desvenlafaxine will improve the level or influence of codeine by affecting hepatic enzyme CYP2D6 metabolism. Use Caution/Watch. Desvenlafaxine inhibits CYP2D6; with higher desvenlafaxine doses (ie, 400 mg) reduce the CYP2D6 substrate dose by as many as fifty%; no dosage adjustment necessary with desvenlafaxine doses 7 consecutive times immediately