desvenlafaxine will raise the amount or effect of codeine by influencing hepatic enzyme CYP2D6 metabolism. Use Warning/Watch. Desvenlafaxine inhibits CYP2D6; with larger desvenlafaxine doses (ie, four hundred mg) reduce the CYP2D6 substrate dose by nearly 50%; no dosage adjustment required with desvenlafaxine doses 7 consecutive days promptly previ