desvenlafaxine will increase the stage or effect of codeine by affecting hepatic enzyme CYP2D6 metabolism. Use Caution/Watch. Desvenlafaxine inhibits CYP2D6; with increased desvenlafaxine doses (ie, four hundred mg) reduce the CYP2D6 substrate dose by approximately fifty%; no dosage adjustment desired with desvenlafaxine doses 7 consecutive days st