Methadone is a synthetic opioid, used medically as an analgesic and a maintenance anti-addictive for use in patients on opioids. Although chemically unlike morphine or heroin, methadone also acts on the opioid receptors and thus produces many of the same effects.

Methadone acts by binding to the µ-opioid receptor, but also has some affinity for the NMDA ionotropic glutamate receptor. Methadone is metabolized by CYP3A4, CYP2B6, CYP2D6 and is a substrate for the P-Glycoprotein efflux protein in intestine and brain. The bioavailability and elimination half-life of methadone is subject to substantial inter-individual variability. Its main route of administration is oral. Adverse effects include sedation, hypoventilation, constipation and miosis, in addition to tolerance, dependence and withdrawal difficulties. Methadone has been shown to reduce neuropathic pain in rat models, primarily through NMDA antagonism. Methadone has a slow metabolism and very high fat solubility, making it longer lasting than morphine-based drugs. Methadone has a typical elimination half-life of 15 to 60 hours with a mean of around 22.