Can Fish Oil Cause Canker Sores?

Can Fish Oil Cause Canker Sores? Less frequently, fish oil may cause bleeding in the stomach or intestines and may cause or worsen ulcers. Can fish cause canker sores? Sensitivities or allergies to certain foods can result in mouth ulcers. Common foods that are problematic include wheat, gluten, eggs, nuts, peanuts, fish, crustaceans, cow’s milk,