There are several brands of food that are provided only by veterinary practices at the sort of prices that make you weep! Of course you want to ensure that your pet has the best and if your vet says that a certain food is necessary due to a particular condition that you pet is suffering from you are unlikley to want to go against that advice.
The Burns food was developed by a veterinary surgeon who is well versed in the requirements of pets with many conditions and no doubt you will be surprised to know that most of the "veterinary only" foods can actually be replaced by one of the Burns varieties.
Burns provide foods with "Low Protein", "Low Phosphorous", "Low Fat", "Low Sodium" and/or "High Fibre" which match most of the specialist foods available and they are free from artificial colours, flavourings, wheat and soya too. There is a chart available from Burns which lists the alternatives to most of the Hills Science and Royal Canine special diet foods.