Prescription diets – how they work

Prescription or therapeutic veterinary diets are quite different from your run of the mill pet food. These diets are designed to treat specific medical conditions and in some cases may form the only treatment for a patient’s condition. These diets have gotten alot of attention recently; many people mistake prescription diets to mean the food contains medications or special supplements that cure diseases – this is simply not how they work, however this misunderstanding was strong enough to lead to a recent class action lawsuit against some of the big brands that make these diets (to little effect).

Prescription diets are also sometimes called therapeutic diets or veterinary exclusive diets – what this means is that these foods are only available from your vet and to be administered under their supervision. But if they don’t contain medications, why must they come from your vet?

Prescription diets require a prescription

With the advent of online shopping, it is now possible in some online pet stores to purchase prescription foods, however they require you to either declare your vet has prescribed it, or your vet must write a prescription for you to upload to the online store prior to purchase. The reason they require prescriptions, is they are forming a medical treatment and to do that, the nutrients in the diet are not always ‘complete and balanced’.

Complete and balanced – for a medical condition

Under the AAFCO and FEDIAF guidelines, prescription diets are not required to be complete and balanced, as they are not formulated for a lifestage – instead, the formulation contains specific nutrients in very different ratios to what you would find in a regular pet food. These are called Key Nutritional Factors and are used to modulate the body’s response to food and induce certain states that are beneficial to the disease or condition they are treating for. A complete and balanced diet for a lifestage would not be suitable for many conditions as the nutrients may be higher or lower than what the patient requires to treat their condition. This is one of the biggest reasons a prescription from your vet is required before starting on a therapeutic diet; some are not designed to be fed long term, instead they are designed to be fed in the short term to induce a healthy state and then the pet can transition back to a regular or slightly modified diet. Some therapeutic diets ARE required to be fed long term, and in these cases it’s recommended that pet owners regularly follow up with their vet to monitor their pet’s progress and response to the diet, as they may need ongoing diagnostic testing to make sure the diet is effective and that the pet’s condition is still being controlled by the diet. It is also important to note that some diets cannot be fed while on certain medications, which is another reason why a vet’s prescription is recommended as it may not be suitable for your pet’s situation.

What is a Key Nutritional Factor?

Key Nutritional Factors are used to describe ‘nutrients of concern’ to allow for the provision of target nutrient levels for disease treatments and prevention. Most commercial foods provide the AAFCO allowances for all nutrients, however Key Nutritional Factors simplify the process of clinical nutrition by allowing veterinary professionals to select diets based on the nutrients of concern for that disease state, as well as the pet’s nutritional risk factors, rather than having to assess based on 40+ nutrients recognised for cats and dogs. When determining a pet’s Key Nutritional Factors, the following must be taken into consideration:

1. Lifestage and physiologic state
2. Environmental conditions such as housing and temperature
3. The nature of disease or injury
4. The known nutrient losses through skin, urine and gastrointestinal tract
5. The interactions of medications and nutrients
6. The known capacity of the body to store nutrients
7. The interrelationships of nutrients

Beyond this, pets may also have specific food-related needs such as managing acid-base balance in the body, kibble or food texture, urine pH, avoiding specific protein sources or ingredients. This is typically the case with therapeutic nutrition.

What about the ingredients?

As I always say, the ingredients are not as important as you might think – it’s the nutrients they provide that is important. This is especially true with therapeutic foods; the ingredients often look very similar to commercial pet foods which makes many people wonder if they are actually doing anything – however, just because the ingredients are the same, does not mean the nutritional analysis is, just like all chicken dog food is not the same. The nutrient values and typical analysis are quite different; when formulating, familiar and heavily researched ingredients are generally used because we know how they will behave and interact with other ingredients in the diet which makes it easier to modify the nutrient values that will trigger a response in the patient after eating. The only time I’d suggest checking the ingredients is to ensure there isn’t an allergen in there which is another reason your vet or nutritionist’s recommendation should be sought prior to purchase.

Can I feed other foods?

These diets are designed to be the sole diet your pet receives as they are carefully crafted to provide very precise nutrition – although they are not medicated, adding another food will reduce the efficacy of the therapeutic diet, or potentially interfere with the nutrients in the therapeutic food. If you wish to provide variety, it’s strongly recommended feeding that prescription range or brand of diets; most will have a few flavour and texture options for your pet that are safe to be used together. You may want to use other prescription branded products for the same condition together, but always check with your vet beforehand if this is okay, as different brands may not be compatible with eachother as they often have different modes of action.

My pet has more than one condition – but there isn’t a diet for them, what are my options?

If you are prepared to homecook for your pet’s condition, you can seek the assistance of a Board Certified Veterinary Nutritionist to formulate a recipe to meet your pet’s requirements, this is especially useful if your pet has multiple conditions, or a condition that doesn’t currently have a commercially prepared therapeutic diet yet. Your vet can also utilise by setting up a veterinary account to create diet recipes specifically for health conditions too. There are some recipes available in veterinary textbooks for health conditions too, but these are computer balanced and not feeding trialled, so may not be suitable for your pet and their individual needs, taste preferences, or safe for longer term feeding.

Does your pet eat a therapeutic diet? Or do you cook your own from a special recipe? Tell us about your experience in the comments below!

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Mark Morris Institute (2019) Small Animal Clinic Nutrition 5th Ed., Chapter 1: An Iterative Process. Available online at:

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