Hemlock poisoning

Hemlock (Latin genus Conium) is a plant that contains several deadly toxins, the most deadly of which is Coniine. Coniine is a neurotoxin that suppresses the function of muscles. As a result, the victim starts to suffer gradually ascending paralysis which, when it reaches the lungs, results in the death of the patient. 
A very small amount of the hemlock plant is more than sufficient to kill an adult. About 6-8 leaves or a small amount of the root or seeds of the plant can result in death within 72 hours. Although the symptoms of the toxin come on quickly, death can come with agonizing slowness, worsened by the fact that the toxin has no effect on consciousness.
The body can metabolize the toxin given time with no lasting ill effects. As such, treatment focusses on respiratory support until the patient recovers. 
Historically, the philosopher Socrates committed suicide by ingesting hemlock in order to carry out the death sentence imposed by the Athenian court after his conviction for corrupting the youth of Athens. 
Hemlock plant at Wikipedia
Coniine at Wikipedia
Hemlock poisoning