Coadministration of nevirapine and efavirenz (600 mg once daily) decreased efavirenz AUC, Cmax and Cmin by 28%, 12% and 32%, respectively. It is not recommended to co-administer efavirenz and nevirapine, because of additive toxicity and no benefit in terms of efficacy over either NNRTI alone.

Coadministration of nevirapine (200 mg once daily for 2 weeks then 200 mg twice daily for 2 weeks) with efavirenz (600 mg once daily) in 17 HIV+ patients caused a 28%, 12% and 32% decrease in efavirenz AUC, Cmax and Cmin, respectively. The effect on nevirapine pharmacokinetics was not significant when compared to historical controls. The appropriate doses of these combinations with respect to safety and efficacy have not been established. Coadministration of nevirapine and efavirenz is not recommended as this combination has been associated with an increase in adverse reactions and no improvement in efficacy.

No studies have been performed with efavirenz in combination with other NNRTIs. Since use of two NNRTIs proved not beneficial in terms of efficacy and safety, coadministration of efavirenz and another NNRTI is not recommended.

Combining two NNRTIs has not been shown to be beneficial. Efavirenz should not be coadministered with other NNRTIs.

A sub analysis of the 2NN study investigated the pharmacokinetics of nevirapine and efavirenz. It was found that concomitant use of nevirapine significantly increased efavirenz oral clearance by 43%.

In a study efavirenz (600 mg once daily) and nevirapine (400 mg once daily) were coadministered. The result was a significant decrease in efavirenz AUC, Cmin and Cmax by 22%, 36% and 17%, respectively. More studies are needed to find the correct dose of efavirenz to use with nevirapine but a dose increase of efavirenz to 800 mg once daily may be warranted. The pharmacokinetic parameters of nevirapine are not affected by coadministration with efavirenz.