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The short answer: Hill’s Prescription Diet z/d Skin/Food Sensitivities earns a B grade (good) on ingredient quality. This is the nuclear option for food allergies — a hydrolyzed protein diet where chicken proteins are broken down into fragments so small that the immune system can’t recognize them as allergens. Corn starch leads the formula as a hypoallergenic energy source. It’s not designed to be nutritionally optimal; it’s designed to stop allergic reactions, and the elimination-diet medical value is well-documented. Our B grade reflects the ingredient-quality lens, not the therapeutic value, which is real and independent.

→ See the live ingredient breakdown for Hill's Prescription Diet z/d

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/75, Hill’s Prescription Diet z/d sits in the upper half of the Hill’s Rx PD line on ingredient quality, a tier above the j/d Joint Care (C/55) and w/d Multi-Benefit (C/55) formulas, just 1 below the k/d Kidney Care (B/76), and even with the standard Hill’s Science Diet (B/75). Note: KibbleIQ scores ingredient quality, not clinical efficacy — the hydrolyzed-protein elimination-diet design serves a clear medical purpose validated in Olivry 2010 and the AAVD 2020 consensus, and that medical value is independent of how the food rates as a quality-ingredient meal.

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 (75/100) 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 (B/78) or Zignature (A/90) as potentially less restrictive alternatives. Shop on Amazon →