Humulin is a short duration and quick acting insulin. Cats absorb and use insulin twice as fast as dogs or humans do, so an insulin like that hits them even harder and faster, so no, it isn't a good insulin for cats.
Unfortunately, it does get prescribed a good bit. I think that may be due to most vets use it on dogs, so why not for cats? On dogs, the dose is determined by weight of the dog. That doesn't work well for cats, and what we usually see is a "high" starting dose based on how much the cat weighs. Consider that most of the time, a cat when diagnosed is on the heavy side of normal.
Lantus is a "human" insulin as well, but it works completely differently than humulin does. It builds a reserve in the cat's body, sort of like a refueling station. Here we call it the "shed". After a couple days of doses, that shed is "full", and then the cat's body will "withdraw" some of it to fight off rises in BG levels. When you shoot the cat with more every 12 hours, you're refilling the shed. Lantus is more gentle, and lasts much longer than humulin does. You don't have huge increases and decreases in blood glucose once you are on a routine dosing schedule.
The thing that was alarming to most with respect to your kitty was that the starting dose was pretty high, which leads me to think maybe your vet determined the dose based on weight? Not sure what the rational was. When you talk to the vet, maybe ask him how he came up with 2.5u? A good safe and generic starting dose is 1 unit, and then you test and collect data for a week or more. At that time, you can make adjustments based on how it is working.
Lantus is a very good insulin, and people here have had terrific success with it. Lots of kitties, if you follow the Lantus protocol, do very very well, and they have a pretty high remission rate.
Hopefully that helps some?
Carl
