In human, the lethal dose of tetrodotoxin is around 1 to 2 mg and the minimum dose necessary to cause symptoms has been estimated to be 0.2 mg. The onset of symptoms of tetrodotoxin intoxication usually occurs from 10 to 45 minutes after ingestion, but may be delayed by three hours or more. Paraesthesia appears in the face and extremities, which may be followed by dizziness or numbness. Nausea, vomiting, diarrhoea and epigastric pain may also be present. Later, respiratory symptoms such as rapid breathing may follow. Low blood pressure, convulsions and irregular heart rate may occur. In most instances, the patients retain consciousness until shortly before death, which usually takes place within the first six hours. All humans are susceptible to tetrodotoxin poisoning. There are currently no known antidotes or antitoxins to tetrodotoxin. The treatment of symptoms is therefore supportive.
There are differences in toxicity and toxin distribution in tissues among different puffer fish species. In addition, seasonal, individual and local variations of toxicity and toxin composition in puffer fish are occasionally observed. Tetrodotoxin poisoning may be caused by ingestion of only a small amount of puffer fish.