Medline Plus, a service of the U.S. National Library of Medicine defines “drug” as:

1a. A substance used as a medication or in the preparation of medication
1b. According to the Food, Drug, and Cosmetic Act:

  1. a substance recognized in an official pharmacopoeia or formulary
  2. a substance intended for use in the diagnosis, cure, mitigation, treatment, or prevention of disease
  3. a substance other than food intended to affect the structure or function of the body
  4. a substance intended for use as a component of a medicine but not a device or a component, part, or accessory of a device

2. Something and often an illicit substance that causes addiction, habituation, or a marked change in consciousness

There are people that believe coffee is a drug when used in the treatment of Parkinson’s Disease. Others use drugs like marijuana to treat pain even though the medical literature clearly indicates that it can cause severe problems if used for any prolonged period.