Drugs that are illegal under international drug control treaties are known as illicit drugs.

Opioids, cocaine, cannabis, and amphetamines are the four main categories of illicit drugs that are often the subject of international statistics. However, a variety of other illegal substances, such as plant-based drugs and synthetic hallucinogens, are covered by international drug control treaties. The "Terminology and Information on Drugs" publication from the UNODC includes a thorough list of illicit drugs.