Opioid

An opioid is any substance that binds to opioid receptors, proteins found on nerve cells in the brain, spinal cord and gut. The group includes opiates taken from the opium poppy, such as morphine and codeine; semisynthetic drugs made from them, such as oxycodone, hydrocodone and heroin; fully synthetic ones, such as fentanyl and methadone; and the body's own endorphins and enkephalins, which these drugs imitate. Of the three classic receptor types, mu, delta and kappa, the mu receptor carries most effects of opioid drugs.

Opioid receptors work through G proteins. When a drug activates them, nerve cells become less excitable and release less neurotransmitter, which stops pain signals at the spinal cord and dulls the sense of pain in the brain. This makes opioids the strongest painkillers, used for severe pain after surgery or injury and in cancer; they also suppress coughing and slow the gut. The same actions cause drowsiness, nausea and constipation, while effects on the brain's reward system bring euphoria. Repeated use leads to tolerance and physical dependence and can lead to opioid use disorder.

The gravest danger is slowed breathing, which is how an overdose kills, especially with very potent synthetic opioids such as illicit fentanyl. Naloxone, a receptor antagonist, rapidly reverses an overdose, and methadone and buprenorphine are used to treat opioid addiction.