We currently have three dogs and prior to this had five, the oldest being 18. A couple of our furkids were given vet prescribed foods for various conditions. Once I went g.f. we put the kids on g.f. food as well, chucking the special pricey vet food. After doing a lot of research we felt that most dog foods, even the "pricey" ones are filled with useless grains ~ rice, wheat, oats, barley. Dogs were not meant to eat grains. Grains make for good food filler but do nothing for the overall health of the creatures. Once we went grain free for our pups skin issues went away, no more flaky itchy skin, tummy upsets went away and the two that had been on "special" diets got better.
That said, and I know some will not agree with me, research the pet foods and then decide for yourself. I, personally, do not believe in vet prescribed foods and would cook for my pets before giving the vet foods to them.