Contrary to popular belief, 'prescription diets' don't need to be prescribed at all. In fact, the laws governing the distribution of prescription diets are the same as for any other foods but as with so many things in the pet food industry, it's all about the marketing.
The manufacturers of prescription diets have done a fantastic job in taking over the veterinary industry. At veterinary college, the module or two spent on nutrition is almost always 'sponsored' by a pet food manufacturer so even before graduating, vets are already inclined to recommend a particular pet food brand. The clinical white packaging and the convenient "if your dog has x, recommend food y" approach of prescription diets are all designed to resonate with vets while blinding consumers with science. Add to that the fact that the customer can only get the 'prescribed' food from the vet and that a large proportion of the extraordinary price goes directly into the practice's coffers, it's easy to see why vets are so keen to recommend their particular prescription diets.
The fact is that most * (see below) prescription diets are simply dog foods, and in general not very good dog foods. Their manufacturers choose to only sell them through vets but if they wanted to they could just as well sell them in pet shops or anywhere else they pleased. You also don't need to be a nutritionist to see that the ingredients of most prescription diets are thoroughly average and certainly don't warrant the often exorbitant price tag.
Nutritionally speaking too, prescription diets aren't all they're cracked up to be. Certainly, they all have certain traits to help with specific conditions but they are in no way medicines and, in general, aren't that different from 'normal' pet foods: Dogs with heart problems, for example, should certainly be fed less salt and less fat but there are plenty of non-prescription diets out there that fulfil these criteria just as effectively as expensive prescription heart diets. Chronic kidney problems too require a low phosphorous, low salt diet, but again, the prescription diets are by no means the only low phosphorus, low salt foods on the market. And as for the prescription diets that are marketed for digestive upsets, joint support and even things like weight loss and 'brain ageing', the right amount of any good quality food is likely to have just as good, if not better, results.
* It should be noted that not all prescription diets can be written off. Some, despite their largely average ingredients, do have certain properties that you will not find elsewhere. Some diets for urinary or kidney stones, for example, have the effect of changing the pH of the urine in order to dissolves the stones.
So, our advice is to treat prescription diets with a degree of skepticism. If your vet can provide a good reason for trying a particular prescription food, then by all means give it a go but if it's simply a matter of finding a food that is low in this or high in that, it may well be worth looking elsewhere.