As a few posters have mentioned (and surprisingly few at that!!), try MAGNESIUM - in the form of Magnesium Chloride oil, used transdermally, not ingested.
I cannot speak highly enough of this substance!!! (it has helped me amazingly - not only with cramps - it also reduced my annoying heart palpitations to almost nil (and assisted with other problems too) ....

Magnesium is soooo essential for the health and well-being of almost every part of a human being and yet probably the majority of Westerners have a Magnesium deficiency!!

Please take the time to check out a website called IMVA (International Medical Veritas Association) run by Dr Mark Sircus, for a veritable goldmine of great information and advice.

There is a link on it too, for the trans-dermal Magnesium Chloride (MgCl2) in the product called "Ancient Minerals" (it's called "oil" - but it's not "oily"!!)

Believe me, it works. Try It!

Cheers, R-J
Down Under