Does anyone know how long a cat may take to display an allergy reaction to a change of food? I changed from royal canin dry to porta 21 eight weeks ago and now one of my cats has licked all of his fur from his tummy. I took him to the vets to check for nasties but he was given the all clear. The vet suggested stress but nothing has changed in the house since January! I have noticed that the cat also starches a lot behind his ears and the fur is shorter there than the rest of his ruff. I was wondering if the porta had given him itchy skin?