desvenlafaxine will improve the level or effect of codeine by impacting hepatic enzyme CYP2D6 metabolism. Use Warning/Keep an eye on. Desvenlafaxine inhibits CYP2D6; with greater desvenlafaxine doses (ie, four hundred mg) lower the CYP2D6 substrate dose by up to 50%; no dosage adjustment desired with desvenlafaxine doses seven consecutive days prom