In Japan, they eat poisonous puffer fish, called fugu. If not prepared properly, you can die:
The fish has a neurotoxin that is quite potent.
It seems to be concentrated in the liver and ovaries, but virtually any part of the fish can contain it.
The poison might be produced by the digestive process. If their diets are reduced in captivity, the fugu fish can have virtually no toxicity.
Of course, this has been attacked, because the researchers who discovered this were most likely using the wrong species of puffer fish, and it is generally believed that the toxin has nothing to do with the fish’s digestive system.
Interestingly, there was an episode of The Simpsons that involved fugu poisoning.
I always wonder how they managed to invent a way to ‘safely’ eat those things in the first place. Try different cuts and feed them to animals, maybe? Or to unlucky serfs?
I assume that they did use the serfs for some experimentation.
Homer Simpson ate fugu, and he thought he was going to die.