Decreased methadone effect after ritonavir initiation

Combination antiretroviral therapy including protease inhibitors such as ritonavir has added significant potency to therapy for human immunodeficiency viral (HIV) infection as well as substantial drug-drug interactions. Methadone metabolism is affected by cytochrome P450 (CYP) 3A4 inhibitors or inducers. Because ritonavir can induce CYP3A, it can decrease methadone plasma levels. An HIV-infected patient receiving methadone maintenance experienced withdrawal symptoms after ritonavir, saquinavir, and stavudine were added to his regimen; the most likely cause was ritonavir.