I don't blame you.
There are several types of insulins that may be prescribed by a vet for your cat's diabetes. On FDMB we sometimes see really strange advice from vets and we see newly diagnosed cat's parents confused about what they just brought home. You can find a more involved primer on Feline Diabetes which I highly recommend you read when you are calmer. But for a short, sweet rundown, here is a short, sweet primer:
Humulin R - Extremely fast acting insulin. Used usually only as a bolster for a cat with unusually high hard to drop numbers in conjunction with another insulin or for ketone cases AT the veterinary's in an effort to get a drop as one part of helping with ketones in DKA. EXTREME CAUTION. Experienced and RARE, CAUTIOUS use only. Only active for about 3 hours but causes FAST drops. Any vet prescribing this should be considered extremely suspect in their experience of feline diabetes (or any diabetes). Use as a bolus should ONLY be undertaken by an extremely experience user and extremely carefully. Very rare need. There, did that scare you? Good.
If the vet gave you this I cannot emphasize enough to find a new vet. This is NOT an insulin that should be prescribed to a new owner. Or actually for anything other than the rarest of circumstances.
Pearl says you may proceed now.
Humulin N (u100)- Often prescribed by vets who may not be used to using newer synthetic insulins or used to treating feline diabetes. Longer lasting than R but tricky for cats. Cats process insulin pretty quickly so this insulin usually has a quick onset and hits the nadir in about 4 hours. MUST be given after feeding because it is fast. Some cats do well on it, usually when a cat parent can really test, monitor and get to know how the cat eats and processes it with the food. Can be used but it's, I think, finicky to use in a cat.
Here is a good primer on how to use with your cat's diabetes. Many cats, it's just not enough. Since it is out of the system eight hours or less, even twice a day dosing is probably going to leave your cat without enough coverage at least half of a day. Also, if you have a vomiter, can be really tricky and leave you worrying about hypo if you can't get the cat to eat again. Or if the cat vomits and you've left for work, again you risk hypo with no one at home to help. Would not ever be my choice of insulin but it works for some.
Vetsulin (u40)- This insulin had some major manufacturing problems several months ago and even the manufacturer sent notices to all vets to change to other insulins. I don't know if they fixed it but it was never great for cats as it was a dog insulin. Vets used it for lack of other insulins but honestly, almost no cat did well on it. If a vet has some lying around and gives it to you, insist on another insulin or find a new vet.
PZI (varying strengths - check with vet and vial) (Protamine Zinc Insulin)- There are only two sources now that I am aware of that produce this insulin. There is BCP PZI by a compounding pharmacy out of Texas. We used this a while. It works better in some cats than others. I didn't find it to be the best for Pearl. Longer lasting than N, probably 5 to 6 hours to nadir, so around a 12 hour cycle. Many vets are leary of it because it is pounded in their heads that compounded insulin is inconsistant. I don't know for sure that is true, but this insulin is more affordable than the other PZI and some cats do great on it. Service in our experience was terrific and fast fast fast. May be worth a try for some. There is also the new ProZinc that replace the Idexx PZI (which was, indeed, a very consistant, good manufactured PZI but was discontinued by the manufacturer for .... well, reasons that are debated). I don't know much about it. Never used it. More people are coming home with it. I'm sure it's pricey but probably works fine. Again, testing and knowing your cat will be key to success as with all insulins. If a cat is not doing well on it, a switch to one of the following insulins should be considered.
Lantus (glargine) (u100)- An excellent long last insulin. This insulin is often great to use, especially in newly diagnosed cats. This insulin, with careful blood monitoring at home with a glucometer and a switch to low carb we food can often result in remission for many many cats. IMO, some vets do not realize that the Queensland protocol says to dose by ideal weight, not the current weight of the cat, resulting in beginning doses that are too high. Nothing against them. Vets are not all versed in specialties, but they do miss this. Not only does beginning with a too high dose risk a hypo of a cat under a new, inexperienced parent but it could also cause chronic rebound which the vet then sees as "not enough" reaction from the insulin and they increase the does, compounding the problem until one day a crisis point is reached and the cat bottoms out and hypos or has other problems from the high blood glucose numbers. In my opinion, the *best* starting dose for any insulin is to begin on the low side and increase. This protocol is very good at describing how and when to change doses in conjunction with home testing.
The awesome thing about lantus and the following insulin, levemir, is that they have a long, slow effect on blood glucose numbers. Instead of the sharper V shaped curves of the shorter insulin there are nice gradual lowerings, in more of a shallow U form. This means, once you get experienced at using this, you can often get a cat on nice mid normal range numbers form most or even all of the day. This is also probably why remission can happen so well on it. It gives the pancreas the rest it needs to repair. The flip side of that is you MUST be testing or one day you could come home to a hypoing cat. Pearl did fantastically on this for over a year, but then did begin to have trouble. I don't know why, but I've heard of this happening with some cats. Others use it fine for years. We switched to the PZIs but then when levemir began being used and people had great results I finally switched to that. Lantus can be purchased in pens or cartridges
that can be used like vials but bring the cost way down because it is much more efficacious for longer.
Levemir (u100)- Uses the same protocol as glargine. Works similarly. I find it is more stable (I never have a vial poop out on me like I did with Lantus). Everything said above about how Lantus works applies here to. I love it. We get the pens:
(only pens, not cartridges, are available in the U.S. but they all can be used with syringes, just like a vial so it's all good), which while it is a higher cost outlay to do, lasts well over 5 months (unless you have a big gulp kitty (rare) or an acromegaly cat rare but less rare than vets have been saying. If your cat is needing ever higher doses of an insulin and the vets are puzzled, even if they say it's too rare to check out, check it out. Make them test. Especially if you have a cat that looks like it has kind of oversized features. It's just not as rare as many believe.) This brings the cost down to probably the most affordable insulin. In my opinion, also the best.
All insulins really require home blood glucose monitoring to get the best results. There is just no doubt about that. Even if your cat has been on insulin 10 years, test. Just the other day a cat who had been getting flat normal numbers on a certain dose of Lantus for YEARs had a hypo. The owner had grown lax in testing because numbers just stayed the same. The cat was in dire shape, her meter was dead and by the time she found batteries and tested the cat was down to a 19 mg/dl reading. That is DEADLY. She caught it in time and using the tools from a Hypo Kit and knowing how to treat a hypo her kitty recovered. Mostly a cat with that number and those symptoms needs the ER immediately!!! So, never doubt testing before shots, even when you get the same ol' same ol'. Treating your cat with insulin and not testing is not unlike, as some have said, driving with a paper sack over your head that has a couple of slits cut out. You may get along but you sure can't see any approaching dangers and your luck could run out very quickly and tragically.
I hope this helps some newly diagnosed parents figure out the mysteries of what the heck the vet just handed you and what it means.