Is your cat scratching, particularly the head or neck? And does your cat often get diarrhea, vomit a lot or experience flatulence as well? If so, then there’s a significant chance that your cat has a food allergy (or rather a food intolerance).
If you suspect an allergy, please do get in touch with your vet first. Finding out whether or not your cat has a food allergy is tricky and time-consuming, and a vet will be able to give guidance in this regard.
Your cat will need to follow a strict diet for 12 weeks, you see. What this means is that he will not be allowed to take any medication or have any snacks. This diet will not contain any substances that cause allergic reactions and will only contain ingredients that the cat has never eaten before. The cat will have to follow this diet until the symptoms disappear. Once the symptoms are gone, then you will be able to gradually start your cat on food containing only chicken (for example). If the symptoms return, then it will be evident that your cat will have to be kept off chicken as a minimum.