A few weeks ago while lying in bed I started to cramp up and I took a small piece (about 1/4)of a 250 mg Magnesium tablet and let it dissolve on my tongue. I might be wrong, cramps do tend to stop all on their own if I stretch and massage a bit, but it seemed to help, and very quickly at that.

I assume that's low dose so it's probably worth trying to see if it works consistently. It'd be nice to find something that does.