desvenlafaxine will improve the amount or outcome of codeine by impacting hepatic enzyme CYP2D6 metabolism. Use Caution/Watch. Desvenlafaxine inhibits CYP2D6; with larger desvenlafaxine doses (ie, four hundred mg) reduce the CYP2D6 substrate dose by around 50%; no dosage adjustment desired with desvenlafaxine doses 7 consecutive days quickly ahead