desvenlafaxine will increase the degree or effect of codeine by impacting hepatic enzyme CYP2D6 metabolism. Use Warning/Observe. Desvenlafaxine inhibits CYP2D6; with bigger desvenlafaxine doses (ie, 400 mg) decrease the CYP2D6 substrate dose by around 50%; no dosage adjustment essential with desvenlafaxine doses seven consecutive times right away p