Long-acting insulin lasts between 24 and 42 hours depending on the specific type. The three main long-acting insulins on the market each have a different duration: insulin detemir lasts up to 24 hours, insulin glargine lasts 24 to 36 hours depending on concentration, and insulin degludec lasts at least 42 hours. These differences matter for how you time your injections, how much flexibility you have if you miss a dose, and how steady your blood sugar stays throughout the day.
Duration by Type of Long-Acting Insulin
Not all long-acting insulins behave the same way once injected. Here’s how the main options compare:
- Insulin detemir (Levemir): Up to 24 hours of action, with onset at about 1 hour and a broad peak between 3 and 14 hours. At lower doses, the duration can be significantly shorter. In clinical testing, the mean duration ranged from just 5.7 hours at the lowest dose to 23.2 hours at the highest. Many people on detemir need two injections per day to maintain coverage.
- Insulin glargine U-100 (Lantus, Basaglar, Semglee): Approximately 24 hours, which is why it’s typically dosed once daily. It has a relatively flat activity profile without a sharp peak.
- Insulin glargine U-300 (Toujeo): Beyond 24 hours, up to 36 hours. This concentrated formulation produces a smoother, more stable glucose-lowering effect compared to the U-100 version.
- Insulin degludec (Tresiba): At least 42 hours after injection, with a half-life of about 25 hours at steady state. This is the longest-lasting insulin currently available.
Why Some Formulations Last Longer
The difference in duration comes down to how each insulin is engineered to slow its own absorption. When you inject regular insulin under the skin, it enters the bloodstream relatively quickly. Long-acting formulations use clever molecular tricks to delay that process by hours or even days.
Insulin glargine works by forming tiny clumps (called precipitates) at the injection site. The solution is acidic in the pen or vial, but once it hits the neutral pH of your body tissue, it crystallizes into small aggregates that dissolve slowly. The concentrated U-300 version takes this further: because the same number of units is packed into one-third the volume, the clump has a smaller surface area and dissolves even more gradually. That’s why Toujeo outlasts Lantus despite containing the same insulin molecule.
Insulin detemir uses a different approach. It has a fatty acid chain attached to each insulin molecule, which causes the molecules to stick together in larger clusters under the skin and then bind reversibly to a protein called albumin in the bloodstream. Both of these steps slow down how quickly the insulin reaches your cells. The fatty acid chains from neighboring clusters also interact with each other, creating an additional brake on absorption.
Insulin degludec combines both strategies. It forms long chains of molecules (called multi-hexamers) at the injection site that break apart extremely slowly, and it also binds to albumin in the blood. The result is an ultra-long duration that builds up a stable depot of insulin in your body over several days of dosing.
Timing Flexibility and Missed Doses
The longer an insulin lasts, the more forgiving it is if your schedule slips. Insulin glargine U-100 lasts roughly 24 hours, so timing doesn’t need to be locked to morning or evening. You can take it at whatever time fits your routine, as long as you’re reasonably consistent from day to day.
Ultra-long formulations like degludec and glargine U-300 offer even more flexibility. If you miss your usual time, you generally have a window of 3 to 8 hours to take the dose without losing background insulin coverage or risking a double-up effect with your next scheduled dose. This can be a real advantage for people with unpredictable schedules, shift workers, or anyone who occasionally forgets a dose.
If you’re on insulin detemir at a moderate dose, the effective window may be shorter than 24 hours. That’s why some people split it into two daily injections, roughly 12 hours apart, to avoid gaps in coverage.
How Dose Size Affects Duration
One detail that catches people off guard: the dose you inject can change how long the insulin actually works. This is most clearly documented with insulin detemir, where clinical data showed the duration nearly quadrupled between the lowest and highest tested doses (from about 6 hours to over 23 hours). At smaller doses, the depot under the skin is smaller and gets absorbed faster.
This effect exists to some degree with all long-acting insulins, though it’s less dramatic with glargine and degludec. If you’ve recently had your dose reduced and notice your blood sugar creeping up toward the end of the day, the shorter effective duration at a lower dose could be a factor worth discussing with your care team.
What “Flat” and “Peakless” Really Mean
You’ll often hear long-acting insulins described as “peakless,” but that’s a simplification. Insulin detemir has a noticeable peak somewhere between 3 and 14 hours. Glargine U-100 is flatter but still has a mild rise in activity. Glargine U-300 and degludec come closest to a truly flat profile, delivering a more even level of insulin around the clock.
A flatter profile means fewer unexpected dips in blood sugar overnight or between meals. It also means basal insulin is doing what it’s supposed to do: quietly keeping your liver’s glucose output in check while you sleep, work, and go about your day. The mealtime spikes are a separate job, handled by rapid-acting insulin or oral medications depending on your treatment plan.
Storing Your Insulin Pen or Vial
Long-acting insulin that you’re actively using can stay at room temperature (between 59°F and 86°F) for up to 28 days. After that, potency starts to decline and the insulin should be discarded even if there’s liquid left. Unopened pens or vials should stay refrigerated until you’re ready to use them. Injecting cold insulin can be uncomfortable, so many people leave their current pen out at room temperature for convenience, which is perfectly fine within that 28-day window.