Tetrodoxin, in simple word it is a globe fish poison
Tetrodoxin, or Tetrodotoxin, is a powerful neurotoxin with no antidote that we know of. It naturally occures in many species of the order of fish called Tetraodontiformes, hence the toxin's name. These include but are not limited to: Pufferfish, Porcupinefish and Mola or Ocean Sunfish. The toxin is also found in a few other sea creatures that are not in this order, but when people gather the poison for medical use, or any other, they gather it from certain kinds of bacteria, Pseudoalteromonas tetraodonis, for example.
How could people use such a deadly toxin in medicine, you ask? Well, it does have its purposes in the right situation, when carefully administered. It has potential use in treating some heart arrhythmias, and, of course, it's a powerful pain killer which can be used in such cases as terminal cancer and other conditions that require the extreme in pain managment.