Codeine phosphate is an opioid drug that is used as a painkiller. It is used to relieve mild to moderate pain. It is usually taken by mouth. Codeine can also treat diarrhoea and reduce coughing.

Co-formulations of codeine and paracetamol are available, since these two drugs enhance each other’s painkilling properties. This combination is called co-codamol. It is also available with aspirin as co-codaprin or with ibuprofen.

Codeine itself does not reduce pain, but it is converted to morphine within the body. However, it is less potent than morphine, since only about 10% of the codeine is converted.

Side-effects of codeine include itching, nausea, vomiting, drowsiness, dry mouth, low blood pressure, difficulty urinating and constipation. It can also cause difficulty breathing in high doses. Codeine can also be abused, leading to dependence and addiction in rare cases. However, its addictive potential is much lower than morphine.

Codeine is not known to interact with any anti-HIV drugs. However, it is metabolised through the CYP2D6 enzymes, suggesting that some HIV drugs may reduce the effectiveness of codeine.