Q. My cats are on a special urinary tract diet I can usually get at my vet’s, but the last time I needed it my vet had run out so they sent me to pet-supply store. When I got there, however, the store refused to sell it to me without a written prescription. How can a pet food require a prescription if it’s not regulated by the FDA?

You’re not alone in your annoyance at this practice. Perhaps it’ll make you feel better if I explain the rationale behind it.

Therapeutic foods are ubiquitous in veterinary medicine. They’re designed to address everything from renal failure and gastrointestinal sensitivity to allergic skin disease and liver disorders.

When these products were developed in the 1970s and ’80s, they were initially marketed as “prescription diets.” The idea was that these foods were not only more scientifically advanced but also that they were so tailored for specific illnesses and were not considered appropriate for healthy pets.

Consequently, they were marketed exclusively through veterinarians. This not only ensured that a vet would be in charge of monitoring a pet’s response to a restricted diet, but that the pet food company had a vet’s expert recommendation, a prized commodity. This arrangement also provided a veterinary hospital with a secure source of significant profits. When clients began asking to purchase these foods from other sources, manufacturers began to require written prescriptions.

Fast-forward 20 or 30 years, and greater competition in the pet food market means expanded distribution online and through big-box retailers. The concession to veterinarians who were unhappy with the competition was to continue to require prescriptions.

As you rightly note, these diets are not regulated by the FDA. They did not have to endure rigorous safety and efficacy trials as drugs do. As such, the requirement of a prescription, though legal, is not strictly necessary.

Dr. Patty Khuly has a veterinary practice in South Miami and blogs at www.dolittler.com. Send questions to khulyp@bellsouth.net, or Dr. Dolittler, Tropical Life, The Miami Herald, 1 Herald Plaza, Miami, FL 33132.