There are several different forms of magnesium. Some give looser stools than others. Some are specifically used as a laxatives' (think Milk of Magnesia).

I sometimes take a magnesium supplement before bed.

A soak in an Epsom Salt bath is one way to get magnesium also. I have done that, but not for quite a while.

Supposedly potassium and magnesium are two minerals a lot of people are deficient of.