Which of these toxins is known to be most lethal to humans?

One billionth of a gram (1 nanogram) of botulinum toxin per kilogram of body weight can kill a human being. This means that a dose of less than 9 nanograms can kill an 84 kg (185 pound) human.

Botulinum toxin is produced by Clostridium botulinum, the anaerobic bacteria than can cause the food poisoning called botulism. Botulinum toxin is at least 10,000 times as poisonous as ricin. Maitotoxin and batrachotoxin are toxic at levels less than botulinum toxin and more than ricin.

The toxin contains compounds that prevent the body from releasing acetylcholine. This is a “neurotransmitter” or signaling molecule, which tells the body’s muscles to contract. Without acetylcholine, muscles simply stop moving; they become paralyzed.

In food poisoning, the victim has trouble swallowing and focusing the eyes. If the toxin spreads, it will eventually cause “respiratory paralysis”, that is, the victim stops breathing. With immediate treatment, recovery is long and difficult and long-term problems—weakness, fatigue, shortness of breath—remain.

This paralyzing effect causes wrinkled skin to relax, which is why Botox, which is modified botulinum toxin, is injected by dermatologists to give patients smooth skin. The substance has other medical uses, as well, including treatment of strabismus, or “crossed eyes”.