Eating pufferfish sushi, or fugu, can be very dangerous. It should only be prepared by a chef who is licensed to do so. When prepared correctly, there is little danger in eating it.
The liver and ovaries of the pufferfish contain tetrodotoxin, which is lethal; just one milligram of this poison can kill a person. Consuming this poison paralyzes the muscles while the person is still conscious, and the person eventually dies of asphyxiation. While preparing the fish, the chef must remove the liver and ovaries. There are smaller amounts of tetrodotoxin in the eyes, skin and testes.
Licensing involves the chef being an apprentice for two to three years, in addition to passing a written test, and preparing and eating the fish. Most chefs do not pass the examination.