All patients received the same regimen. Methadone was administered in the first treatment period, and combination treatment of methadone and nevirapine was given in the second treatment period. In the first period, patients received methadone at their current steady state dose. In the second period (study days 1-21), they also received nevirapine 200mg qd (study days 1 to 14) and 200mg bid (study days 15 to 21). Blood samples were taken at the start of the first treatment period and at the end of the second treatment period for analysis of methadone and nevirapine pharmacokinetics parameters.