Hill's vs Royal Canin for Dog Allergies: Complete Comparison

By Gary, founder of Pet Allergy Scanner. 7+ years managing pet food allergies with my Cockapoo.

Your vet just said your dog needs a hydrolyzed protein diet. You leave the clinic with two options written on a sticky note: Hill's z/d or Royal Canin HP. Both cost around $100 for a 25-pound bag. Both require a prescription. Both promise to solve your dog's allergies. For most dogs they are equally effective — the table below is where the meaningful differences live.

Side-by-side

| Factor | Hill's z/d | Royal Canin HP | |--------|-----------|----------------| | Protein source | Hydrolyzed chicken | Hydrolyzed soy | | Success rate (severe allergies) | 85–90% | 85–90% | | Crude protein | 18% | 21% | | Cost, 25 lb bag | ~$95 | ~$105 | | Matching hydrolyzed treats | No | Yes | | Starting point if… | No specific reason to avoid chicken | Chicken-suspected or training-heavy trial |

Hill's z/d has the longest research track record, is slightly cheaper, and is what most clinics reach for by default. Royal Canin HP uses hydrolyzed soy (so it's the better choice if chicken is already on the suspect list), runs slightly higher protein, and has matching hydrolyzed treats — which makes staying strict during an elimination trial far easier. Clinical success rates are a wash.

Which one to buy

• Start with Hill's z/d if this is your first elimination trial, your vet recommended it, and there's no specific reason to avoid chicken.
• Start with Royal Canin HP if chicken is already suspected, your dog needs the higher 21% protein, palatability has been an issue before, or training treats matter during the trial.
• Move up to Royal Canin Ultamino only if a standard hydrolyzed diet has failed after 12+ weeks of strict compliance. Expensive, but 95%+ success on the cases where everything else stopped.
• Switch to a novel-protein diet (d/d, PD/PR) once hydrolyzed has worked, the trigger is identified, and you're ready to reintroduce whole proteins. For the broader landscape, see the best dog food for allergies guide.

The Background (Brief)

Both companies are veterinarian-founded and research-heavy. Hill's (1939, now owned by Colgate-Palmolive) has the longest track record and the most published studies—z/d is essentially the default prescription in most vet clinics. Royal Canin (1968, now owned by Mars) offers more options, including the ultra-hypoallergenic Ultamino for dogs who fail everything else.

The key difference in their allergy lines:

Hill's prescription products: z/d (hydrolyzed chicken), d/d (novel proteins like venison/duck), Derm Complete (skin-focused)

Royal Canin prescription products: Hydrolyzed Protein HP (hydrolyzed soy), Ultamino (amino acid-based), Selected Protein PD/PR (novel proteins)

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The Prescription Formulas: What You're Actually Choosing Between

Hill's Prescription Diet z/d

This is the one most vets reach for first. It's been around forever, has the most research behind it, and it works for most dogs.

The protein source is hydrolyzed chicken liver—broken down so thoroughly that the immune system can't recognize it as chicken anymore. We're talking molecular weights below 10 kilodaltons (kDa), which is science-speak for "too small to cause a reaction."

You'll find it in dry kibble, a canned stew formula, and a small bites version for smaller mouths. The z/d ULTRA was even more broken down, though it's been discontinued in some markets.

What's inside: Corn starch is the first ingredient, followed by the hydrolyzed chicken liver. You'll also find powdered cellulose, soybean oil, coconut oil, and fish oil for omega-3s. Protein sits at 18% minimum, fat at 13.5%—fairly moderate on both counts. The omega-6 to omega-3 ratio is solid for skin health.

What the research says: Around 88% of dogs see symptom resolution during elimination trials on z/d. It's also the diet most dermatology studies use as a control, which tells you something about its reliability.

Check Hill's z/d Price on Amazon →

Royal Canin Hydrolyzed Protein HP

Royal Canin took a different approach—their hydrolyzed protein comes from soy, not chicken. If your dog is one of those ultra-sensitive cases that reacts to chicken even when it's been through extensive hydrolysis, this is the logical alternative.

