desvenlafaxine will improve the amount or outcome of codeine by influencing hepatic enzyme CYP2D6 metabolism. Use Warning/Observe. Desvenlafaxine inhibits CYP2D6; with bigger desvenlafaxine doses (ie, 400 mg) reduce the CYP2D6 substrate dose by up to fifty%; no dosage adjustment desired with desvenlafaxine doses seven consecutive times immediately