strychnine
a natural poison that causes the body to go into convulsions and spasms.It is one of the most bitter substances in the world.Eventually causes the body to collapse from complete exhaustion or go into asphixia followed by instant rigor mortis. A common urban ledgend is drug dealers mixing it with drugs such as LSD which would not be possible since it would most likely be fatal.
Dr. Thomas Neill Cream killed many people, mainly prostitutes via strychnine poisoning.