A 65-year-old woman died after eating puffer fish and 10 more fell sick yesterday in Jaintapur upazila of the district.
Five people, including two children, died on Tuesday due to the same reason.
District Civil Surgeon Md Habibur Rahman said an alert has been circulated in the upazila and the surrounding areas prohibiting sale and purchase of puffer fish and asking people not to eat the fish.
A medical board headed by Professor Dr Enayet Hossain, head of the Department of Medicine in Sylhet MAG Osmani Medical College Hospital, has been formed to investigate the matter.
Dr Enayet said the poison in the puffer fish cannot be treated with any medicine, and they are trying to save affected people in various ways.
Director of the hospital Brigadier General Abdus Sabur Mia said puffer fish are more poisonous than cyanide.