A drug is any substance used to treat, prevent, or diagnose disease or aid recovery from injury. The word is also used to describe illegal, or controlled substances, such as cocaine and ecstasy.


Still today many drugs are obtained from natural sources: plants, animals, or minerals. Many antibiotics, for instance, have been developed from microbes present in the soil. However, since the time of Paul Ehrlich, the German scientist who coined the term "magic bullet", scientists have sought to develop synthetic drugs that will work only on target cells or microorganisms, leaving other tissues unaffected.


There is a vast range of drugs in circulation. The most obvious magic bullet effect is that of the antibiotics, which destroy invading bacteria. Some drugs interfere in physiological processes – for instance, anticoagulants render the blood less prone to clotting. Drugs may be given to make good some deficiency, such as hormone preparations that compensate for an under-active gland.


There are a number of routes of drug delivery: orally, by injection, as a lotion or ointment, or by inhalation, pessary, or transdermal patch. As well as direct effects, many drugs, too, are associated with side-effects. If these become too problematic, the dosage may be have to be adjusted or a different drug substituted. Also, some people have an allergy to particular drugs. Increasingly, new drugs are being developed and prescribed to combat the effects of certain psychological disorders.


The use of some drugs on both psychological and physiological conditions can result in a dependence on that drug, a situation doctors are very careful to avoid where possible. Drugs are subject to extensive clinical trials before being licensed for use.