Fish is one of the most common causes of food allergy, particularly in adults and in Scandinavian countries. Fish may find their way into processed foods in raw, powder or oil form. In the majority of instances, this substance is clearly labeled as “fish” or with another obvious descriptor. However, fish allergens may be go unlisted if added as part of an oil. Fish products are not usually hidden ingredients but can go unspecified in Caesar salad dressing or in Worcestershire sauce if it contains anchovies.

Some seafood flavors (e.g., shrimp) are added to food in the form of a powder manufactured from the seafood's shell. Shrimp antigen II is heat stable. A variety of antigens are shared by several crustaceans including shrimp, prawns, crabs, lobsters and crayfish (crawfish). These antigens are not eradicated by cooking.

At present, some manufacturers are researching the possibility of adding fish meal (flour) to bread as a source of omega-3-fatty acids (personal communication, M. M. Melnyczuk).  Skin prick tests and RASTs indicate extensive cross-reactivity among fish species, but recent research suggests that patients may be able to consume some species of fish despite positive test responses to one or two. However, it is generally recommended that patients allergic to fish avoid all fish species.   An epicutaneous test can confirm a fish allergy.