The molecular weight target is the same (<10 kDa), but there's zero chicken or poultry-derived ingredients anywhere in the formula.

It comes in dry kibble for adults and small dogs, a loaf-in-sauce canned option, and—here's a real advantage—actual HP treats. If you've ever tried to train a dog on an elimination diet, you know how valuable safe treats are.

What's inside: Again, corn starch leads, followed by hydrolyzed soy protein. Protein is higher at 21% minimum, fat at 16%. The omega profile is slightly richer than z/d.

What the research says: Clinical trials show around 90% improvement rates. It's particularly effective for dogs who failed chicken-based formulas, and vets note that many dogs find it more palatable than z/d.

Check Royal Canin HP Price on Amazon →

Royal Canin Ultamino

This is the nuclear option—for dogs who fail standard hydrolyzed diets.

Ultamino doesn't use hydrolyzed proteins at all. Instead, it's made from free amino acids derived from feather keratin. We're talking molecular sizes under 1 kDa—essentially, it cannot trigger an allergic response because there's nothing intact enough to be recognized.

The downside: it only comes in dry food, the bag sizes are limited, and it's the most expensive option by a significant margin. But if your dog has failed everything else, Ultamino has a 95%+ success rate even in the most extreme cases.

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Why the Protein Source Matters

Here's where the practical difference lies:

Hill's z/d uses chicken liver. For the vast majority of dogs, hydrolyzed chicken doesn't trigger reactions—the protein fragments are simply too small. But occasionally, you'll encounter a dog so sensitive that even thoroughly hydrolyzed chicken causes problems. It's rare, but it happens.

Royal Canin HP uses soy. There's no chicken anywhere in the formula. If your dog is one of those extreme chicken-sensitive cases, HP eliminates that concern entirely. And here's something worth knowing: true soy allergies in dogs are actually uncommon. Most dogs blamed for "soy allergies" are actually reacting to other ingredients in soy-containing foods.

Royal Canin Ultamino uses amino acids. Not protein fragments—actual individual amino acids. The molecular weight is under 1 kDa compared to under 10 kDa for standard hydrolyzed formulas. It physically cannot trigger an allergic response. This is for dogs who have failed literally everything else.

Are Both Brands Equally Good at Hydrolysis?

Yes. Independent testing confirms that both Hill's and Royal Canin achieve molecular weights below the allergenic threshold consistently. Their manufacturing quality control is comparable. For standard food allergy cases, clinical outcomes between z/d and HP are essentially equivalent.

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What About Over-the-Counter Options?

A quick reality check: if your dog has true food allergies, the OTC "sensitive" formulas from either brand aren't going to cut it.

Hill's Science Diet Sensitive Skin & Stomach is designed for dogs with delicate digestion, not allergies. The first ingredient is chicken—if your dog reacts to chicken (one of the most common allergens), this won't help. It's a good food for easily upset stomachs, but don't confuse "sensitive" with "hypoallergenic."

Royal Canin's OTC options are even more limited for allergy sufferers. Their breed-specific and size-specific formulas aren't designed for allergies at all. If you need serious allergy management with Royal Canin, you're looking at the prescription Hydrolyzed Protein line.

The honest truth: for real food allergies, you need the prescription diets. The OTC options might work for mild sensitivities, but they're not in the same category.

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Not sure which ingredients are causing problems? Use the free Pet Allergy Scanner to check any pet food label for hidden allergens in seconds.

Novel Protein Options (When Hydrolysis Isn't Necessary)

If your dog only reacts to specific proteins—say, just chicken—you might not need hydrolyzed food. Novel protein diets use proteins your dog has never encountered, avoiding the allergy without breaking proteins down.

Hill's d/d comes in venison, duck, or salmon—all paired with potato. Protein runs around 16%, fat around 13%. These work well for dogs with confirmed single-allergen issues, or as a step-down diet after you've stabilized on z/d and want to identify exactly what's safe.

