desvenlafaxine will improve the amount or effect of codeine by influencing hepatic enzyme CYP2D6 metabolism. Use Caution/Keep an eye on. Desvenlafaxine inhibits CYP2D6; with greater desvenlafaxine doses (ie, four hundred mg) reduce the CYP2D6 substrate dose by approximately 50%; no dosage adjustment wanted with desvenlafaxine doses 7 consecutive da