Chemical intervention can alter the course of disease. A drug is a chemical substance used to diagnose, prevent, treat, or cure disease or to modify physiological function for the benefit of the recipient.

Drugs may be derived from natural sources or synthesized. They act on receptors, enzymes, ion channels, or other molecular targets. Development proceeds through laboratory, animal, and staged human trials before regulatory approval. Once marketed, ongoing surveillance detects rare adverse effects.

The same substance can be a medicine or a poison depending on dose and context. Controlled substances require additional safeguards because of abuse potential. Rational drug use matches the right agent to the right patient at the right time and dose.

• Chemical substance used for diagnosis, prevention, or treatment
• Acts on specific biological targets
• Developed through rigorous testing and regulation
• Benefit depends on correct selection and dosing