This occurs from eating poorly preserved tuna, mackerel and related ‘scombroid’ fish. A toxic histamine-like substance is produced by the action of bacteria or histidine, a normal component of fish flesh. This toxin produces nausea, vomiting, headache, upper abdominal pain, difficulty in swallowing, thirst, itching and sometimes urticaria. The condition settles as a rule in 12 hours, antihistamine drugs may help.