Swapping to Hills z/d

My cat is on Royal Canins Hypoallergenic biscuits. He's been having them for about a year now and we've recently been to the vet because he's breaking out in scabs again which is what happens when he doesn't have he's prescription biscuits.
Now the vet seems to think he's developed an allergy to the biscuits and suggests we change over to hills z/d... My question is... Will switching really help? I'm definitely willing to give it a go but in my experience swapping he's food on him just made him have an upset tummy, he's also very fussy and at first he wouldn't even eat the royal canins.

Switching any dry food if not done slowly can cause an upset stomach in any cat. The Hill's pet is a superior choice of poor choices anyway, as the Royal Canin hypoallergenic is basically offering your cat (an obligate carnivore) a vegan diet. At least the Hill's pet is made with hydrolyzed chicken.

To make the switch, add just a little bit of the new food to the old one. The transition to 100% new food should take one or two weeks to avoid stomach upset. Yes, add just a little new and take out a little old VERY slowly.

But cats do much better on wet food anyway. Can you afford to buy the canned version of the z/d? This would be much healthier for your kitty.

This doesn't address allergies, but it does discuss the needs of cats in general: http://www.catinfo.org/

Many people that have kitties with allergies opt to try limited ingredient diets with novel proteins rather than feed the prescription food. You might want to discuss this with your vet.

Just some thoughts...

1.) I had 2 cats on the hypo diet about a year or so ago. Started out on Purina HA (cats began to have diarrhea) switched to Royal Canin (they didn't like this) and then ended up on z/d. All went well for a few months and then the cats started having diarrhea. Did fecal tests, both in house and sent to lab...(I work for a rescue, thought I might have brought something home.). All tests came back negative. So, I called the dermatologist and told him the problem. He had us stop z/d immediately as he was seeing an increase in z/d causing gastrointestinal problems.


2.) I think it could be possible your cat has developed a grain allergy. This is basically all the hypo diet is to rule out protein allergies, though there IS chicken fat in the diet. I have a foster kitty with a severe chicken allergy, this includes chicken fat.

Do you know what proteins your cat is allergic to? If not, now might be the time to begin a food trial. Start by introducing a grain free food with a single protein source.

Thanks for the advise, I'll definitely do a slow change over to make sure Jekyll doesn't get sick.
I understand that they do better on wet food it's just my vet has given specific orders to keep him on a dry food diet only.
I'd love to give him wet food, money is not an issue. I'll definitely bring it up again at the next vet visit which is Friday.

We're not exactly sure what it is that's causing him problems each time we go to the vet were just told that he's 'allergic to food' which doesn't give us much to go on.

We'll be booking him into see a dermatologist in about 2 weeks time and hopefully they can help us work out what's going on with him.

Apparently Royal Canin has various hypoallergenic diets. I am not familiar with the non-meat one that @LDG mentioned. If your cat is on the RC hypoallergenic "selected protein" rabbit or venison, then it is possible that he has developed an allergy to one of those proteins.