I use these:
http://www.boots.com/webapp/wcs/stor...oductId=335251
Cramp in the Uterus muscle has put me in hospital 3 times, but since I started on these 2 years ago, not an iota of cramp. I take one the night before a longish race, one on the morning of it, one during & one after.
Previously discussed this somewhere on here. I use them in road marathons & on long fell races & if it is really sunny.