Basal Insulin: What It Is, How It Works, and Which Types Are Available

Medically reviewed by Dr Sultan Linjawi, Endocrinologist & Diabetes Specialist — December 2025

Basal insulin is the background insulin that keeps blood glucose stable between meals and overnight. It works quietly in the background, controlling fasting blood sugar levels when you are not eating. Long-acting insulins such as Lantus, Toujeo, Tresiba and NPH are commonly used to provide this steady insulin coverage.

What is basal insulin?

In someone without diabetes, the pancreas releases small amounts of insulin all day and all night. This constant trickle of insulin keeps blood glucose stable when you are asleep, between meals, or not eating at all.

Basal insulin is designed to replace that background insulin. Unlike mealtime (bolus) insulin, it is not meant to cover food. Instead, it controls fasting glucose levels and reduces glucose released from the liver.

For many people with type 2 diabetes, basal insulin is the first insulin introduced. For people with type 1 diabetes, basal insulin is essential from diagnosis.

Why do some people need basal insulin?

Over time, the pancreas may struggle to produce enough insulin to control blood glucose levels, particularly overnight and before breakfast. This often shows up as a rising fasting blood sugar or HbA1c, even when diet and tablets are optimised.

Basal insulin helps by:

  • Reducing overnight and fasting blood glucose levels
  • Lowering HbA1c without large glucose swings
  • Allowing other medications (such as metformin or GLP-1 medications) to work more effectively

Importantly, starting basal insulin does not mean you have failed. It simply means your body needs additional support.

Types of basal insulin

Basal insulins differ in how long they last, how evenly they work, and how flexible they are with timing. These differences matter in day-to-day life.

Modern long-acting basal insulins

These insulins are designed to provide smoother, more predictable insulin coverage with fewer overnight lows:

Intermediate-acting basal insulin

Older insulins such as NPH tend to have a stronger peak and shorter duration, which can increase the risk of hypoglycaemia overnight.

Types of basal insulin

Basal insulins differ in how long they last, how evenly they work, and how flexible they are with timing. These differences matter in day-to-day life.

Modern long-acting basal insulins

These insulins are designed to provide smoother, more predictable insulin coverage with fewer overnight lows:

Intermediate-acting basal insulin

Older insulins such as NPH tend to have a stronger peak and shorter duration, which can increase the risk of hypoglycaemia overnight.

How long does basal insulin last?

Not all basal insulins last the same length of time:

  • Lantus: around 14-20 hours
  • Toujeo: up to 16-22 hours with a flatter profile
  • Tresiba: over 42 hours, offering the most flexibility
  • NPH: 12–18 hours with a noticeable peak

This is why some insulins are injected once daily, while others may need twice-daily dosing.

Basal insulin vs other insulin types

Basal insulin is only one part of insulin therapy.

  • Bolus (mealtime) insulin covers food and corrects high glucose after meals – for example Fiasp or NovoRapid
  • Mixed insulin combines basal and bolus insulin in one injection – for example Ryzodeg

Some people only need basal insulin. Others require a basal-bolus regimen. The right approach depends on your glucose patterns and lifestyle.

Common questions about basal insulin

When should basal insulin be injected?

This depends on the insulin type. Some are taken in the morning, others at night, and some can be taken at flexible times. Your doctor or diabetes educator will guide this.

How much basal insulin do I need?

Doses are individual and adjusted gradually. This process is called titration and is explained in detail in our basal insulin titration guide.

Does basal insulin cause weight gain?

Weight gain is possible, but modern basal insulins are less likely to cause significant weight gain when used appropriately.

Where does basal insulin fit in diabetes treatment?

Basal insulin is often used alongside lifestyle changes and other medications. It may be introduced before mealtime insulin and can delay or reduce the need for more complex regimens.

If you are newly diagnosed or struggling with control, see our guide to treatment options for type 2 diabetes.