprenavir calcium with drugs that induce CYP3A4 may decrease amprenavir (active metabolite) concentrations leading to potential loss of virologic activity.
Coadministration of fosamprenavir calcium and drugs that induce CYP3A4, such as rifampin, may decrease amprenavir concentrations and reduce its therapeutic effect.
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Open in the checkerNot medical advice. Interaction Checker quotes FDA label text and NIH fact sheets. Whether an interaction matters for you depends on doses, timing, kidney and liver function, and other conditions. Confirm with a pharmacist or prescriber.