err...just found this ;p

"The intestines control the amount of magnesium absorbed, since any overdosing with magnesium causes diarrhea, which expels the overdose. This is the way Milk of Magnesia works, resulting in evacuation of the entire bowel, because this salt is virtually insoluble and combines with water to act as a laxative. If you are taking Mg Supplements and take too much you will develop an episode of watery diarrhea that will last about an hour. You should then decrease your next dose by 50% as indicated on the Mg Dosing page"