Codeine or 3-methylmorphine is an opiate used for its analgesic, antitussive, antidiarrheal, and anxiolytic/antidepressant and sedative properties. Codeine is the second-most predominant alkaloid in opium.

Codeine is closely related to morphine being an alkaloid of opium but possessing around 1:5th–6th of the strength. It is a drug with a long history that is commonly used recreationally, for self medication, in the control of mild to moderate pain, and less commonly in the UK in drug treatment programmes. The usual clinical formulation is codeine phosphate. It is metabolized into morphine, albeit in small quantities in drug testing. On the issue of ‘Self- medication’, this is a term used to describe when an individual opts without individual medical endorsement to use a psychoactive substance to reduce pain of a medical or psychological nature. It will be noted that codeine has anxiolytic/antidepressant properties. In the case of self-medication with codeine the sought after effects are usually to reduce stress, anxiety, existential pain and depression. As the drug clearly is highly efficacious for this purpose, the question is whether the negative consequences outweigh the benefits. Modern medical thinking appears to be a largely consensual endorsement that they do and the benficial properties of the drug rarely justify the risk of addiction. Codeine tolerances can increase across time and it is not unheard of to encounter users tolerant to quantities of well over a gram per day. When this is achieved in combination formulations the amount of ibuprofen or paracetamol ingested can cause serious health difficulties. Codeine is very rarely encountered outside of pharmaceutical formulations.