The generic term applied to alkaloids from the opium poppy (Papaver somniferum), their synthetic analogues, and compounds synthesized in the body. All of these substances interact with the same specific receptors in the brain, have the capacity to relieve pain and produce a sense of well-being (euphoria). The opium alkaloids and their synthetic analogues also cause stupor, coma and respiratory depression in high doses.

Opium alkaloids and their semisynthetic derivatives include morphine, diacetylmorphine (diamorphine, heroin), hydromorphine, codeine and oxycodone. Synthetic opioids include levorphanol, propoxyphene, fentanyl, methadone, pethidine (meperidine) and the agonist–antagonist pentazocine. Endogenously occurring compounds with opioid actions include the endorphins and enkephalins.

The most commonly used opioids (such as morphine, heroin, hydromorphine, methadone and pethidine) bind preferentially to the μ-receptors; they produce analgesia, mood changes (e.g. euphoria, which may change to apathy or dysphoria), respiratory depression, drowsiness, psychomotor retardation, slurred speech and impaired concentration, memory and judgement.

Physical consequences of opioid use (mostly resulting from intravenous administration) include hepatitis B, hepatitis C, HIV, septicaemia, endocarditis (inflammation of the inner layer of the heart), pneumonia and lung abscess, thrombophlebitis (blood clots causing vein inflammation) and rhabdomyolysis (breakdown of muscle fibres). Psychological and social impairment is common, reflecting the illicit nature of non-medical opioid use.