An alkaloid C18H21NO3 found in opium. It is structurally similar to morphine, from which it is produced, and is used in the form of the sulphate or phosphate as a painkiller and cough medicine. Codeine is converted into morphine in the liver. It is used to some extent as a recreational drug. In the UK it is a class B drug but can be obtained in composite over-the-counter preparations in which it has a low concentration and is combined with paracetamol or ibuprofen. See also opioids.