desvenlafaxine will increase the stage or effect of codeine by affecting hepatic enzyme CYP2D6 metabolism. Use Warning/Keep an eye on. Desvenlafaxine inhibits CYP2D6; with bigger desvenlafaxine doses (ie, 400 mg) decrease the CYP2D6 substrate dose by approximately fifty%; no dosage adjustment desired with desvenlafaxine doses seven consecutive days