The most toxic substance known is the Botulinum toxin, a neurotoxic protein produced by the bacterium Clostridium botulinum. Ingestion of this toxin produces botulism.
A typical lethal dose = 0.00000002 grams, meaning that one hundred grams could kill every human on earth. Another way to consider its toxicity to realize it is 40 million times more powerful than cyanide.
By blocking neurons, the toxin renders the muscles unable to contract for a period of 4 to 6 months. Death is generally caused by suffocation due to the paralysis of respiratory muscles.
Oddly enough, the muscle paralyzing properties of the Botulinum toxin has been utilized in medicine to treat: muscle spasms, migraines, excessive salivation, and uncontrollable blinking.
During the use of the Botulinum toxic to treat eye muscle disorders, it was noticed that the frown lines between the eyebrows were softened. This effect was quickly capitalized on to produce one of the most common cosmetic procedures in the United State, Botox.
Strange Chemistry 