Codeine is an old drug that is still widely used to treat mild and moderate pain. It is mainly metabolised by glucuronidation, but minor pathways are N-demethylation to norcodeine and O-demethylation to morphine. The latter pathway depends on the genetically polymorphic CYP2D6 which is absent in 7% of the white population (PM) and present in the remainder (EM). Lack of influence of codeine on experimental pain in PM as well as in EM treated with the CYP2D6 blocker quinidine, who are both practically unable to convert codeine to morphine, has supported an old hypothesis that codeine acts through metabolically formed morphine. Possibly, local codeine O-demethylation in the CNS is of major importance for its hypoalgesic effect. Such a local morphine formation from codeine, which supposedly is also catalysed by CYP2D6, could explain why the hypoalgesic effect of codeine stems from morphine despite relatively low plasma levels of morphine after standard hypoalgesic doses of codeine. Dependence of codeine hypoalgesia on morphine formation via CYP2D6 makes this effect liable to interaction with drugs that are inhibitors of CYP2D6. Examples of potent inhibitors of CYP2D6 are quinidine, some selective serotonin reuptake inhibitors and some neuroleptics. Less potent inhibitors, such as tricyclic antidepressants, will probably also reduce the pain relieving effect of codeine, since codeine has a low affinity for CYP2D6. Biosynthesis of morphine in humans may also include steps catalysed by CYP2D6. Experimental studies in large groups of EM and PM indicate that this may lead to interphenotype differences in pain tolerance.