Drugs are chemical compounds intended for use in the diagnosis and treatment of diseases. More informally, a drug is any non-food substance that causes changes in the structure or function of the body.
Drugs have played a major role in the history of medicine. Perhaps the most famous example is the accidental discovery of penicillin, a drug which proved to be a highly effective treatment for many infectious diseases, by the biologist Alexander Fleming.
Drug abuse occurs when the steady use of one or more addictive drugs, prescription or illegal, results in overuse with negative consequences. Even familiar drugs like aspirin can have significant side effects. Several blogs offer useful overviews on drugs, including Brain Blogger and Rx Blog.