Hill’s Prescription Diet is the global market leader in clinical veterinary nutrition โ a range of therapeutic pet foods designed to manage specific diagnosed medical conditions through targeted dietary modification. Unlike mainstream premium foods, the Prescription Diet range is not about ingredient aesthetics or lifestyle positioning. It is about measurable clinical outcomes: reducing creatinine in kidney disease, dissolving struvite crystals in urinary conditions, restoring GI function after inflammatory episodes, and managing blood glucose in diabetic animals.
In the UK, Hill’s Prescription Diet is available without a formal veterinary prescription from ZooPlus โ but this accessibility does not mean these foods should be used without veterinary guidance. Each formula Code (k/d, c/d, i/d, z/d, r/d, m/d) represents a specific nutritional profile optimised for a narrow clinical application. Feeding the wrong formula to a healthy animal can cause nutritional imbalances over time. Feed the right formula to the right animal and you have one of the most powerful non-pharmaceutical clinical tools available in veterinary practice.
This guide explains every major Hill’s Prescription Diet code, what it does, and which dogs and cats it is designed for.
Understanding the Hill’s Formula Codes
Each code corresponds to an original condition. The codes are consistent across dog and cat formulas:
| Code | Condition | Key Modification | Available For |
|---|---|---|---|
| k/d | Kidney / Renal | Low phosphorus, restricted protein (high quality) | Dog + Cat |
| c/d | Urinary / Struvite | Controlled minerals to dissolve / prevent crystals | Dog + Cat |
| i/d | Gastrointestinal | Highly digestible, low fat, GI microbiome support | Dog + Cat |
| z/d | Allergies / Sensitivities | Hydrolysed protein (immune system cannot react) | Dog + Cat |
| r/d | Obesity / Weight Loss | Very high fibre, controlled calorie, L-carnitine | Dog + Cat |
| m/d | Diabetes Mellitus | Low carbohydrate, high protein, controlled glycaemic index | Dog + Cat |
| d/d | Dermatology / Derm | Single novel protein + rice (duck, salmon, or venison) | Dog + Cat |
| t/d | Dental Health | Large fibre-matrix kibble that mechanically cleans teeth | Dog + Cat |
The Most Commonly Prescribed Formulas
Hill’s k/d โ Kidney Support (Most Prescribed Overall)
Chronic kidney disease (CKD) is the leading cause of death in cats over 10 years old and extremely common in older dogs. The kidneys lose their phosphorus filtration capacity as they decline, meaning dietary phosphorus restriction can dramatically slow progression.
Hill’s k/d restricts phosphorus to approximately 0.2% on a dry matter basis (compared to 0.6-0.8% in standard foods) while maintaining protein quality and palatability. The protein restriction in k/d is also significant: reducing the nitrogenous waste the kidneys must filter reduces the progression of uraemia โ but the protein that is included is high biological value to prevent muscle wasting.
When your vet recommends it: After a diagnosis of CKD stages 2-4 (IRIS staging), typically when serum creatinine and SDMA markers are elevated consistently on two or more blood tests.
View Hill’s k/d Kidney Care at ZooPlus โHill’s c/d โ Urinary Stress Care
Feline lower urinary tract disease (FLUTD) affects approximately 1-3% of all cats and is the most common reason for emergency veterinary visits in cats under 10 years. The c/d Stress Urinary Care formula addresses the two most common urinary presentations in cats simultaneously: struvite crystal dissolution (controlled magnesium and urinary acidification) and stress-associated inflammation (inclusion of L-tryptophan and milk protein hydrolysate to reduce cortisol-driven inflammatory episodes).
When your vet recommends it: After a diagnosis of idiopathic cystitis, struvite crystalluria, or recurrent FLUTD. Cats with calcium oxalate crystals require a different formula (k/d can also help).
View Hill’s c/d Stress Urinary Care at ZooPlus โHill’s i/d GI Biome โ Gastrointestinal Support
The i/d Gastrointestinal Biome formula represents Hill’s most advanced GI product โ combining a highly digestible protein and fat matrix (for immediate symptom management) with a proprietary prebiotic blend targeting microbiome restoration. Independent research has shown that the specific fibre combination in i/d Biome increases the production of short-chain fatty acids by colonic bacteria, directly supporting the intestinal mucosal barrier repair.
When your vet recommends it: After episodes of acute gastroenteritis, chronic enteropathy, small intestinal bacterial overgrowth, or as a post-antibiotic course intervention to restore microbiome diversity.
Hill’s z/d โ Hydrolysed Protein for Allergies
The z/d formula uses chicken liver as its protein source, but hydrolysed โ broken down by enzymatic hydrolysis into molecular fragments so small that the immune system’s antibody recognition mechanism cannot bind to them. The result is a protein source that provides full amino acid nutrition while being immunologically invisible to a food-sensitised animal.
When your vet recommends it: After single-protein elimination diets (like James Wellbeloved) have failed to resolve food allergy symptoms, or when a dog or cat is allergic to multiple proteins making novel protein selection impossible.
Hill’s Derm Complete โ Skin & Allergy
The Derm Complete formula is a skin-focused variant that uses rice and egg as the single carbohydrate and protein โ both virtually zero-allergen food ingredients โ with an elevated omega-6:omega-3 ratio targeting the epidermal lipid barrier specifically. It does not use hydrolysed protein (unlike z/d) but the exclusion of common allergens combined with the barrier-support nutrition makes it effective for environmental as well as dietary allergy presentations.
View Hill’s Derm Complete at ZooPlus โDo I Need a Prescription to Buy Hill’s Prescription Diet?
In the UK, you can legally purchase Hill’s Prescription Diet foods without a veterinary prescription โ including through ZooPlus. However, we strongly recommend obtaining a veterinary diagnosis before starting any prescription diet. The reasons are practical:
- Using k/d (kidney diet) on a healthy young dog will restrict phosphorus to levels that may be marginally suboptimal for an animal with healthy kidney function long-term.
- Using c/d on a cat with oxalate crystals (instead of struvite) will not dissolve the crystals and may actually worsen the condition.
- Using z/d will not resolve environmental allergies, wasted skin conditions, or hormonal skin disorders that may appear similar to food allergy on the surface.
The ZooPlus listing availability is a convenience โ not a recommendation to self-diagnose. If your pet shows signs that might indicate a need for prescription diet, a vet visit first will get you to the right formula faster and more safely. View the full Hill’s Prescription Diet range at ZooPlus โ
For mainstream Hill’s food (no prescription required), see our Hill’s Science Plan guide. For an alternative to z/d hydrolysed protein, see James Wellbeloved (single novel protein, no prescription needed).