The Sip that Killed Socrates: Poison Hemlock
Hemlock contains Coniine, Methyl-coniine, and Succus conii. The toxins block signals in the nervous system which lead to the cessation of breathing and consequently suffocation. The poison will also lead to salivation, twitching, pupil dilation, and an increased heart rate which eventually decreases to, you guessed it, 0 beats per minute. No one in Ancient Greece had a chance against such a poison, very few have a chance today.