Adverse drug reactions occur when the body reacts in unexpected ways to what is considered a normal dose of a drug. Adverse drug reactions are one of the leading causes of hospitalizations and results in thousands of unnecessary deaths in the US every year. ADRs may be caused be the lack of an enzyme need to metabolize a drug allowing the build-up of the drug in the body; they may be caused by the metabolizing of a drug in an unexpected way resulting in a toxic metabolite; they may be caused by prescription of usually safe drugs in normal amounts, but the presence of the second drug (or smoking, or particular foods) affects the body’s ability to metabolize one of the drugs. In some cases a particular drug may inhibit the action of a metabolizing enzyme on another drug, or induce greater activity. In other cases, it’s competitive—one drug is metabolized more aggressively, leaving more of the second drug in the body.
These sites have detailed information on adverse drug effects.