Methadone and opiate derivatives: co-administration of methadone with amprenavir resulted in a decrease in the Cmax and AUC of the active methadone enantiomer (R-enantiomer) of 25% and 13% respectively, whilst the Cmax, AUC and Cmin of the inactive methadone enantiomer (S-enantiomer) were decreased by 48%, 40% and 23% respectively.