Royal Canin's Selected Protein line offers more variety: duck, rabbit, whitefish, or venison, all paired with potato. Same concept, slightly different protein options.

The advantage of novel proteins over hydrolyzed: they're generally more palatable because they taste like actual meat. The disadvantage: if your dog reacts to multiple proteins, you might just be trading one allergen for another.

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Let's Talk About the Money

These aren't cheap foods. Let's be honest about what you're getting into.

Hill's z/d runs around $110 for a 25-pound bag, working out to roughly $4.40 per pound. The small bites version is slightly more expensive per pound because you're buying a smaller bag.

Royal Canin HP is a bit pricier—about $120 for a 25-pound bag, or $4.74 per pound. Their small dog formula runs higher per pound as usual with smaller bags.

Royal Canin Ultamino is where it gets expensive: around $140 for a 20-pound bag, or over $7 per pound. You're paying for that amino acid technology.

What Does This Mean Monthly?

For a 40-pound dog, expect to spend:

• Hill's z/d: $90-110/month
• Royal Canin HP: $95-120/month
• Royal Canin Ultamino: $140-170/month
• Hill's d/d (novel protein): $80-100/month

Hill's z/d offers the best value among hydrolyzed options. Royal Canin HP costs more overall, though the higher protein content means slightly smaller portions. If you end up needing Ultamino, the cost jumps significantly—but at that point, you're out of alternatives.

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What Do Vets Actually Recommend?

When you look at prescription patterns, Hill's z/d edges out Royal Canin HP in general veterinary practice—roughly 55% of vets reach for z/d first, about 35% start with HP, and the rest alternate based on the individual case.

Why the Hill's preference? It's not necessarily that z/d is better. It's that z/d has been around longer, has more published research, and vets have decades of experience with it. When a formula works consistently for 20+ years, it becomes the default.

Veterinary dermatologists—the specialists who see the tough cases—are more nuanced. They'll often start with z/d for general food allergy trials, but they're quicker to switch to Royal Canin HP if they suspect chicken sensitivity, and they keep Ultamino in their back pocket for dogs who fail everything else.

When Vets Lean Toward Each Brand

Hill's z/d makes sense when:

• You're starting an elimination trial and want the most-studied option
• Cost matters (it's slightly cheaper)
• Your clinic stocks it and your vet can monitor easily
• There's no specific reason to avoid chicken

Royal Canin HP makes sense when:

• Your dog has known or suspected chicken sensitivity
• You need higher protein (athletes, muscle wasting)
• Palatability has been an issue with other foods
• You want hydrolyzed treats for training—Royal Canin's HP treats are a real advantage

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Will Your Dog Actually Eat It?

Here's something nobody tells you when they hand you that prescription: hydrolyzed proteins taste weird. The process that makes them safe for allergic dogs also strips out the flavors that make food appealing.

Hill's z/d has moderate palatability. Some dogs take to it fine. Others turn their nose up completely. The canned stew formula tends to be better accepted than the dry kibble—probably because it's moister and smells stronger. If your dog is hesitant, try warming the food or adding warm water to create a gravy.

Royal Canin HP generally scores better on palatability. More dogs accept it without a dramatic transition struggle, and the loaf-in-sauce canned version is particularly well-liked. The fact that Royal Canin makes HP treats is a real practical advantage—you can reward good behavior without breaking the diet.

Royal Canin Ultamino is the hardest sell. Individual amino acids just don't taste like food. Many dogs need an extended transition, and honestly, sometimes you just need to wait until they're hungry enough. If your dog is extraordinarily picky, Ultamino might be a challenge—but then again, if you're trying Ultamino, you've probably run out of other options.

Tips for Picky Eaters

For any of these foods:

• Warm it up—heat releases aroma
• Add warm water to create gravy with dry food
• Try mixing canned and dry formulas
• Extend the transition period to 14+ days if needed
• Cut back on treats so actual hunger helps motivate
• Hand-feed for the first few meals if your dog needs encouragement

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How Well Do They Actually Work?

