Insulin for cats: types and how to inject

In October 2025, our cat Manishka was diagnosed with diabetes. Our first evening went into searching: “insulin for cats,” “how to inject a cat with insulin,” “Lantus for cats.” I dismissed other people’s numbers right away: the dose is calculated by the vet for this particular cat — based on weight, lab results, and the response to the first dose. No dose tables, no borrowed regimens. What follows covers the types of insulin, syringes, injection technique, and what to keep track of.

Types of Insulin

Feline insulins fall into two baskets. The first is veterinary products: porcine lente insulin (known under the brand names Caninsulin and Vetsulin), licensed for cats, at a U40 concentration, and protamine zinc insulin (PZI), also U40. The second is long-acting human analogs that a vet may use in cats when appropriate: glargine (Lantus) and detemir (Levemir).

For an owner holding a vial, three things matter: the product’s name, its concentration (U40 or U100), and what you draw the vial up with. Everything else is the vet’s job. Products differ in how long they act in a cat: according to ISFM data, lente insulin in cats often works for a shorter time than glargine, detemir, or PZI, so it doesn’t always become the first choice. Different owners have different vials in their refrigerators — and that’s normal. In some countries, regulations may limit the first choice to veterinary products. And more concentrated glargine is not “the same Lantus but stronger” — it’s a different product that behaves differently.

The product is chosen based on availability in the country and at the pharmacy, on coexisting conditions, on whether you can inject twice a day at consistent times, and on how this particular cat responds. Manishka’s regimen wasn’t prescribed from a search engine: I came to the vet with questions, not a printout of someone else’s chat.

U40 and U100 Syringes

U40 and U100 are not needle sizes — they are concentrations: how many units of insulin are in one milliliter. A U100 vial contains 2.5 times as much insulin as a U40 vial. That’s why the syringe has to match the concentration: a U40 syringe for U40, U100 for U100. Mix them up, and the same mark on the syringe will hold a different amount of insulin: 2.5 times less or more than needed. Too little, and blood sugar stays high. Too much, and blood sugar drops too low — this is called hypoglycemia: it develops quickly and requires immediate action. Discuss its signs and first aid with your vet in advance — the detailed algorithm is in our overview article on diabetes in cats.

At the pharmacy, you may be offered “just insulin syringes.” Look for the U40 or U100 marking on the box and match it to the vial, not to the habit of “that’s what we used to inject a person in the family.” If the vet has prescribed an insulin pen, it has its own needles, and you can’t pour the insulin into it by eye — the dose is set by the pen’s mechanism, not by eyeballing with a syringe.

Check how your particular product is prepared for drawing up. Each form has its own routine: porcine lente is shaken vigorously until it’s a uniform milky white, PZI is rolled gently, and clear glargine is not shaken at all — it doesn’t need mixing. Our vet showed us this at the clinic, but ask them specifically how to prepare your vial. And one more rule: if the contents of the vial look different from yesterday, don’t inject on a hunch — ask your vet or replace the vial.

How to Inject

A vet at the clinic showed me the technique on Manishka herself — not from a video and not from an article. The injection is given under the skin: usually into the scruff or a fold on the flank, wherever the vet placed your hand. Tent the skin, insert the needle at the angle you were shown, not into the muscle. After injecting, hold for a second or two so no insulin leaks back out. It seemed logical to me to inject in the same spot every time, but the vet explained that a week of injections into one point causes thickening and unpredictable absorption, so I alternated sides (morning — one, evening — the other) and shifted the spot by a couple of fingers.

There are situations best discussed with your vet before the first injection. Not sure the injection actually went in — don’t give a second one “just in case” on your own judgment: ask the vet how to handle this and write down the answer. Missed the time — don’t double the next dose yourself: changing the dose or frequency without the vet’s instruction is not allowed.

And if the diagnosis has only just been made and you’re at the start of the road, begin with the first-week checklist: your cat has been diagnosed with diabetes. Ask in advance how to time the injection with meals and what to do if your cat won’t touch the bowl. How to check the result of an injection with a home measurement is covered in our article on measuring your cat’s blood sugar at home.

What to Ask Your Vet

Instead of a dose table from the internet, I carried a list with me. Copy it and add your own questions:

• Which insulin, and why this one and not the one next to it at the pharmacy?
• U40 or U100 — which syringe or pen?
• How often to inject, and how to time it with meals?
• If my cat isn’t eating or ate only a little — inject or not?
• If I’m not sure the injection went in, or I missed the time?
• Signs of blood sugar that’s too low, and what to do in the first minutes?
• When is the follow-up visit, and what records should I bring to it?
• How to store the vial, and how long it lasts once opened?
• What must not be changed on my own: the product, the interval, the injection site, the food?

What to Record After Every Injection

Without records, the vet has nothing to adjust the regimen with. After every injection, the same set: the time; the insulin’s name exactly as on the vial; the dose in the vet’s wording; whether the cat ate; the injection site; whether I’m sure the insulin went under the skin; the blood sugar, if I measured it; her behavior over the next few hours. For us, these very records became the basis of the curve the vet used to adjust the dose; we kept them in Diabnostic — the records are always at hand and easy to show to the vet.

Manishka’s story is separate: her story. I won’t promise you can replicate it one to one: every animal’s diabetes runs its own course.

Sources

1. Sparkes A. et al. ISFM Consensus Guidelines on the Practical Management of Diabetes Mellitus in Cats. J Feline Med Surg. 2015;17(3):235–250 — choice of insulin, duration of action in cats, handling of the preparation.
2. iCatCare consensus guidelines on the diagnosis and management of diabetes mellitus in cats. J Feline Med Surg. 2025;27(11) — the table of insulins and concentrations, preparing suspensions for drawing up.
3. AAHA Diabetes Management Guidelines for Cats. J Am Anim Hosp Assoc. 2026;62:65–93 — U-40/U-100 syringes, the ban on changing the dose on your own.