There seem to be genetic variances in how well you deal with caffeine. Some people definitely have problems with it. 23andMe claims a slightly increased risk for heart attacks for me in relation to caffeine, though I don't think it is so bad. But I do have a problem with caffeine. (According to 23andMe, I am a slow caffeine metabolizer - I suppose that means that I can't gulp down 8 cups without side effects, like some people claim doing).