desvenlafaxine will enhance the stage or impact of codeine by affecting hepatic enzyme CYP2D6 metabolism. Use Warning/Keep track of. Desvenlafaxine inhibits CYP2D6; with increased desvenlafaxine doses (ie, 400 mg) minimize the CYP2D6 substrate dose by nearly 50%; no dosage adjustment required with desvenlafaxine doses 7 consecutive days immediately