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 relatively safe when used as directed, and when the benefits outweigh their risks. Thus, some very dangerous drugs are approved because they are necessary to treat serious illness. Digitalis, which causes the heart muscle to contract, is a dangerous drug, but doctors are permitted to use it because it is vital for treating patients whose heart muscle is weak. A drug as potent as digitalis would not be approved to treat such minor ailments as temporary fatigue because the risks outweigh the benefits. Many persons suffer ill effects from drugs even though they take the drug exactly as directed by the doctor or the label. The human population, unlike a colony of ants or bees, contains a great variety of genetic variation. Drugs are tested on at most a few thousand people. 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 severe psychological and physical disturbances when the drug is taken away. Morphine, cocaine, and Benzedrine are common habit-forming drugs. Finally, drugs often have unwanted side effects. These usually cause only minor discomfort such as a skin rash, headache, or drowsiness. Certain drugs, however, can produce serious, even life-threatening adverse reactions. For example, the drug Thalidomide was once called one of the safest sedatives ever developed, but thousands of women in the United Kingdom who took it during pregnancy gave birth to seriously deformed babies. Other adverse reactions stem from mixing drugs. Thus, taking aspirin, which has blood-thinning qualities, for a headache can be very harmful if one is also taking other blood-thinning drugs such as heparin or dicumarol. . 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. common habit-forming drugs. Finally, drugs often have unwanted side effects. These usually cause only minor discomfort such as a skin rash, headache, or drowsiness. Certain drugs, however, can. Other adverse reactions stem from mixing drugs. Thus, taking aspirin, which has blood-thinning qualities, for a headache can be very harmful if one is also taking other blood-thinning drugs such