Most drugs have potential adverse effects or side effects that must be evaluated before being prescribed. In addition, there may be contraindications, or reasons not to use a particular drug for a specific individual based on that person's medical conditions, current medications, sensitivity, or family history. Also, while a patient is under treatment, it is important to be alert for signs of adverse effects such as digestive upset, changes in the blood, or signs of allergy, such as hives or skin rashes. Anaphylaxis is an immediate and severe allergic reaction that may be caused by a drug. It can lead to life-threatening respiratory distress and circulatory collapse.

Because drugs given in combination may interact, the prescriber must know of any drugs the patient is taking before prescribing another. In some cases, a combination may result in synergy or potentiation, meaning that the drugs together have a greater effect than either of the drugs acting alone. In other cases, one drug may act as an antagonist of another, interfering with its action. Drugs may also react adversely with certain foods or substances used socially, such as alcohol and tobacco.

Drugs that act on the central nervous system may lead to a psychological or physical substance dependence, in which a person has a chronic or compulsive need for a drug regardless of its bad effects. With repeated use, a person may develop a drug tolerance, whereby a constant dose has less effect and the dose must be increased to produce the original response. Cessation of the drug then leads to symptoms of substance withdrawal, a state that results from reduction or removal of a drug. Certain symptoms are associated with withdrawal from specific drugs.

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