What is the poison in a blowfish?

The poison in a blowfish is called tetrodotoxin (TTX). It is a neurotoxin that blocks the voltage-gated sodium channels in nerve cells, inhibiting the transmission of nerve impulses and leading to paralysis. TTX is one of the most potent natural toxins known, with a lethal dose of less than 2 milligrams for humans. It is found in the liver, viscera, and skin of blowfish, and can cause severe illness or even death if ingested.