There are different varieties of magnesium: Magnesium Oxide, Magnesium Citrate, Magnesium Glycinate, etc. I was already taking it for my osteoporosis and I've read quite a bit about it on some of those sites.
Magnesium Oxide is probably the cheapest (and easy to find at Walmart or Walgreens or CVS) but supposedly is not as well absorbed as some of the other varieties. And too much of any magnesium can cause loose stools.
I've taken all three of the varieties that I mentioned above. I had switched to the citrate and glycinate varieties because they didn't seem to cause me as many problems with stools. But that was before I knew that it could also be useful for VM.
I have seen on at least one VM site (can't remember for sure where) where they specifically say to take Magnesium Oxide. But on most of the other sites it just says "magnesium." So I am NOT SURE now whether the "variety" of magnesium makes any difference for VM.
I may try going back to the Magnesium Oxide again and see whether I can tolerate the full 400mg (I split it into two doses, one morning and one evening). If not then maybe I'll try one dose of Magnesium Oxide and one dose of Magnesium Glycinate.