A drug can be one of the medical profession's most valuable tools.  Doctors prescribe drugs to treat or prevent many diseases.  Vaccines prevent such diseases as measles, polio, and smallpox.  Analgesics lesson or eliminate pain.  Many other drugs have helped millions of people to live longer, healthier lives than would otherwise have been possible.

World Book Encyclopedia. Vol. 5, p. 350

Drug abuse:

Drug abuse is the nonmedical use of a drug that interferes with a healthy and productive life.  Drug abuse occurs at all economic levels of society, from wealthy to the impoverished, and among young people as well as adults. Any drug may be abused, including tobacco and alcohol, medications and other substances.  Drug abuse can also be called substance abuse or substance dependence, which together are called substance use disorders.

World Book Encyclopedia, Vol. 5. p. 361