Vet recommended and our kitty loves it! Update 3/14/17: we started giving this to our kitties as two of ours got eye ulcers, something that can be a genetic defect. They are siblings, so we started giving these supplements to all of the kitties (we have 7 rescues, 4 from the same family). Lysine is a preventative supplement, and clearly it is working, as we have not had an ulcer issue in almost 2 years. Although it is a little pricey feeding them this daily, it is much, much less expensive that multiple trips to see a specialist, and it is worth it to avoid having our kitties suffer.