A controlled substance is generally a drug or chemical whose manufacture, possession, and use are regulated by a government. This may include illegal drugs and prescription medications (designated Controlled Drug in the United Kingdom).

In the United States, the DEA is responsible for suppressing illegal drug use and distribution by enforcing the Controlled Substances Act. Some precursor chemicals used for the production of illegal drugs are also controlled substances in many countries, even though they may lack the pharmacological effects of the drugs themselves. Substances are classified according to schedules and consist primarily of potentially psychoactive substances. The controlled substances do not include many prescription items such as antibiotics.

Some US states have statutes against health care providers self-prescribing and/or administering substances listed in the Controlled Substance Act schedules. This does not forbid licensed providers from self-prescribing medications not on the schedules.