Try getting it in Transdermally! I found oral methods not very good at all.

I have used Epsom salts in very strong concentration in a hot bath, but it uses a lot, so now I dry off straight after a shower, and spray myself with a warm, very strong solution of Epsom Salts, and rub it in thoroughly and consistently for ten or fifteen minutes until it is dry.

Gets a bit dusty, but stops my cramping!

First got the idea from Dr Myhill, and adapted to suit, I drew the line at Magnesium enemas!!...
http://www.drmyhill.co.uk/wiki/Mine...amins_in_creams