Based on published veterinary research, here's what you can realistically expect:

Hill's z/d resolves symptoms in 85-88% of dogs with food allergies, typically within 6-8 weeks. Royal Canin HP has slightly higher success rates at 88-90%, also in the 6-8 week range. They're comparable—the difference isn't statistically meaningful for most dogs.

Royal Canin Ultamino is in a different category entirely: 95%+ success rate, often working within 4-8 weeks. When standard hydrolyzed diets fail, Ultamino usually succeeds.

Novel protein diets (Hill's d/d, Royal Canin PD/PR) have lower success rates—70-85%—and take longer to show results (8-12 weeks). That's because they only work if your dog hasn't developed an allergy to that specific protein yet.

When to Switch Between Brands

Consider switching from z/d to HP if: You've done 12 weeks on z/d and your dog improved but didn't fully resolve. Some dogs need a completely chicken-free option—even hydrolyzed chicken can occasionally cause issues in ultra-sensitive cases.

Consider switching from HP to Ultamino if: 12 weeks on HP with strict compliance produced no improvement, or if you're seeing signs of soy sensitivity (rare but possible). This is usually a dermatologist's call.

Consider moving to novel protein diets if: The hydrolyzed diet worked great and you want to try reintroducing whole proteins to find what's actually safe long-term—or if cost is becoming prohibitive and you've identified specific allergens.

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Honest Take

My take: Here's what nobody tells you about Hill's z/d vs Royal Canin HP: for most dogs, they're equally effective. Both achieve 85-90% success rates in clinical trials. The real decision comes down to practical factors — Hill's z/d is slightly cheaper and more widely available, Royal Canin HP is generally more palatable and comes in a small breed formula. If your dog fails one, try the other before giving up on hydrolyzed protein entirely. And if both fail, Royal Canin Ultamino is the nuclear option with the highest success rate but the highest price and worst palatability. Start with whichever your vet stocks, switch if needed, and budget $100-130/month for the long haul.

Can Pet Insurance Help?

Most basic pet insurance policies don't cover prescription diets—they're considered ongoing maintenance, not acute treatment. However, some plans offer wellness riders or prescription diet coverage that can offset costs.

If you have pet insurance, here's what you'll need for reimbursement:

• A formal diagnosis of food allergy from your vet
• The actual prescription
• Itemized receipts showing the food purchases
• Some insurers want proof that you're feeding the diet exclusively

Annual limits typically range from $500-1,500 for prescription diet coverage, which covers maybe half a year for most dogs. Worth claiming if you have it, but don't expect full reimbursement.

Ways to Reduce the Cost

These diets are expensive, but there are legitimate ways to save:

Buy the biggest bag possible. Per-pound pricing drops significantly with larger sizes. Just make sure you can use it before it goes stale—6-8 weeks is reasonable for an opened bag stored properly.

Subscribe and save. Chewy and the manufacturers' direct sites offer 5-10% discounts on autoship. It's not dramatic, but it adds up over months.

Compare prices. Veterinary clinics often charge more than online pharmacies for the same product. Ask your vet if they'll price match, or get the prescription and order online. Some clinics will match to keep your business.

Watch for manufacturer promotions. Both Hill's and Royal Canin occasionally run rebates or coupons. Sign up for their mailing lists.

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How to Transition to Prescription Food

Don't switch overnight—gradual transitions prevent digestive upset and improve acceptance.

Week 1: Start with 25% prescription food mixed with 75% current food. Watch stool quality and make sure your dog is drinking normally.

Week 2: Move to 50/50. You might notice some changes in scratching or energy—keep mental notes.

Week 3: Now you're at 75% prescription, 25% old food. Most dogs are fully adjusted by this point.

Week 4 onward: 100% prescription diet. This is when the elimination trial officially starts. From here on, strict compliance matters—no treats (unless they're the prescription brand's treats), no flavored medications, no table scraps, nothing.

