desvenlafaxine will improve the degree or result of codeine by affecting hepatic enzyme CYP2D6 metabolism. Use Warning/Check. Desvenlafaxine inhibits CYP2D6; with greater desvenlafaxine doses (ie, 400 mg) lower the CYP2D6 substrate dose by nearly fifty%; no dosage adjustment needed with desvenlafaxine doses 7 consecutive days immediately just befor