desvenlafaxine will increase the degree or effect of codeine by affecting hepatic enzyme CYP2D6 metabolism. Use Warning/Monitor. Desvenlafaxine inhibits CYP2D6; with higher desvenlafaxine doses (ie, four hundred mg) reduce the CYP2D6 substrate dose by around fifty%; no dosage adjustment required with desvenlafaxine doses seven consecutive days imme