Not surprisingly, adverse drug reactions are common. Most adverse drug reactions are relatively mild, and many disappear when the drug is stopped or the dose is changed. Some gradually subside as the body adjusts to the drug. Other adverse drug reactions are more serious and last longer. About 3 to 7% of all hospital admissions in the United States are for treatment of adverse drug reactions. Adverse drug reactions occur during 10 to 20% of hospital admissions, and about 10 to 20% of these reactions are severe.

Digestive disturbances—loss of appetite, nausea, a bloating sensation, constipation, and diarrhea—are particularly common adverse drug reactions, because most drugs are taken by mouth and pass through the digestive tract. However, almost any organ system can be affected. In older people (see Aging and Drugs), the brain is commonly affected, often resulting in drowsiness and confusion.

Some adverse drug reactions are identified when a drug is being tested before it is submitted to the Food and Drug Administration (FDA) for approval. Other adverse drug reactions, typically those that are uncommon, are not detected until the drug has been on the market long enough to be used by a large number of people. Thus, doctors are required to report suspected adverse drug reactions to the FDA.