Insulin glargine is a long-acting, synthetic version of human insulin used to manage blood sugar in people with diabetes. It works as a “basal” insulin, meaning it provides a steady, low level of insulin throughout the day and night, mimicking the background insulin that a healthy pancreas releases between meals and during sleep. It’s approved for adults and children (age 6 and older) with type 1 diabetes, and for adults with type 2 diabetes.
How Insulin Glargine Works
When you inject insulin glargine under the skin, it forms a tiny solid deposit at the injection site. This happens because the solution is slightly acidic, and when it meets your body’s neutral pH, the insulin molecules clump together into a compact mass. Your body then slowly dissolves this deposit over many hours, releasing small, consistent amounts of insulin into your bloodstream.
This slow-release design is what makes glargine different from rapid-acting insulins you’d take at mealtimes. It doesn’t spike and fade. Instead, it provides a flat, reliable level of background insulin for roughly 24 hours, with an onset of about 2 to 4 hours after injection and no pronounced peak. That peakless profile is one of its most important features, because it reduces the risk of blood sugar dropping dangerously low in the middle of the night.
Brand Names and Formulations
Insulin glargine is sold under two primary brand names: Lantus and Toujeo. While both contain the same insulin molecule, they differ in concentration and behavior.
- Lantus (U-100): Contains 100 units of insulin per milliliter. This is the standard formulation and has been available since 2000.
- Toujeo (U-300): Contains 300 units per milliliter, meaning the same dose fits into one-third the injection volume. The smaller, more compact deposit under the skin dissolves more slowly, giving Toujeo a flatter and longer-lasting activity profile that extends beyond 24 hours.
These two formulations are not interchangeable. Because the concentrated version releases insulin differently, switching between them requires a dose adjustment supervised by your prescriber. The units used for Toujeo are also specific to that product and not directly equivalent to units of other insulins.
Why It Replaced Older Basal Insulins
Before glargine, the standard long-acting option was NPH insulin, an intermediate-acting insulin with a noticeable peak a few hours after injection. That peak created a practical problem: it increased the chance of blood sugar dropping too low, especially overnight while sleeping.
Head-to-head comparisons show the difference clearly. In studies of people with type 2 diabetes, glargine reduced the risk of severe hypoglycemia by 46% and severe nighttime hypoglycemia by 59% compared to NPH. Nearly 25% more patients on glargine reached their blood sugar target (an A1C of 7% or below) without experiencing nighttime low blood sugar. For people with type 2 diabetes taking glargine alongside an oral medication, the rate of symptomatic nighttime lows was about a third of what it was with NPH (1.1 versus 3.1 episodes per patient per year). These advantages made glargine the default basal insulin for most people with diabetes.
Who Uses Insulin Glargine
People with type 1 diabetes need insulin glargine (or another basal insulin) as part of their daily regimen, typically alongside rapid-acting insulin taken at meals. Since the pancreas produces no insulin in type 1 diabetes, basal insulin covers the body’s constant background need for insulin between meals and overnight.
For people with type 2 diabetes, glargine often enters the picture when oral medications and lifestyle changes aren’t keeping blood sugar in range. A single daily injection of basal insulin can bring fasting blood sugar under control, and many people stay on their oral medications alongside it. Children under 6 with type 1 diabetes and children with type 2 diabetes don’t have established safety data for glargine, so it isn’t approved for those groups.
How It’s Taken
Insulin glargine is injected once daily under the skin, usually at the same time each day. Common injection sites include the abdomen, thigh, and upper arm. Rotating the specific spot within those areas helps prevent the skin from thickening or developing lumps over time, which can interfere with insulin absorption.
It should never be mixed in the same syringe with other insulins, and it’s not used in insulin pumps. The injection itself takes only a few seconds with a pen device or syringe.
Storage and Shelf Life
Unopened insulin glargine should be kept in the refrigerator between 36°F and 46°F, where it stays good until the expiration date printed on the package. Once you start using a vial or pen, it can be kept at room temperature (between 59°F and 86°F) for up to 28 days. After that, it should be discarded even if there’s insulin left. Heat degrades insulin quickly, so leaving it in a hot car or direct sunlight can make it ineffective well before the 28-day mark.
Side Effects
The most common side effect of any insulin, including glargine, is hypoglycemia, or low blood sugar. Symptoms include shakiness, sweating, confusion, a fast heartbeat, and irritability. While glargine’s flat activity profile makes severe lows less likely than with older insulins, they can still happen, particularly if you skip a meal, exercise more than usual, or take too high a dose.
Injection site reactions like redness, swelling, or itching occur in some people but are usually mild and temporary. Weight gain is another common effect of insulin therapy in general, since insulin helps the body store glucose that was previously being lost in urine. Rare but serious allergic reactions are possible and would show up as a rash over the whole body, difficulty breathing, or a rapid drop in blood pressure.
What Makes It Different From Other Insulins
Insulin comes in several categories based on how fast it works and how long it lasts. Rapid-acting insulins start working in minutes and cover meals. Intermediate-acting NPH lasts about 12 to 16 hours and has a peak. Insulin glargine sits at the long end of the spectrum: 2 to 4 hours to start, no peak, and a full 24 hours of coverage (longer for Toujeo). This makes it specifically a basal insulin, not a mealtime one.
Other long-acting options like insulin detemir and insulin degludec fill a similar role but have slightly different duration profiles and dosing schedules. Degludec, for instance, lasts beyond 42 hours and offers even more flexibility in injection timing. Your prescriber chooses among these based on your blood sugar patterns, lifestyle, insurance coverage, and how your body responds.