What’s actually in Hill’s Prescription Diet z/d?
We analyzed Hill’s Prescription Diet z/d Skin/Food Sensitivities Hydrolyzed Chicken Flavor dry dog food. The formula leads with corn starch — a highly refined, hypoallergenic carbohydrate that provides energy without triggering immune responses. Hydrolyzed chicken liver at position two and hydrolyzed chicken at position three are the protein sources: chicken proteins enzymatically broken down into peptides too small to be recognized by allergy-triggering antibodies.
Ground pecan shells at position four is unusual — it’s an insoluble fiber source chosen because nut shells are hypoallergenic (the allergenic proteins are in the nut meat, not the shell). Powdered cellulose at five adds more fiber. The formula includes both coconut oil and fish oil for fatty acids, flaxseed for additional omega-3s, and pressed cranberries as a whole-food antioxidant source. Shop on Amazon →
The good stuff
The hydrolyzed protein approach is genuinely innovative. By breaking chicken proteins down to molecular weights below 10,000 daltons, the formula eliminates the immune system’s ability to mount an allergic response. This is backed by clinical research, not marketing. Two hydrolyzed protein sources (chicken liver and chicken) provide a reasonable protein profile despite the restriction.
Flaxseed, coconut oil, and fish oil together create a robust fatty acid profile — important because skin health is a primary concern for allergy dogs. Pressed cranberries are a whole-food antioxidant. Dried beet pulp provides prebiotic fiber for gut health. The formula avoids common allergens like soy, wheat, and standard chicken proteins. Mixed tocopherols for natural preservation.
The not-so-good stuff
Corn starch is pure refined starch with virtually no nutritional value beyond calories — it’s chosen for hypoallergenicity, not nutrition. Ground pecan shells are literally ground-up nut shells — fiber with zero nutritional value. Powdered cellulose is wood pulp. Positions one, four, and five in this formula are all nutritionally empty fillers. That’s a hard pill to swallow at prescription diet prices.
Soybean oil is an unusual inclusion in a hypoallergenic diet since soy is a common allergen in dogs, though the oil form contains minimal protein. No probiotics despite being formulated for dogs with GI-related food sensitivities. No chelated minerals. Dicalcium phosphate is a cheap mineral source. The formula relies heavily on synthetic amino acid supplementation (DL-Methionine, L-Tryptophan).
How it compares
At B/76, Hill’s Prescription Diet z/d ties with the k/d Kidney Care (B/76) and j/d Joint Care (B/76) formulas, and scores well above the standard Hill’s Science Diet (C/61).
See the full Hill’s Prescription Diet z/d vs Hill’s Science Diet comparison for a detailed side-by-side breakdown.
Comparing z/d to regular dog foods is meaningless — if your dog has confirmed food allergies, standard foods with intact proteins are the problem. Among hypoallergenic diets, z/d is one of the most aggressive approaches available. The only peer-level comparison would be Royal Canin Hydrolyzed Protein, which uses soy-based hydrolyzed proteins instead of chicken-based.
The bottom line
Hill’s Prescription Diet z/d earns a B grade (76/100) from KibbleIQ. The ingredient quality as a regular food would score lower, but the hydrolyzed protein technology and hypoallergenic formulation serve a clear medical purpose. If your vet prescribed z/d as an elimination diet or long-term allergy management food, trust that recommendation — the science behind hydrolyzed proteins is well-established. If your dog’s allergies are mild, ask your vet about limited ingredient diets like Natural Balance (C/66) or Zignature (C/73) as potentially less restrictive alternatives. Shop on Amazon →