Most drugs must pass through the liver, which is the primary site for drug metabolism. Once in the liver, enzymes convert prodrugs to active metabolites or convert active drugs to inactive forms. The liver’s primary mechanism for metabolizing drugs is via a specific group of cytochrome P-450 enzymes. The level of these enzymes controls the rate at which many drugs are metabolized. The capacity of the enzymes to metabolize is limited, so they can become overloaded when blood levels of a drug are high. Many substances (such as drugs and foods) affect the cytochrome P-450 enzymes. If these substances decrease the ability of the enzymes to break down a drug, then that drug's effects (including side effects) are increased.

Source: Merck Manual