http://www.peoplespharmacy.com/2010/10/11/is-quinine-in-tonic-water-safe/

I used to race motorcycles in the Mohave desert.  A hundred miles of whoops is a good way to get bad cramps.  In addition to what kirk said, quinine really helped.  I used a big pill that the USDA has now banned for any use besides malaria.  I never noticed any negative side effects.