desvenlafaxine will raise the degree or influence of codeine by affecting hepatic enzyme CYP2D6 metabolism. Use Warning/Keep track of. Desvenlafaxine inhibits CYP2D6; with bigger desvenlafaxine doses (ie, four hundred mg) reduce the CYP2D6 substrate dose by nearly 50%; no dosage adjustment necessary with desvenlafaxine doses seven consecutive days