desvenlafaxine will enhance the amount or result of codeine by influencing hepatic enzyme CYP2D6 metabolism. Use Warning/Check. Desvenlafaxine inhibits CYP2D6; with greater desvenlafaxine doses (ie, four hundred mg) lessen the CYP2D6 substrate dose by as much as fifty%; no dosage adjustment necessary with desvenlafaxine doses seven consecutive time