desvenlafaxine will improve the level or influence of codeine by affecting hepatic enzyme CYP2D6 metabolism. Use Caution/Watch. Desvenlafaxine inhibits CYP2D6; with better desvenlafaxine doses (ie, four hundred mg) decrease the CYP2D6 substrate dose by around 50%; no dosage adjustment necessary with desvenlafaxine doses 7 consecutive days right awa