desvenlafaxine will boost the amount or impact of codeine by influencing hepatic enzyme CYP2D6 metabolism. Use Warning/Watch. Desvenlafaxine inhibits CYP2D6; with bigger desvenlafaxine doses (ie, 400 mg) reduce the CYP2D6 substrate dose by as many as 50%; no dosage adjustment desired with desvenlafaxine doses 7 consecutive times immediately previou