desvenlafaxine will boost the level or effect of codeine by influencing hepatic enzyme CYP2D6 metabolism. Use Warning/Check. Desvenlafaxine inhibits CYP2D6; with higher desvenlafaxine doses (ie, four hundred mg) lessen the CYP2D6 substrate dose by nearly fifty%; no dosage adjustment essential with desvenlafaxine doses 7 consecutive days right away