Insulin: what it does, why it’s prescribed, and how it’s used in diabetes

Insulin is a hormone that allows glucose (sugar) to move from the bloodstream into the body’s cells, where it can be used for energy. When insulin is absent, insufficient, or not working effectively, blood glucose levels rise — leading to diabetes.

In people with type 1 diabetes, the body no longer produces insulin at all. In people with type 2 diabetes, insulin is still made, but it may not work properly (insulin resistance), and over time the body may not produce enough to keep blood glucose under control. In both situations, insulin therapy may be required to protect long-term health.

Chemically, insulin is a protein made up of amino acids. Because proteins are broken down by stomach acid and digestive enzymes, insulin cannot be taken as a tablet — it would be destroyed before reaching the bloodstream. This is why insulin must currently be given by injection or infusion under the skin.

How insulin works in the body

Once injected under the skin, insulin is absorbed into the bloodstream and acts like a “key,” unlocking cells so glucose can enter. Different insulin preparations are designed to work at different speeds and for different lengths of time, allowing treatment to be tailored to how the body normally releases insulin.

Basal (long-acting) insulin

Basal insulin provides a steady background level of insulin throughout the day and night. It is designed to control blood glucose between meals and overnight.

Examples include Lantus (insulin glargine) and Toujeo. These insulins last around 24 hours or longer and are usually injected once daily. Choosing the best time to take basal insulin and adjusting the dose carefully is important for stable control.

Rapid-acting (mealtime) insulin

Rapid-acting insulins are taken before or with meals to control the rise in blood glucose after eating. These insulins work quickly and last for only a few hours.

An example is Fiasp (fast-acting insulin aspart), which begins working within minutes. These insulins are commonly used in people with type 1 diabetes and in some people with type 2 diabetes who require more intensive treatment.

Combination insulins

Some insulins combine a long-acting component with a rapid-acting component in a single injection. These are designed to simplify treatment for people who need both background and mealtime insulin.

Examples include Ryzodeg, which combines insulin degludec with insulin aspart, and Xultophy, which combines insulin with a GLP-1 medication.

A brief history of insulin

Insulin was first discovered in 1921 through research involving dogs, which demonstrated that a substance produced by the pancreas could lower blood glucose. This discovery transformed diabetes from a fatal condition into a manageable one almost overnight.

Early insulin treatments were extracted from animal sources — initially cows and pigs. Pharmaceutical companies such as Eli Lilly and the organisation that would later become Novo Nordisk helped scale insulin production worldwide.

Over time, insulin technology advanced significantly. The development of human insulin and later insulin analogues allowed insulin to act more predictably, last longer, and reduce the risk of hypoglycaemia. Modern insulins are far safer, more consistent, and easier to use than early preparations.

Is insulin safe?

When used correctly, insulin is a safe and life-saving medication. The most common side effect is low blood glucose (hypoglycaemia), which can usually be prevented with appropriate dosing, monitoring, and education. You can learn more about treating low blood glucose safely in our guide on treating a hypo.

Weight gain can occur with insulin, particularly if doses are higher than needed. Modern treatment approaches focus on balancing insulin with diet, physical activity, and — where appropriate — other medications.

Common questions about insulin

How much insulin do I need?

There is no single “right” dose of insulin. Dosing depends on body weight, insulin sensitivity, food intake, physical activity, and blood glucose patterns. This is why insulin doses are usually adjusted gradually, as explained in our dose titration guide.

How do I inject insulin?

Most people inject insulin using a pen device with a very fine needle. Insulin is injected under the skin, usually into the abdomen, thigh, or upper arm. Storage and handling are also important — learn more about how to store insulin correctly.

Do all people with type 2 diabetes need insulin?

No. Many people with type 2 diabetes can manage their condition for years without insulin. However, insulin may be required if other treatments are no longer sufficient. We explain this in more detail in managing diabetes with insulin — is it necessary?

Explore insulin-related topics

This page provides an overview of insulin and how it is used. For more detailed guidance, you may wish to explore:

• Basal insulin dosing and timing (Lantus and Toujeo)
• Rapid-acting insulin for meals
• Combination insulins such as Ryzodeg and Xultophy
• Insulin pumps and advanced insulin delivery systems

Additional in-depth insulin guides — including insulin and illness, insulin during pregnancy, and preventing overnight hypoglycaemia — are planned and will be added here as they are published.