Quinine
"I've learned the warning sign of an impending leg cramp and I always keep a small supply of 200 mg quinine sulfate accessible.  I use very small containers and have one in the car, one in my backpack when hiking and one in my pocket when out and about. My leg cramps are so painful that it seems that the muscle is tearing itself apart and luckily the pills are within easy reach so the tension ends within a couple of minutes."