Insulin glargine: what it treats, how to inject it, side effects
Last updated September 3, 2026.
Insulin glargine is a long-acting insulin - a man-made version of the hormone your body uses to move sugar out of the blood. You may know it as Lantus; biosimilars like Semglee and Basaglar work the same way. One injection a day covers your background insulin needs around the clock.
What does it treat?
Type 1 diabetes in adults and children, always paired with a short-acting mealtime insulin, and type 2 diabetes in adults when the body's own insulin plus other medications are not enough.
How do you take it?
One injection under the skin once a day, at the same time every day - any time of day works, but it must be the same time. It goes into the abdomen, thigh, or upper arm, rotating spots within the region; never into a vein or muscle, and never in an insulin pump. Three never-rules come straight from the label: never dilute or mix it with any other insulin or solution, never use it unless the liquid is clear and colorless with no particles, and never share or reuse needles, syringes, or pens. Read the label before every injection - mix-ups between look-alike insulin products are a real error pattern. Dosing is individual: for type 2 diabetes, the label's starting point is 0.2 units per kilogram or up to 10 units once daily, adjusted by your doctor based on your glucose logs. Never inject when your blood sugar is low. Missed dose: this one is planned in advance - MedlinePlus says to ask your doctor before you start exactly what to do if you forget a dose or take an extra one, and to write the directions down.
Side effects to know
- Common: redness, swelling, pain, or itching at the injection site; skin thickening or small dents where you inject (the reason rotation matters); and cold-like infection signs.
- Serious: low blood sugar is the main one - know your symptoms and your treatment plan before the first dose. Allergic reactions mean immediate care: rash, hives, or itching all over the body, wheezing, trouble breathing or swallowing, fast pulse, sweating, and swelling of the eyes, face, lips, tongue, or throat. Weakness, muscle cramps, and an abnormal heartbeat point to low potassium - also immediate.
Who should not take it?
Anyone whose blood sugar is low at that moment - that is the one absolute rule. Past that, it is about the conversation: kidney or liver disease changes dosing, and illness, unusual stress, and changes in diet, exercise, or activity change how much insulin you need - get sick-day rules from your doctor in advance. Alcohol can swing blood sugar, so ask for the explicit rule. Low blood sugar affects your ability to drive; ask your doctor whether you should check before getting behind the wheel. Meals matter: eat about the same amounts of the same kinds of food at about the same times each day, because skipping or delaying meals on insulin causes real problems. If you become pregnant, call your doctor.
When is it an emergency?
Severe low blood sugar - confusion, inability to wake, seizure - is a 911 call, and the people close to you should know it too. Trouble breathing, throat swelling, or a racing pulse after an injection means immediate care. For an overdose, call Poison Control at 1-800-222-1222. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
Why can't I mix insulin glargine with my other insulin in one syringe?
Chemistry. The label is flat: do not dilute or mix insulin glargine with any other insulin or solution - the mixture changes how both insulins behave. Your long-acting and your mealtime insulin are two separate injections, always.
Why does it have to be the same time every day?
Because it is your background insulin. Glargine is built to cover a full 24 hours with a steady profile, so each injection hands off to the last. A drifting schedule creates gaps and overlaps - the label says any time of day is fine, but the same time every day.
How do I store insulin glargine?
Unopened vials and pens live in the refrigerator until the date on the label - never frozen, and insulin that froze and thawed goes in the trash. Once in use: vials keep 28 days at room temperature or in the fridge; pens stay at room temperature and last 28 days after first use. Anything exposed to extreme heat or cold is done. Traveling without a fridge, room temperature and shade works for up to 28 days.
Can I drink alcohol while using insulin glargine?
Ask your doctor - MedlinePlus says alcohol may cause a change in blood sugar, and the change can hit hours after drinking. Whatever rule your doctor sets becomes part of the routine.
What do I do if I forget a dose of insulin glargine?
Follow the plan you wrote down. MedlinePlus's guidance for this insulin is to ask your doctor before you start exactly what to do about a forgotten dose or an accidental extra dose, and to keep the written directions. There is no safe generic rule - the answer depends on your dose, your schedule, and your numbers.
Why do I check the label before every injection?
Because insulin errors are usually swap errors. Several glargine products exist - Lantus, Semglee, Basaglar - plus rapid-acting insulins in similar-looking pens. MedlinePlus says to always check that you have the correct insulin before each dose. Right product, clear and colorless, every time.