Cat insulin falls into 3 duration classes, and the class decides the injection schedule more than the brand does. Most newly diagnosed cats do best on an intermediate or long-acting product given twice daily, 12 hours apart. Which product, what starting amount, and every later change are decisions for your veterinarian: a small excess can pull a cat under 60 mg/dL within 2 hours.
Why a cat is not a small dog
Cats clear insulin quickly and often show a sharp early drop. A suspension that holds a dog for 24 hours may fade in a cat in 10 to 14 hours, which is why twice-daily dosing is the default and why only a full-day curve shows the real duration in your cat. Absorption also shifts with the injection site: the scruff covers a wide area, so pick 2 or 3 fixed spots and rotate between them instead of moving at random.
The 3 duration classes at a glance
| Class | Onset | Peak | Lasts | Typical use in cats | Cost per month (USD) |
|---|---|---|---|---|---|
| Rapid | 10-30 min | 1-2 h | 3-6 h | Clinic only, ketoacidosis | 40-120 |
| Short (regular) | 30-60 min | 2-4 h | 4-8 h | Clinic, rare home use | 25-70 |
| Intermediate (NPH) | 1-2 h | 4-8 h | 8-12 h | Twice daily, 12 h apart | 30-90 |
| Lente or PZI | 1-2 h | 6-10 h | 12-18 h | Twice daily, some once | 35-95 |
| Glargine | 1-3 h | Flat | 16-24 h | Twice daily, gentle curve | 60-160 |
| Detemir | 1-2 h | Flat | 12-20 h | Twice daily, gentle curve | 70-180 |
Read the table as a map, not a prescription. The same product can last 8 hours in one cat and 20 in another, so the label range is a hypothesis your vet tests with a curve. Long-acting products trend toward higher cost but fewer dramatic swings, which matters in a household where nobody is home at midday.
Check 1: the 12-hour glucose curve
A curve means a reading every 2 hours from just before the morning injection to just before the evening one, so 7 readings. Most clinics repeat it 5 to 7 days after any change, then every 3 to 6 months once stable. Common targets are a nadir (lowest point) of 80 to 150 mg/dL with most of the day below 250 mg/dL. A nadir under 80 mg/dL, or a fall of more than 150 mg/dL between 2 readings, means the amount gets reviewed before the next injection.
Check 2: water, weight, litter
Weigh the cat every 2 weeks on the same scale, at the same time of day, and log 24-hour water intake. Normal intake is roughly 45 mL per kg per day, so a 4.5 kg cat drinking 350 mL is not regulated, even if one curve looked acceptable. Note urine clump size in the litter box: 3 or more large clumps a day points the same way. A cat that gains 100 g in 2 weeks while eating well is improving; one that loses 200 g is not.
The syringe trap: 40 versus 100
Insulin is sold at 2 strengths in most markets, 40 units per mL and 100 units per mL, and the syringes are matched to the strength. Drawing a 40-unit syringe from a 100-unit vial gives 2.5 times the intended amount. Keep 1 syringe type per cat, store the box beside the vial, and ask the clinic to print the strength on the label in letters 10 mm high.
Storage numbers worth taping to the fridge
Unopened vials: 2 to 8 C (36 to 46 F) until the printed date. In-use vial: 28 to 42 days at room temperature below 25 C (77 F), then replace even if half full. Never freeze (0 C ruins the protein), never shake (roll 10 times instead), keep it out of sunlight, and never park a pen in the fridge door where it cycles through 0 to 4 C.
Hypoglycemia: the 60 mg/dL line
Below 80 mg/dL, watch for lethargy, head tilt, and twitching. Below 60 mg/dL, treat as an emergency. If the cat is conscious, rub 1 teaspoon of honey or glucose gel on the gums, not down the throat, and call the clinic. If it is unconscious or seizing, go in now and do not wait for a phone call back. Skip the next injection and restart only when the vet clears it.
Call the clinic today if
Glucose stays above 350 mg/dL on 3 consecutive days; urine ketones read above trace; the cat vomits 2 or more times in 24 hours; it refuses food for 24 hours; or it hides and stops grooming for 2 days. Diabetic ketoacidosis can build in 24 to 48 hours and needs IV fluids, not a dose tweak at home.
What a month of supplies costs
Vial 25 to 90 USD, 100 syringes 15 to 35 USD, a glucose meter 30 to 60 USD with strips at 1 to 2 USD each, and a curve visit 80 to 200 USD. Home testing with a meter costs about 30 USD a month and replaces roughly 2 of every 3 clinic curves.
Four steps to run this week
- Write the product class and strength on the box lid, not on a note you will lose.
- Fix 2 injection spots and alternate them; mark the calendar with the side used.
- Start a 14-day log of weight, water, and litter clumps before the next curve.
- Ask for the nadir number at every visit and keep the last 4 in one place.
A cat on insulin is a numbers project: 1 product class, 7 points per curve, 2 weekly measurements, and a clear line at 60 mg/dL. Track those and most problems surface weeks before the cat looks sick. If you are still choosing a clinic, remission is possible in a share of cats and worth asking about at diagnosis, and any suspected toxin exposure goes to the poison list first.
Terms mentioned in this article
Frequently asked
Can I switch my cat to a cheaper insulin on my own?
No. Changing the class changes onset, peak, and duration at the same time, so the old injection schedule and the old curve both stop applying. A switch needs a new starting plan from your veterinarian and a fresh 12-hour curve 5 to 7 days later. Buying a different product online without telling the clinic is the most common route to a severe hypoglycemic episode.
How do I know the insulin still works after 30 days in the fridge?
Look for cloudiness, clumps, or a color shift, and check that the rubber stopper is intact. Beyond 28 to 42 days in use, potency drops even when the liquid looks normal, so replace it on schedule rather than waiting for high glucose to prove the point. Write the opening date on the vial with a marker.
Is one injection a day ever enough?
Rarely. Most cats need 2 injections 12 hours apart because the available products fade in 8 to 18 hours. A once-daily plan is sometimes tried with a long-acting product in a cat with a very flat curve, but that decision follows a 24-hour curve, not a guess, and it needs a vet who will recheck within 7 days.