I completely agree with Judge12345. Firstly, always change a dogs diet over 7 days - much kinder to the dog and the dogs tummy! Are you feeding wet or dry? Either way, look around for a good one with a high percentage of fresh meat as opposed to the usual "animal derivatives" that you can pick up in most supermarkets - if your dog has an intollerance to a particular source of protein you'll never work out what the allergy is to whilst feeding derivatives. Good pet shop staff are more likely to know what food is best than staff in Tescos or Sainsburys because they have a genuine interest in pet health care. Some larger pet shops are pushing their own brands now but, personally I'd stick to ones that have been around for longer with a good reputation . Once you find the one that settles him / her, I'd keep them on it and avoid changes. I think each food brand can affect each dog differently.