Man are those damn things strong I know they say 2 servings but so does Rockstar or Monster drinks. So I popped a whole one, and on an empty stomach no less. I was wound up unbelievably within a few minutes, and it lasted a good 3 hours, maybe more. Mind you thats 3 hours of jittery spasticness. made for a nice workout in some ways. I need to try 1/2 a bottle precardio next time I decide to do cardio