desvenlafaxine will increase the level or result of codeine by influencing hepatic enzyme CYP2D6 metabolism. Use Caution/Check. Desvenlafaxine inhibits CYP2D6; with higher desvenlafaxine doses (ie, four hundred mg) lessen the CYP2D6 substrate dose by as much as 50%; no dosage adjustment desired with desvenlafaxine doses seven consecutive days right