desvenlafaxine will improve the level or impact of codeine by influencing hepatic enzyme CYP2D6 metabolism. Use Warning/Check. Desvenlafaxine inhibits CYP2D6; with greater desvenlafaxine doses (ie, four hundred mg) reduce the CYP2D6 substrate dose by as many as fifty%; no dosage adjustment needed with desvenlafaxine doses 7 consecutive days promptl