Insulin starts lowering blood sugar in most cats within about 1 to 2 hours of injection, but the type of insulin your cat is on determines when it hits peak effect and how long it lasts. The full picture matters more than onset alone, because the timing of peak action is what shapes your cat’s daily routine, feeding schedule, and risk of low blood sugar.
Onset, Peak, and Duration by Insulin Type
Four insulin types are commonly prescribed for cats, and each has a distinct timing profile. The differences can be significant, so it helps to know exactly what your cat is receiving.
Glargine (Lantus) begins working roughly 1.3 hours after injection. In a controlled study comparing it with another long-acting insulin, glargine’s onset averaged about 1 hour and 18 minutes. Peak blood sugar lowering occurs around 5 to 12 hours after the shot, depending on the dose and individual cat. In healthy cats given a single daily dose, blood sugar reached its lowest point at an average of about 12 hours. In half the cats studied, blood sugar hadn’t returned to baseline even after 24 hours, meaning the effect can last a full day or longer. Glargine is typically given every 12 hours.
Detemir (Levemir) has a slightly slower start, with onset averaging about 1.8 hours. Peak action comes at roughly 7 hours, and the total duration averages around 13.5 hours. Like glargine, it’s usually given twice daily.
ProZinc (protamine zinc insulin, or PZI) reaches its peak effect at about 6 hours on average, though the range spans 3 to 9 hours. The glucose-lowering effect lasts a minimum of about 9 hours in most cats after the first injection. It’s dosed every 12 hours.
Vetsulin (porcine lente insulin) is the fastest to peak, hitting its lowest blood sugar point anywhere from 2 to 8 hours after injection. Its total duration is shorter too, lasting roughly 8 to 14 hours. Some cats on Vetsulin need injections every 8 to 12 hours to maintain coverage.
What “Working” Actually Looks Like
It’s important to separate what happens in the first hours from what you’ll notice at home. Insulin begins pulling blood sugar down within the first couple of hours, but you won’t see obvious changes in your cat’s behavior that quickly. The clinical signs of diabetes, such as excessive thirst, frequent urination, and weight loss, improve gradually over days to weeks of consistent treatment, not minutes after a single shot.
If your cat was recently diagnosed, the first one to two weeks of insulin therapy are largely about finding the right dose. Your veterinarian will likely want to see a blood glucose curve, a series of blood sugar readings taken every 1 to 2 hours over a 12-hour period, to map how your cat’s body responds. The lowest reading on that curve (called the nadir) tells your vet whether the dose is too high, too low, or in the right range. One important wrinkle: blood sugar curves can vary quite a bit from day to day in the same cat, even with the same dose and feeding schedule. This variability is well documented, so a single curve doesn’t always tell the whole story.
Why the Same Insulin Works Differently in Different Cats
Two cats on identical doses of the same insulin can have very different responses. Several factors explain this.
Obesity increases insulin resistance, meaning the insulin your cat receives has a harder time doing its job. Overweight cats often need higher doses or take longer to reach good blood sugar control. On top of that, persistently high blood sugar and high blood fat levels create a cycle called glucose and lipid toxicity. These elevated levels make insulin resistance worse, which in turn keeps blood sugar and blood fats high. The encouraging part is that this cycle is at least partly reversible. As insulin treatment brings blood sugar down, the cat’s body gradually becomes more sensitive to insulin again, so the medication works better over time.
This is one reason your vet may start with a conservative dose and increase it slowly. A dose that seems insufficient in week one may become more effective by week three as your cat’s metabolism stabilizes.
Feeding and Injection Timing
The recommended approach is to feed your cat just before each insulin injection. This ensures food is already in the stomach when insulin starts working, which helps prevent blood sugar from dropping too low. Most cats on twice-daily insulin follow a simple rhythm: meal, then shot, roughly every 12 hours.
Consistency matters as much as timing. Keeping meals the same size at the same times, giving insulin at regular intervals, and minimizing disruptions to your cat’s routine all contribute to more predictable blood sugar levels. Cats that eat erratically or skip meals are at higher risk for dangerous blood sugar drops.
Recognizing Low Blood Sugar
Because insulin’s peak effect can occur several hours after injection, low blood sugar (hypoglycemia) doesn’t always happen right after the shot. With glargine, the window of greatest risk is roughly 5 to 14 hours post-injection. With Vetsulin, it can be as early as 2 hours.
In a study of 30 hypoglycemic episodes in diabetic cats, the most common signs owners noticed were inability to walk, lethargy, weakness, failure to respond when called or touched, and muscle twitching. Less common signs included vocalization, seizures, vomiting, panting, and drooling. Nearly half the cats in that study were already in a state of severe neurologic dysfunction by the time their owners recognized something was wrong. The early warning signs, like trembling and restlessness, were reported by fewer than half of owners, probably because they’re subtle and easy to miss in cats.
The most common triggers were a cat eating less food than usual or being accidentally double-dosed. If your cat skips a meal or eats significantly less than normal, contact your vet before giving the next dose.
Tracking Insulin’s Effect at Home
Home blood glucose monitoring has become far more practical with flash glucose monitors like the FreeStyle Libre. The small sensor attaches to the cat’s skin (often on the neck or between the shoulder blades), reads glucose levels from the fluid under the skin, and can stay in place for up to 14 days without needing calibration. In a study of 20 cats, the sensor started producing readings within 60 minutes of application, and most owners found it quick and painless to apply.
These monitors are accurate enough for day-to-day diabetes management, but they do have a built-in lag. The sensor measures glucose in the tissue fluid, not directly in the blood, and tissue readings can trail behind actual blood sugar by up to 30 minutes during rapid changes. During stable or slowly changing blood sugar periods, the sensor closely tracks actual blood glucose, though it tends to read slightly lower (by about 23 mg/dL on average). If the sensor shows a reading in the low or hypoglycemic range, it’s worth confirming with a fingerstick-style blood glucose meter before making dose changes.
The real advantage of continuous monitoring is seeing the full shape of your cat’s glucose curve over several days and nights, including the exact timing of the nadir and how long the insulin effect lasts. This gives your vet far more data than a single in-clinic glucose curve, and it captures the natural day-to-day variation that a one-time test would miss.