Once It's Working

If the diet resolves your dog's symptoms, keep going. This becomes their long-term food, at least until you've had time to do controlled food challenges to identify exactly which ingredients are safe.

Monitor weight and adjust portions as needed. Check in with your vet every 6-12 months. And if symptoms ever return, don't assume the diet stopped working—first check for dietary indiscretions. Did someone slip your dog a treat? Get into the cat's food? Find something outside? Human error is more common than diet failure.

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Sources & Further Reading

For more information from trusted veterinary and pet health organizations:

• American Kennel Club - Food Allergies in Dogs
• Merck Veterinary Manual - Food Allergy Diagnosis
• Cummings Veterinary Medical Center at Tufts - Nutrition

Ready to choose the right food? Check out the Best Limited Ingredient Dog Food for specific product recommendations.

Final Thoughts

Both Hill's and Royal Canin make excellent hydrolyzed protein diets. The debates you'll find online about which is "better" mostly miss the point—for the majority of dogs, they work equally well.

The real differences:

• Hill's z/d: More research behind it, uses chicken (which is fine for most dogs even when hydrolyzed), slightly cheaper
• Royal Canin HP: No chicken at all, higher protein, often better palatability, treats available
• Royal Canin Ultamino: For dogs who fail everything else—expensive but incredibly effective

Start with whichever your vet recommends. If it doesn't work after 12 weeks of strict compliance, try the other. If neither works, Ultamino is waiting. And if that fails, food might not be the problem.

Work with your vet, commit fully to the elimination protocol, and give it time. Most dogs get relief. Yours probably will too.

Need help comparing food ingredients? Use the free Pet Allergy Scanner to check any dog food for potential allergens.

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Prescription diets require veterinary authorization. This information is for educational purposes and doesn't replace professional veterinary guidance.

Take action today: Use the free Pet Allergy Scanner to check your current pet food for hidden allergens and find safer alternatives.

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• Best Dog Food for Allergies
• Blue Buffalo vs Purina Pro Plan
• Best Prescription Dog Food for Allergies
• Hypoallergenic Dog Food Brand Comparison
• Best Limited Ingredient Dog Food

Frequently Asked Questions

Is Hill's or Royal Canin better for dog allergies?

For most dogs, no. The clinical outcomes are comparable. Hill's z/d has a longer track record and more published research. Royal Canin HP offers no chicken-derived ingredients and higher protein content. The "better" choice depends entirely on your individual dog's needs and response.

Can I switch brands during an elimination trial?

Don't do this. Switching mid-trial makes it impossible to interpret results. Pick one and commit for the full 8-12 weeks. If it doesn't work, you can try the other—but treat it as a separate, fresh trial.

My dog won't touch this stuff. Now what?

First, try the palatability tricks—warming, adding water for gravy, mixing canned with dry, extending the transition period. If your dog is still refusing after 2 weeks of patient effort, talk to your vet about trying the other brand. Some dogs that hate z/d will happily eat HP, and vice versa.

If my dog is allergic to chicken, is hydrolyzed chicken really safe?

Usually, yes. Proper hydrolysis breaks chicken into fragments so small the immune system can't recognize them. However, a tiny fraction of extremely sensitive dogs may still react. If you're dealing with that level of sensitivity, Royal Canin HP's soy-based formula eliminates the concern entirely.

Can my puppy eat prescription hypoallergenic food?

Hill's z/d is approved for all life stages, so puppies can eat it. Royal Canin makes a specific Hydrolyzed Protein Puppy formula. Just verify the specific product is appropriate for growing dogs before you commit.

What if neither brand works?

If you've done a proper 12-week trial on hydrolyzed food with strict compliance and seen no improvement, food might not be the problem—or at least not the only problem. Environmental allergies, bacterial infections, and other conditions can cause identical symptoms. This is when a veterinary dermatologist becomes essential.

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