The term "narcotic"", based on the Greek word for narcosis, the term used by Hippocrates for the process of numbing or the numbed state, is believed to have been coined by the Greek physician Galen to refer to agents that numb or deaden, causing loss of feeling or paralysis.

Galen listed mandrake root, altercus (eclata) seeds, and poppy juice (opium) as the chief examples.

Narcotics originally referred medically to any psychoactive compound with sleep-inducing properties.

In the United States, it has since become associated with opiates and opioids, commonly morphine and heroin, as well as derivatives of many of the compounds found within raw opium latex.

The primary three are morphine, codeine, and thebaine (while thebaine itself is only very mildly psychoactive, it is a crucial precursor in the vast majority of semi-synthetic opioids, such as hydrocodone).

The term opiate should be differentiated from the broader term opioid, which includes all drugs with morphine-like effects, including opiates, semi-synthetic opioids derived from opiates (such as heroin, hydrocodone, hydromorphone, oxycodone, and oxymorphone), and synthetic opioids that are not derived from opiates (such as fentanyl, buprenorphine, and methadone).