When given with efavirenz for HIV-1 treatment, the dose of Descovy is 200/25 mg once daily. Coadministration of efavirenz (600 mg once daily) and tenofovir alafenamide (40 mg once daily, given as Descovy) decreased tenofovir alafenamide AUC and Cmax by 14% and 22%.

Based on drug interaction studies conducted with the components of Descovy, no clinically significant drug interactions have been either observed or are expected when Descovy is combined with efavirenz. Coadministration of efavirenz (600 mg once daily) and tenofovir alafenamide (40 mg once daily, dosed as Descovy) was studied in 11 subjects. Tenofovir alafenamide Cmax and AUC decreased by 22% and 14%.

Specific interaction studies have not been performed with efavirenz and NRTIs other than lamivudine, zidovudine, and tenofovir disoproxil fumarate. Clinically significant interactions are not expected since the NRTIs are metabolised via a different route than efavirenz and would be unlikely to compete for the same metabolic enzymes and elimination pathways. No dosage adjustment is necessary for either medicinal product.

No dosage adjustment is recommended when efavirenz is given with emtricitabine.

Coadministration of efavirenz (600 mg once daily) with emtricitabine/tenofovir alafenamide (200/40 mg once daily) was studied in 11 healthy volunteers in a crossover study. Tenofovir alafenamide AUC and Cmax decreased by 14% and 22%, respectively. Tenofovir AUC and Cmax decreased by 20% and 24%, respectively. Efavirenz pharmacokinetic parameters were similar to historical controls. The authors hypothesise that the decrease in tenofovir alafenamide and tenofovir exposures are due to intestinal induction of P-gp by efavirenz and that this reduction is not clinically relevant, as equivalent tenofovir exposures were associated with efficacy in phase 3 trials.