Triples with object Drug Interactions Ritonavir is an inhibitor of cytochrome P450 3A (CYP3A) both in vitro and in vivo. Ritonavir also inhibits CYP2D6 in vitro, but to a lesser extent than CYP3A. Co-administration of ritonavir and drugs primarily metabolized by CYP3A or CYP2D6 may result in increased plasma concentrations of other drugs that could increase or prolong its therapeutic and adverse effects