Littlewish, mrsd is quite correct, the amount necessary to cause weakness in a myasthenic is a very high dose. The amt of magnesium found, for example, in a vitamin pill, is just not a high enough dose to cause any problems. Actually, a certain amount of magnesium is not only beneficial, it's absolutely necessary for good health. I've taken a multivitamin for years that contains magnesium, and have never had any problems with it at all. Remember, even something as necessary and beneficial as is Mestinon to an MGer will cause weakness if one takes too much.
Hugs,