desvenlafaxine will boost the degree or influence of codeine by impacting hepatic enzyme CYP2D6 metabolism. Use Warning/Check. Desvenlafaxine inhibits CYP2D6; with bigger desvenlafaxine doses (ie, 400 mg) reduce the CYP2D6 substrate dose by nearly 50%; no dosage adjustment required with desvenlafaxine doses seven consecutive days straight away prio