The word drug covers a lot of ground. Aspirin, heroin, insulin, caffeine, and penicillin are all drugs. So is alcohol, by some definitions. At its core, a drug is a chemical substance that changes how the body works. In medicine, the term usually means a substance used to treat, cure, or prevent disease, but the same molecule can be a medicine in one context and a substance of abuse in another.

Drugs come from many sources. Some are extracted from plants, like morphine from poppies and digoxin from foxglove. Some are made by bacteria or fungi, like penicillin. Some are synthesized in laboratories, like most modern pharmaceuticals. Some are produced by living cells, like monoclonal antibodies. The boundary between a drug and a biologic is regulatory, not chemical. Both are used to treat disease, but they follow different approval pathways.

What defines a drug:

• Biological effect. It changes a physiological process.
• Dose dependence. The effect varies with the amount taken.
• Intended use. Medical, recreational, or both.
• Regulation. Most countries control the sale and use of drugs.

Prescription drugs require a clinician's order. Over-the-counter drugs do not. Illegal drugs are controlled substances, often because of abuse potential or lack of accepted medical use.