desvenlafaxine will improve the degree or influence of codeine by influencing hepatic enzyme CYP2D6 metabolism. Use Caution/Observe. Desvenlafaxine inhibits CYP2D6; with greater desvenlafaxine doses (ie, four hundred mg) minimize the CYP2D6 substrate dose by around fifty%; no dosage adjustment desired with desvenlafaxine doses seven consecutive day