desvenlafaxine will improve the amount or impact of codeine by affecting hepatic enzyme CYP2D6 metabolism. Use Caution/Keep an eye on. Desvenlafaxine inhibits CYP2D6; with bigger desvenlafaxine doses (ie, 400 mg) minimize the CYP2D6 substrate dose by nearly 50%; no dosage adjustment essential with desvenlafaxine doses seven consecutive times straig