Man is a valetudinary animal, and is probably unique in this behavior. The availability of drugst for the treatment of disease offers an opportunity to indulge in this activity. Although it is usually thought that the consequences of taking drugs will be beneficial, or at least pleasurable, the desired effects do not necessarily occur; pills and potions may even be harmful. Iatrogenic or drug-induced diseases are, indeed, becoming quite common. The ultimate effect of a drug will depend primarily on its chemical nature, but it will also be influenced by a variety of other factors, including the quantity used, the state of the person's health, age, sex, diet, heredity, and prior and concurrent use of other types of drugs.

Drugs have undoubtedly provided ways and means for the successful treatment of many human diseases, and the use of drugs for health, pleasure, and profit has increased explosively in the last 30 years. However, drugs are not always used effectively or appropriately, and they are often abused. This situation not only results from their illicit use by those with little or no knowledge of their nature or actions, but it is also the result of an insufficient basic understanding about what may happen to drugs in the body. Although various drugs may behave differently, there is an underlying, common pattern in the behavior of most of them. A knowledge of these principles may provide a rational basis for the therapeutic use of drugs and allow prediction, or at least an understanding, of the responses that may occur.