Three people needed hospital treatment but all have been discharged, according to Public Health Wales.
The agency said it is leading the investigation into the outbreak from laverbread in the Swansea, Neath Port Talbot and Carmarthenshire areas.
However, it added that product sampling to date has not proved a firm link between the products and the outbreak.
Ten people reported eating laverbread in the days before they became unwell.
Laverbread is a national delicacy made from seaweed, it is washed and then cooked to a soft greenish black paste.
Penclawdd Shellfish Processing, a local company, has voluntarily withdrawn its laverbread from sale.
The product is available at high street retailers and Penclawdd is the only supplier of laverbread to Tesco.
Public Health Wales said it is recommending that anyone who has laverbread at home that was bought on or before 25 March and may have been produced by Penclawdd throws it away and does not eat it.
The agency is investigating with Environmental Health Officers from Swansea, Neath Port Talbot and Carmarthenshire councils.
Laverbread suspected source
Dr Jörg Hoffmann, consultant in communicable disease control for Public Health Wales, said: "We know that the majority of cases have eaten laverbread produced by Penclawdd,” he said.
“The company has cooperated fully with our investigations and has chosen to voluntarily withdraw the product from sale as a precaution.”
Hoffmann said laverbread is generally a safe product to eat, and it remains unclear whether it is indeed the source of this outbreak.
"However, given that we cannot rule out laverbread at present, it is very important that, to avoid the risk of illness, anyone with this product at home does not eat it,” he said.