desvenlafaxine will enhance the degree or influence of codeine by impacting hepatic enzyme CYP2D6 metabolism. Use Warning/Observe. Desvenlafaxine inhibits CYP2D6; with better desvenlafaxine doses (ie, four hundred mg) reduce the CYP2D6 substrate dose by around 50%; no dosage adjustment needed with desvenlafaxine doses seven consecutive times immedi