The
(2018) AAHA Diabetes Management Guidelines for Dogs and Cats has this to say about glipizide.
"Sulfonylureas such as glipizide promote insulin secretion from the pancreas and can be used in cats. Oral glipizide has been used successfully in cats with DM, with benefits being reported in approximately 40% of cats. Transdermal application is unreliable. Adverse effects following oral administration include cholestasis, hypoglycemia, and vomiting. There is concern that glipizide may contribute to progression of DM and pancreatic amyloidosis.
The TaskForce only recommends glipizide for use in cats with owners who refuse insulin therapy, and only with concurrent dietary therapy."
The bold is mine.
So, what diet did your vet recommend, for your diabetic cat?
We don't recommend glipizide here, because it basically makes the remaining insulin producing cells in the pancreas work harder and irreversibly damages them over time. Damage that results in the cat needing insulin therapy anyway.