• Most poisonous substance known to man
• LD50 oral dose 1 mcg/kg
• Heat labile single polypeptide chain undergoes proteolytic clevage irreverisibly binds  and blocks cholinergic transmission causing a deadly neuroparalytic syndrome
• Rx: Botulin antitoxin (equine derived against Clostriduim botulinum A,B,E)

• Highly toxic, restricted availability is rapidly absorbed and metabolized to methylmercury crosses CNS
• LD50 of 50 mcg/kg means a dose as little as 0.1ml can result in severe poisoning
• Death of Darmouth inorganic chemist Karen Wetterhahn who spilled a few drops on back of her latex gloved hand, quickly permeated, and absorbed causing severe neurotoxocity and death 10 months later
• Rx: Chelation

VX ("venomous agent X") 

• Organophosphate nerve agent has been used as chemical weapon
• Colorless, odorless, low volatility, and high lipophilicity
• LD50 of 0.04mg/kg (10 mg). Death can occur within 15 minutes after absorption
• Blocks acetylcholinesterase enzyme causing excess accumulation of acetylcholine at the neurojunction and cholinergic poisoning
• Rx: Decontamination, Atropine, 2-PAM

• 100 fresh and salt water varieties (pufferlike fish/blue ringed octopus, frogs)
• Heat stable, water soluble found in fish skin, liver, ovaries,intestine, and muscle
• 25 mg (0.000881 oz) expected to be lethal to a 75 kg person
• Neurotoxicity by inhibition of Na-K pump and blockade neuromuscular transmission
• Rx: Supportive measures

LD50 expresses the dose at which 50% of exposed population will die as a result of exposure.

References