Codeine

Codeine is an opioid used to treat mild-to-moderate pain and, in some products, to suppress cough. It is generally milder than morphine, but it belongs to the same drug class and carries the same fundamental opioid risks — dependence, and dangerous-to-fatal respiratory depression in overdose or in combination with other depressants. Codeine is found in prescription tablets and in combination cough and pain products; some preparations have historically been available with fewer restrictions in certain places, which has contributed to misuse (for example in sweetened drink mixtures). Because the body converts codeine into morphine, how strongly it affects a given person depends partly on individual metabolism, which makes its effects less predictable than they might seem.

Codeine is itself relatively weak at opioid receptors; most of its effect comes from being converted in the body (by the liver enzyme CYP2D6) into morphine, which then activates mu-opioid receptors to relieve pain and produce sedation and euphoria. Because people vary widely in how fast they make this conversion, the same dose can produce very different effects: "ultra-rapid metabolizers" can generate unexpectedly high morphine levels and are at greater risk of dangerous breathing suppression, which is why codeine is considered especially hazardous in children and in some individuals. Like all opioids, codeine suppresses the brain-stem control of breathing, and repeated use produces tolerance and physical dependence.

At usual doses codeine relieves pain and suppresses cough, and commonly causes drowsiness, constipation, nausea, dry mouth, and a mild sense of calm or euphoria. Higher doses bring stronger sedation, confusion, and the opioid-class danger of slowed, shallow breathing. With repeated use, tolerance and physical dependence develop, and stopping after regular use brings opioid withdrawal — restlessness, aches, insomnia, sweating, nausea, and craving. Because effects depend on individual metabolism, some people experience stronger-than-expected effects from a standard dose.

Although codeine is milder than morphine, it is still an opioid, and the central danger remains respiratory depression — slowed or stopped breathing — which becomes much more likely at high doses, in fast metabolizers, and when codeine is combined with other depressants. Mixing codeine with alcohol, benzodiazepines, or other opioids compounds the suppression of breathing and can be fatal; a particular hazard is combining codeine cough syrup with alcohol or sedatives. Codeine is especially dangerous for children and for people who metabolize it rapidly. Naloxone (Narcan) reverses an opioid overdose, is available over the counter in the U.S. since 2023, and is safe to give even if opioids turn out not to be involved — give it if available and call 911 for slow or stopped breathing or unresponsiveness. Anything sold as codeine outside a pharmacy carries the broader illicit-supply risk of contamination with stronger opioids such as fentanyl. For poisoning guidance call Poison Control at 1-800-222-1222; for substance-use support the SAMHSA National Helpline is 1-800-662-4357.