Long-acting insulin is a type of insulin you inject once or twice a day to provide a steady, low level of insulin around the clock. It mimics what a healthy pancreas does naturally: releasing small amounts of insulin continuously between meals and overnight to keep blood sugar stable. This background insulin supply is also called basal insulin, and it forms the foundation of insulin therapy for both type 1 and type 2 diabetes.
How Long-Acting Insulin Works
A functioning pancreas produces insulin in two ways. It releases a constant trickle of insulin throughout the day and night, and it sends out larger bursts when blood sugar rises after eating. Long-acting insulin replaces that first job, the constant trickle. It doesn’t handle the spikes from meals, which is why some people also need a rapid-acting insulin at mealtimes.
What makes long-acting insulin “long-acting” is its chemistry. Each type uses a different trick to slow its absorption from the injection site into the bloodstream. Insulin glargine forms tiny crystals under the skin that dissolve gradually. Insulin detemir binds to a protein called albumin near the injection site, which slows its release. Insulin degludec takes a different approach: after injection, its molecules link together into long chains called multi-hexamers. As zinc slowly diffuses away from those chains, individual insulin molecules break free and enter the bloodstream one by one. The result in all three cases is the same: a slow, steady supply of insulin rather than a sudden flood.
Types Available Today
Several long-acting insulins are on the market, each with slightly different timing profiles.
- Insulin glargine U-100 (Lantus) is the most widely known. It begins working about 2 hours after injection, has no pronounced peak, and lasts up to 24 hours. Two biosimilar versions, Semglee and Rezvoglar, are FDA-designated as interchangeable with Lantus.
- Insulin glargine U-300 (Toujeo) is a more concentrated version. It provides a flatter, longer profile than U-100 glargine, lasting up to about 36 hours. It comes only in prefilled pens with a maximum dose of 80 units per injection.
- Insulin degludec (Tresiba) is classified as ultra-long-acting. It takes about 6 hours to start working, has no peak, and lasts 36 hours or longer. The U-200 concentration can last up to 42 hours, and its pen allows up to 160 units per injection.
Insulin detemir (Levemir), which was once a popular option, was discontinued in the U.S. as of December 31, 2024. If you were previously on Levemir, your provider has likely switched you to one of the alternatives above.
How It Differs From Other Insulins
The key distinction is timing. Rapid-acting insulin starts working within minutes and is used to cover meals. Short-acting (regular) insulin works within 30 minutes and also covers food. Long-acting insulin operates on a completely different schedule, providing background coverage that keeps blood sugar from climbing between meals and while you sleep.
Older intermediate-acting insulin (NPH) served a similar basal role but has a noticeable peak several hours after injection, which raises the risk of blood sugar dropping too low, particularly overnight. Long-acting analogs like glargine and degludec were specifically designed to avoid that peak. Clinical trials have shown they modestly reduce the risk of nocturnal hypoglycemia compared to NPH.
Timing and Dosing Basics
Most long-acting insulins are injected once daily, ideally at the same time each day. Some people take their dose in the morning, others at bedtime. Consistency matters because it keeps insulin levels as steady as possible over each 24-hour cycle. Ultra-long-acting options like Tresiba offer more flexibility: because their effects last well beyond 24 hours, a few hours’ variation in your injection time is less likely to cause a gap in coverage.
Your dose is determined by your blood sugar patterns, not by a one-size-fits-all number. Providers typically start with a conservative dose and adjust upward every few days based on fasting blood sugar readings, the number you see first thing in the morning before eating.
Injection Sites and Rotation
Common injection areas include the abdomen, thighs, and backs of the upper arms. Rotating your injection site within each area is important because repeated injections in the exact same spot can cause a buildup of fatty tissue under the skin. That buildup changes how quickly insulin absorbs, which can make your blood sugar less predictable.
A practical approach: use one area for your morning injection and a different area for your evening injection if you take two doses. Many people find it easiest to inject in their thigh in the morning while getting dressed and use their abdomen or arm in the evening. Avoid injecting into a muscle group you’re about to exercise, since increased blood flow to that area can speed up absorption and cause a blood sugar drop.
Side Effects
The most common side effect of any insulin is hypoglycemia, when blood sugar drops too low. Symptoms include dizziness, confusion, shakiness, and vision changes. Long-acting insulin carries a lower risk of sudden hypoglycemia than rapid-acting types because it enters the bloodstream gradually rather than all at once, but the risk still exists, especially if you skip a meal or are more physically active than usual.
Weight gain is another common effect. Insulin helps your body store glucose, and over time that can translate to added pounds. Injection site reactions like redness, bumps, or mild swelling are possible but usually temporary, especially when you rotate sites consistently.
Storage and Handling
Unopened insulin pens and vials should be kept in the refrigerator between 36°F and 46°F. Once you start using a pen or vial, it can stay at room temperature (59°F to 86°F) for up to 28 days. After that, it should be discarded even if insulin remains. Never freeze insulin, and avoid leaving it in a hot car or in direct sunlight, as heat breaks down the protein and makes it less effective.
Once-Weekly Insulin
The FDA approved insulin icodec (brand name Awiqli) for adults with type 2 diabetes, with a projected U.S. launch in the second half of 2026. It’s injected once a week on the same day using a prefilled pen, which could simplify routines for people who find daily injections burdensome. It is not currently approved for type 1 diabetes or for children and adolescents. The FDA declined a type 1 indication due to concerns about a higher risk of severe hypoglycemia in that population.