Some people get jumpy after drinking a single cup of coffee, while others can gulp down a Venti Americano [what is this?] without feeling a thing. Part of that variability is due to the development of tolerance by regular coffee drinkers; but there are genetic differences in how people metabolize caffeine as well.
The following results are based on Preliminary Research for 1 reported marker.
I find this interesting, because if I drink a cup of coffee, which I don't regularly at all, let alone a Venti Americano, I get so 'jittery' I can't stand it. Also, I once took a dose of a drug called theophylline for a study and I could not come close to sleeping! See if you can find out how theophylline is related to caffeine!
At any rate, my genetics say I am a 'fast caffeine metabolizer'. Thank goodness!