caffeine.
I don't know why, but if even 1mg of caffeine enters my bloodstream my entire nervous system goes into defcon 1 for hours. I have the worst panic attacks where my heart rate can go to 140bpm for hours.
It all happened about about 3ish years ago after smoking weed and having a terrible panic attack, since then I've had severe heart palpitations that have only gotten worse, about a year ago is when I started to loose my tolerance for energy drinks and couldn't drink them anymore, and only about 6 months ago have I not even been able to drink caffeine. I had a bunch of pepsi one night then went to bed, didn't have any issues until I woke up at around 3am with an insanely high heart rate and couldn't breathe, it was so bad I had to go to the emergency room where they literally sedated me so I'd be able to calm down