I used to drink a lot of caffeine, and it had basically zero effect on me. Even like string energy drinks.
Now whenever I even drink a like half a can of coke early in the day, I get all anxious and can't sleep till like two am. It's crazy. Even sounds inside my head change from just normal sounds I hear to be really loud and sound angry, if that makes sense.
I get anxious a little bit otherwise, but for some reason caffeine really does me in.
My father-in-law has the same forreal. He's the kindest gentlest man alive until caffeine.. and then he growls and it goes downhill fast.