can develop over a matter of days in patients who have multiple functional alleles of CYP2D6, resulting in ultrarapid metabolism of opioids such as codeine into morphine.

Studies on codeine's analgesic effect are consistent with the idea that metabolism by CYP2D6 to morphine is important, but some studies show no major differences between those who are poor metabolizers and extensive metabolizers. Evidence supporting the hypothesis that ultrarapid metabolizers may get greater analgesia from codeine due to increased morphine formation