When taken together, drugs can interact with one another and produce desirable or undesirable results. Some drugs have an additive effect-that is, they increase the effect of other drugs. For example, alcoholic beverages intensify the drowsiness-producing effect of some sedatives. Drugs that displace, or take the place of other drugs present in blood proteins, make the displaced drugs more active in the body, increasing their effect. Other drugs have a reducing effect-that is, they interfere with the action of drugs already present in the body. For example, antacids prevent antibiotics from being absorbed by the stomach. Some drugs combine with other drugs to create a substance that has no medical benefit. In some cases, however, drug interactions can produce desirable results. Doctors have found that using three drugs to fight AIDS is more effective than one drug used alone.

Drugs are most effective when properly prescribed by physicians and taken correctly by patients. Missing doses, taking drugs at the wrong time of the day or with instead of before meals, and stopping drug use too soon can markedly reduce the medical benefits of many drugs.