Codeine is an opioid medication that is used as a pain reliever and cough suppressant. Codeine works by binding to the mu-opioid receptors in the brain and spinal cord, which helps to reduce pain and suppress coughing. Codeine is similar in chemical structure and pharmacological effects to other opioids such as morphine and hydrocodone.

In addition, a small amount of codeine is metabolized (broken down) by the liver into morphine, which is a more potent opioid medication. This conversion process is mediated by an enzyme called CYP2D6, which is highly variable in different individuals.