desvenlafaxine will raise the amount or impact of codeine by impacting hepatic enzyme CYP2D6 metabolism. Use Warning/Observe. Desvenlafaxine inhibits CYP2D6; with increased desvenlafaxine doses (ie, four hundred mg) lessen the CYP2D6 substrate dose by approximately 50%; no dosage adjustment needed with desvenlafaxine doses seven consecutive days im