The body must process (chemically alter, or metabolize) drugs to be able to use and eliminate them. Most of this processing occurs in the liver, done by liver enzymes. Thus, drugs and the liver can affect each other in several ways:
Drugs can affect how quickly certain other drugs are metabolized in the liver. If a drug is metabolized more quickly, it may be broken down and eliminated before it can do what it is supposed to do. In slower drug metabolism, side effects are more likely.