How is unabsorbed magnesium in milk of magnesia (magnesium hydroxide) excreted by body? thanks.

Magnesium: Magnesium is fairly efficiently (30 to 40%) absorbed by the body. Any that is absorbed in excess can be excreted easily by the kidneys, assuming normal renal function. Any excess will be excreted in the stool, usually causing diarrhea if it is present in any significant amount. Most people do not need magnesium supplements, since it is present in many foods.