Tetrodotoxin is a potent neurotoxin, which shuts down electrical signalling in nerves by binding to the pores of sodium channel proteins in nerve cell membranes. Its name derives from Tetraodontidae, the scientific family name of the puffer fishes, some species of which carry the toxin. Although tetrodotoxin was discovered in these fish, it is actually a product of Pseudoalteromonas haloplanktis tetraodonis bacteria, which live inside them, as well as inside some other sea creatures.

The effects of tetrodotoxin poisoning include shortness of breath, numbness, tingling, lightheadedness, paralysis and irregular heartbeat. Symptoms typically onset quickly, minor ones instantaneously. Death may occur in minutes and is the usual outcome. Although the toxin unbinds from channels as its concentration around nerves diminishes, the molecules are exceptionally potent and unbind only very slowly. Treatment usually consists of respiratory assistance. Nothing equivalent to an antivenom has been developed--presumably because the toxin acts quickly and binds with an affinity that isn't easily overcome.

Common causes of tetrodotoxin poisoning include the eating of the puffer fish known as fugu, which is a popular but rare delicacy in Japan and often contains significant amounts of toxin in its liver and other viscera. Cone snail stings represent another common source, which most often affect divers. Blue-ringed octopus, which inhabit tidepools, also sometimes contain the toxin, and people have died from their bites.