Beginning of Essay
In order for a chemical to be considered a drug it must have the capacity to affect how the body works--to be biologically active. No substance that has the power to do this is completely safe, and drugs are approved only after they demonstrate that they are relat ....

Middle of Essay
.... When that same drug is taken by millions, some people may not respond in a predictable way to the drug. A person who has a so-called idiosyncratic response to a particular sedative, for example, may become excited rather than relaxed. Others may be hypersensitive, or extremely sensitive, to certain drugs, suffering reactions that resemble allergies. A patient may also acquire a tolerance for a certain drug. This means that ever-larger doses are necessary to produce the desired therapeutic effect. Tolerance may lead to habituation, in which the person becomes so dependent upon the drug that he or she becomes addicted to it. Addiction causes s ....