Introduction
In the early 1900s, the German scientist Paul Ehrlich described an ideal drug as a "magic bullet"; such a drug would be aimed precisely at a disease site and would not harm healthy tissues. Although many new drugs are aimed more accurately than their predecessors, none of them, as of yet, hit the target precisely.
Most drugs produce several effects, but usually only one effect--the therapeutic effect--is wanted for the treatment of a disorder. The other effects may be regarded as unwanted, whether they are intrinsically harmful or not. For example, certain antihistamines cause drowsiness as well as control the symptoms of allergies. When an over-the-counter sleep aid containing an antihistamine is taken, drowsiness is considered a therapeutic effect. But when an antihistamine is taken to control allergy symptoms during the daytime, drowsiness is considered an annoying, unwanted effect.
Most people, including health care practitioners, refer to unwanted effects as side effects; another term used is adverse drug event. However, the term adverse drug reaction is technically more appropriate for drug effects that are unwanted, unpleasant, noxious, or potentially harmful.
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. Between 3% and 7% of hospital admissions in the United States are estimated to be for treatment of adverse drug reactions. Each time a person is hospitalized, the risk of having at least one adverse drug reaction is 10 to 20%.
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, the brain is commonly affected, often resulting in drowsiness and confusion.
 See the table Some Serious Adverse Drug Reactions.
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