Here's some more info (altho yeah, I probably repeated alot of Glok's, so, oops...) :

Magnesium Supplements: Benefits, Deficiency, Dosage, Effects, and More

"Magnesium is also the main ingredient in many antacids and laxatives." EEK.

Aaand that's why I won't take it, although it was suggested to me for help with anxiety/depression. I know there is already some in my multivitamin. If you can, I think the healthiest way (as usual) is the increase your intake of foods with a higher magnesium content (some are listed in the above info.)

WebMd does say if you take proton pump inhibitors on a long-term basis that your dr. might need to test your magnesium level.