desvenlafaxine will increase the degree or influence of codeine by affecting hepatic enzyme CYP2D6 metabolism. Use Caution/Check. Desvenlafaxine inhibits CYP2D6; with bigger desvenlafaxine doses (ie, four hundred mg) lessen the CYP2D6 substrate dose by as much as fifty%; no dosage adjustment wanted with desvenlafaxine doses seven consecutive times