Why should I purchase a hypoallergenic diet via the veterinarian and not at the pet store?

In some cases claims of ‘hypoallergenic’ foods on packages are misleading. The producers of the food claim the food to be hypoallergenic, but when you read the ingredients listed on the bag, this is not always the case. Sometimes multiple protein sources are mentioned, in contrary to just one. The definition of a hypoallergenic food is a diet that minimises the risk of a hypersensitivity reaction. The greater the number of protein sources, the greater the chance of an allergic reaction.