Hill’s Prescription Diet z/d is a veterinary diet for dogs with suspected food allergies or food intolerances. Its protein is hydrolyzed, which means it is broken into pieces small enough that a dog’s immune system is unlikely to react to them. It is a well-regarded choice for a food allergy trial, but it is only sold with a veterinarian’s authorization, and it costs more than regular dog food.
This review covers what is in z/d, the nutrition numbers, the pros and cons, and how to use it properly.
What Is Hill’s z/d?
z/d is part of Hill’s Prescription Diet line. Hill’s sells it for dogs with skin and digestive signs caused by adverse reactions to food, such as:
- Itchy skin, paws, or ears
- Repeat ear or skin infections
- Vomiting, diarrhea, or soft stools linked to food
According to VCA Animal Hospitals, the most common food triggers in dogs are proteins, including dairy, beef, chicken, chicken eggs, soy, and wheat gluten. A hydrolyzed diet like z/d gets around the problem by breaking the protein down until the immune system no longer recognizes it.
z/d comes in three forms: the original dry food, a Small Bites dry food, and a canned version.
Ingredients
Hill’s lists these as the first ingredients in the z/d dry food:
- Corn starch: a purified carbohydrate with almost no protein, which is why it is used in allergy diets.
- Hydrolyzed chicken liver and hydrolyzed chicken: the only animal protein sources, broken down to reduce the chance of a reaction.
- Ground pecan shells, powdered cellulose, dried beet pulp, and dried citrus pulp: fiber sources. Hill’s calls its prebiotic fiber blend ActivBiome+.
- Flaxseed, soybean oil, coconut oil, and fish oil: fats, including omega-3 and omega-6 fatty acids for skin and coat.
The rest of the list is vitamins, minerals, amino acids, and mixed tocopherols as a preservative.
Two things surprise some owners. First, z/d is not grain-free, and corn starch is the first ingredient. That is by design, not a flaw. Second, the protein comes from chicken, a common allergen. Because it is hydrolyzed, many chicken-allergic dogs still tolerate it, but not all do. Your vet will tell you if a different diet makes more sense.
Nutrition Numbers
These are Hill’s published figures for the z/d dry food. Formulas change, so check the current bag.
| Nutrient (dry matter basis) | Amount |
|---|---|
| Protein | 20.1% |
| Fat | 14.5% |
| Crude fiber | 6.4% |
| Carbohydrate | 53.4% |
| Calories | 348 kcal per cup |
Protein is moderate and carbohydrate is high compared with most regular kibble. This food is built for a medical job, not to win an ingredient-list contest. It is complete and balanced for adult dogs.
Hill’s sells the dry food in 8 lb and 25 lb bags. Its feeding guide runs from about 5/8 cup a day for a 5 lb dog to about 6 1/2 cups for a 120 lb dog. Use the amount your vet recommends.
Pros and Cons
Pros
- Hydrolyzed protein made for dogs with food allergies
- Single animal protein source and a purified carbohydrate
- Added omega fatty acids for skin and coat
- Available as dry, small-bite dry, and canned
- Complete and balanced, so it can be fed long term if your vet agrees
Cons
- Requires veterinary authorization
- Costs more than over-the-counter food
- Some dogs find hydrolyzed diets less tasty
- Does not work for every dog, including some with chicken allergy
- Not made for puppies unless your vet says otherwise
How Vets Use z/d: The Elimination Trial
There is no reliable blood or saliva test for food allergies in dogs. The standard way to diagnose one is an elimination diet trial. VCA describes it this way:
- Feed only the trial diet for 8 to 12 weeks.
- Give nothing else: no treats, table scraps, chews, or flavored supplements. Some flavored medications and parasite preventives count too, so ask your vet about substitutes.
- If the signs clear up, your vet may reintroduce the old food. If the signs come back, that confirms a food allergy.
The trial only works if it is strict. One flavored chew can undo weeks of progress. Tell everyone in the house, and keep other pets’ food out of reach.
When you start z/d, switch gradually over about a week unless your vet tells you otherwise.
Alternatives to Ask Your Vet About
- Other hydrolyzed diets: Royal Canin Hydrolyzed Protein and Purina Pro Plan Veterinary Diets HA.
- Novel protein diets: veterinary diets built around a protein your dog has never eaten.
- Digestive diets: if the problem is a sensitive stomach and not an allergy, your vet may suggest something like Hill’s i/d instead.
Over-the-counter limited-ingredient foods are not the same thing. They can pick up traces of other proteins during manufacturing, which makes them unreliable for a diagnosis. They can still be a reasonable everyday choice once you know what your dog reacts to. Our guide to dog food without corn, wheat, or soy covers some options.
Bottom Line
Hill’s z/d does one job well: it gives dogs with suspected food allergies a diet their immune system is unlikely to react to. The ingredient list looks plain because it has to be. If your dog has itchy skin, ear trouble, or stomach upset that will not go away, ask your vet whether a strict 8 to 12 week trial on z/d or a similar diet is the right next step.
This article is for general information only and is not a substitute for veterinary advice. Hill’s z/d is a therapeutic diet. Talk to your veterinarian before starting or stopping it.
