desvenlafaxine will improve the level or effect of codeine by influencing hepatic enzyme CYP2D6 metabolism. Use Caution/Check. Desvenlafaxine inhibits CYP2D6; with larger desvenlafaxine doses (ie, four hundred mg) lower the CYP2D6 substrate dose by as many as 50%; no dosage adjustment necessary with desvenlafaxine doses seven consecutive times quic