Increase of R-/S-methadone enantiomer concentration ratio in serum of patients treated with either nevirapine or efavirenz.

An increased methadone enantiomer ratio (R/S) was associated to both nevirapine (179%, n=5) and efavirenz (36%, n=9) treatments when compared with that of controls (n=52). Additionally, in four follow-up patients, both R- and S-methadone normalized concentrations decreased (19%-93%) while R/S increased (22%-314%) following nevirapine/ efavirenz treatment. R/S decreased (42%) after non-compliance with efavirenz treatment. Therefore, the methadonemaintenance- treatment outcome should be evaluated when patients are treated with drugs which are supposed to induce CYP3A4 and CYP2B6 isoforms.