Isn't magnesium hydroxide the ingredient in Milk of Magnesia which is a laxative because it is not well absorbed? Mag oxide and mag citrate are not well absorbed either.
Some forms which change more easily into the absorbable ionic form include:
mag chloride
mag acetate
mag gluconate
Some swear by magnesium sulphate soaks for a quick boost, but I believe it gets excreted quickly by the kidneys.