Personal experience? Panic attack. Nothing quite matches the apparent absolute truth that you can no longer breathe faced with the utter fact that you still are. They are completely horrifying and terrifying. I never want one again (but I'm fairly sure it will, someday, happen again).
Hypothetically? Needles all over my body. I freaking hate needles.
Hypothetically? Needles all over my body. I freaking hate needles.