Answer: When you get right down to it, everything is poisonous. Water will kill you if you drink too much of it. Oxygen is a deadly poison, yet we need it to live. However, there are some chemicals that we are better off not encountering. Here's a list of the most poisonous chemicals known. Keep in mind, toxicity varies from one species to another (i.e., what may be poisonous for a mouse may be more/less poisonous to a human) and within a species (i.e., age, sex, genetics all affect susceptibility to a toxin). I've listed the name of the toxin, its source, approximate average lethal dose per kilogram of body weight (LD50), and the species.

1. tetanus
   1 nanogram/kg
   mouse, human
2. botulinal neurotoxin (bacteria)
   1 nanogram/kg
   mouse, human
3. shigella (bacteria)
   1 nanogram/kg
   monkey, human
4. palytoxin (coral)
   60 nanogram/kg
   dog (iv)
5. diphtheria (bacteria)
   100 nanogram/kg
   human
6. ricin (from castor beans)
   1 microgram/kg
   human
7. aflatoxins (mold which grows on nuts, legumes, seeds)
   1-784 micrograms, depending on type of aflatoxin
   duckling (oral)
8. shigella (bacteria)
   1 microgram/kg
   mouse
9. saxitoxin (shellfish)
   3-5 micrograms
   mouse (iv), about 50x higher dose orally
10. tetrodotoxin (fugu pufferfish)
    10 micrograms
    mouse (ip)
11. diphtheria (bacteria)
    1.6 milligram/kg
    mouse