These are clinical diets for diagnosed conditions, not premium dog food. They work by restricting or adjusting things a healthy dog needs, which is why they should only be fed on a vet's recommendation and why some are actively unsuitable for a well dog.
If your vet has named a diet, this is where you reorder it. If they haven't, book an appointment rather than working backwards from the symptoms.
Buy exactly what your vet named
Four brands here, and each names its diets differently, which is why matching the note from the practice matters more than it looks.
Hills Prescription Diet uses two-letter codes: c/d for urinary, i/d for digestive, j/d for joints, k/d for kidney, l/d for liver, w/d for low fat, z/d and d/d for food sensitivities, plus Metabolic for weight.
Royal Canin Veterinary uses words: Gastrointestinal, Renal, Urinary, Hypoallergenic, Satiety.
Pro Plan Veterinary Diets uses short abbreviations: EN for gastrointestinal, HA for hypoallergenic, OM for obesity management, NF for renal function.
Eukanuba Veterinary we carry in a very small range, so if your vet has named one, check it's here before you rely on it.
Similar names are different foods. Hills i/d and i/d Low Fat are not interchangeable, and Royal Canin's veterinary Urinary is not the same as any urinary recipe from the ordinary retail range.
Can you swap between brands?
Not without asking. Two diets for the same condition often work in different ways, and with urinary diets in particular some are formulated to dissolve existing stones while others prevent them forming.
If the one your vet named is out of stock, ring the practice and ask what else they're happy with rather than picking the nearest-sounding alternative.
Treats are the thing that undoes it
Most of these diets only work if they're all your dog eats, and dogs get given far more extras than cats do. A therapeutic diet plus the usual chews, training treats and a corner of toast is not the diet your vet prescribed.
This matters most with the food sensitivity, urinary and weight diets. If you want to give your dog something, ask your vet what's allowed. Several brands make treats designed to go alongside their therapeutic ranges, and some of these diets can be baked into treats using the kibble itself.
Weight diets need the portion right
A weight loss diet fed at the old portion does very little. These foods work by changing the balance of what's in them, not by being magic, so the amount matters as much as the food.
Weigh it rather than using a scoop, and take the treats out of the daily total rather than adding them on top. Ask your practice to weigh your dog every few weeks. Most will do it free, and it's the only way to know whether it's working.
Give it time
Most therapeutic diets need six to eight weeks before you can judge them. Skin and joint diets are the slowest. Stopping after a fortnight because nothing has changed is the commonest mistake, and it's an expensive one because you've paid for the bag and got none of the benefit.
Buy the small bag first
These diets are dear, particularly for a big dog, and you can't return opened food. If your dog is starting something new, buy the smallest size, find out whether they'll eat it, then order properly.
Introduce it over a week or two mixed with the old food unless your vet has said otherwise. Most of these come in wet as well as dry, so if the kibble is being ignored, try the wet version of the same diet before abandoning it.
Supplements alongside
Some conditions are managed with a diet and a supplement together, joints especially. Those sit under dog health next to the diets for each condition, so you can see the options in one place.