When you take too much magnesium it has a laxative effect. The amount that causes this is different for different people and is different for different forms of magnesium. Some laxatives take advantage of this effect, such as milk of magnesia or the strong magnesium citrate laxatives they use in the hospitals before some medical procedures.
Some magnesium supplements even tell you to start low and increase the dosage until you start experiencing loose stools and then backing off a bit to find the dose that is best for you.
A rough guideline is that forms such as magnesium citrate and even oxide can cause laxative effects at lower doses and ones like glycinate, taurate, threonate are less likely to cause this. Oxide is used often cause it is cheap and higher in elemental magnesium than the others, but is considered to be less bioavailable and can cause laxative effects in some people right around the RDA which is 400mg.
Edited by MrSpud, 04 April 2015 - 10:38 PM.