I've been trying magnesium for the last couple of months to combat various problems which had been developing for about a year: light periods, dizziness, nausea, hot flushes and insomnia, all especially at PMS time. I also came off my BCP in order to have an FSH test, so I couldn't say exactly which did the trick, but all in all I do feel a lot better. (The FSH test was normal, by the way). I have lost a lot of water weight in the process too, which I do definitely attribute to the BCP!
I've been taking a supplement which gives me 250 mg per day - since the RDA is about 300 mg, I figure I should pick the rest up in my diet. I may try going to alternating between one and two tablets per day just to pick up the remaining symptoms. I also take a general multivitamin/mineral (which doesn't have magnesium in) and saw palmetto, which I think I'm also starting to see results with.