An analysis of several studies concluded that magnesium may help reduce the risk of stroke. People who consumed 100 mg of magnesium more per day than average (the average being about 300 mg) had an 8% lower risk of strokes of any kind and a 9% lower risk of ischemic stroke (Larsson, Am J Clin Nutr 2012). This finding is based on total magnesium in the diet — it does not mean that 100 mg of magnesium from a supplement will necessarily have the same risk-lowering effect, but getting a total of at least 400 mg of magnesium from your diet and supplementation per day may be beneficial.
You want to make sure your supplements contain a chelated form of magnesium and only a small portion of the salt form. Look for magnesium citrate or Amino Acid chelate. Most of the store brands contain magnesium oxide. These forms of minerals are very poorly absorbed as compared to the chelated forms, of course the chelated forms are much more costly, hence most companies go for the cheaper magnesium oxide form, even it is does little good for you. My mulit contains 300 mg of magnesium citrate per day. On the other hand, the #1 selling multi only contains 100 mg of magnesium oxide.
One study looked at the bioavailability of magnesium citrate vs magnesium oxide. They found that in humans absorb 36 times more magnesium in the citrate from vs the oxide form. (http://www.ncbi.nlm.nih.gov/pubmed/2407766) So if your multi or your calcium/magnesium supplement contains magnesium oxide, then you’re wasting your money. This same applies for all the salt forms of minerals (-oxide, -chloride, -iodide, etc ending in -ide). If your supplement doesn’t list the form of the mineral and only lists say Magnesium, Calcium, Potassium, etc and doesn’t list the form (magnesium citrate or magnesium oxide) either on the line or in the “Ingredients” section below the nutritional fact panel, then it is safe to assume it only contains the salt, or oxide form as if they put the more costly chelated form (citrate) in then they’d make sure to list it on the label.