desvenlafaxine will boost the level or influence of codeine by impacting hepatic enzyme CYP2D6 metabolism. Use Warning/Check. Desvenlafaxine inhibits CYP2D6; with larger desvenlafaxine doses (ie, 400 mg) reduce the CYP2D6 substrate dose by as many as 50%; no dosage adjustment wanted with desvenlafaxine doses seven consecutive times instantly just b