I've heard that they can counter act the effects of creatine. First time I bought xpand guy at the store told me not to take caffiene and the like, because it would reduce it's effectiveness. If taking creatine mono I wouldn't worry about it because it's cheap, but soemthin expencive like xpand I wouldn't want to risk it, waste of money. Besides I don't take the xpand while cutting.. and that's the only time I'll take the dieretics.