Other drugs, as well as supplements or steroids that are processed by the liver, can also interact with HIV drugs. That complicates the issue of drug interactions, which may lead to side effects. When you swallow a drug, it goes from your stomach into your intestinal tract, then into the liver . That is called inhibition. A p450 inhibitor is any drug that inhibits the metabolism of another drug by an enzyme in the p450 family.
Inhibitors usually increase concentrations of other drugs.
Once a p450 inhibitor is cleared from the system, the inhibition stops. The protease inhibitor ritonavir , for example, is a p450 inhibitor that increases the blood level of another drug, rifabutin , by nearly fourfold. Some HIV drugs get broken down by more than one p450 enzyme, making them p450 inhibitors and inducers. Unfortunately, it's almost impossible for even the experts to predict how a given combination of HIV drugs will work together. This is particularly true for people whose old regimens are failing.