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How Insulin Works

Reviewed by: HU Medical Review Board | Last reviewed: August 2023

Insulin is a common treatment for people living with type 2 diabetes (T2D). This is because people with T2D slowly lose their ability to produce insulin on their own or cannot effectively use the insulin their body produces (insulin resistance).1,2

How does insulin work?

Insulin is a hormone produced in the pancreas. Insulin helps manage the body’s blood sugar (glucose). Your body naturally contains its own glucose and you get more when you eat.2,3

Insulin transports glucose from the blood to cells throughout the body. The glucose is then used for energy. Without insulin, too much glucose builds up in the blood. Having unhealthily high levels of blood glucose is called hyperglycemia.2,3

What are the different types of insulin?

The insulin US doctors prescribe is formulated in 2 ways:1,2

  • Human insulin – made in a lab to act exactly like human insulin.
  • Analog insulin – also made in a lab. This is very similar to human insulin but with slight changes at the molecular level. These changes make the analog insulin absorb and take effect in your body more quickly.

There are several types of insulin. They vary in how quickly they take effect (onset), when they peak, and how long they continue to act (duration).1-4


Rapid-acting insulin takes effect within 5 to 20 minutes, peaks after 1 hour, and continues to act for 2 to 4 hours. Brands include:2,4

  • Novolog® (insulin aspart)
  • Fiasp® (insulin aspart)
  • Apidra® (insulin glulisine)
  • Admelog® (insulin lispro)
  • Humalog® (insulin lispro)
  • Lyumjev™ (insulin lispro-aabc)
  • Afrezza® (insulin human) inhalation powder

Short-acting (regular)

Regular insulin is a short-acting insulin that takes effect within 30 to 45 minutes, peaks at 2 to 3 hours, and continues to act for 3 to 6 hours. Brands include:2,4

  • Humulin® R
  • Novolin® R


Intermediate-acting insulin takes effect within 2 hours, peaks around 4 to 12 hours, and continues to act for 14 to 24 hours. Brands include:2,4

  • Humulin® N (insulin neutral protamine Hagedorn [NPH])
  • Novolin® N (insulin NPH)
  • ReliOn™ (insulin NPH)


Long-acting insulin takes effect within 1 hour, peaks around 3 to 14 hours, and continues to act up to 24 hours. Brands include:2,4

  • Tresiba® (insulin degludec)
  • Levemir® (insulin detemir)
  • Basaglar® (insulin glargine)
  • Lantus® (insulin glargine)

Ultra long-acting

Ultra long-acting insulin takes effect within about 6 hours. This type of insulin takes longer to reach your bloodstream. Instead of peaking, it steadily provides insulin for up to 2 days. One brand is Toujeo® (insulin glargine).2-4

It is important to note that the times listed above for onset, peak, and duration are averages. That means they can vary from person to person.1-4

In practice, you may take an intermediate- or long-acting insulin to provide general glucose control throughout the day. You might then use a rapid- or short-acting insulin around mealtime. The rapid- or short-acting insulin is added to cover the boost of carbohydrates you get when you eat, which cause blood sugar levels to rise.1-4


Most premixed insulin treatments combine intermediate-acting NPH insulin with either rapid-acting or short-acting insulin. These treatments may allow tighter control of blood sugar in some people. No manual mixing of insulin is needed with this type. But when doses are increased or decreased, both types of insulin are changed. Brands include:3,4

  • Humalog® Mix 50/50
  • Humalog® Mix 75/25
  • Humulin® 70/30
  • Novolin® 70/30

The premixed treatment Novolog® Mix 70/30 does not have NPH insulin. This drug has 70 percent insulin aspart protamine (intermediate-acting) and 30 percent insulin aspart (rapid-acting).5

How do you take insulin?

Taking insulin has advanced in recent years. It is far less difficult, thanks to simpler delivery methods.2,6

If you are prescribed insulin, you will work with your doctor and a registered dietitian to understand the best way to manage your T2D.2-4,6

Insulin is taken a few different ways:1-3,6

  • Subcutaneous injection – Insulin is injected just below the skin (not into the muscle). These can take the form of a vial and syringe or an autoinjector pen.
  • Insulin pump – Insulin is delivered by a small computerized device that attaches to your body (via a catheter). The pump can deliver insulin in 2 ways. It can dispense small doses continuously (“basal” insulin). It can also provide a correction or supplemental (“bolus”) dose when you need it, usually around mealtime.
  • Inhaled insulin – Insulin is inhaled as a powder.

What are the possible side effects?

Side effects can vary depending on the specific insulin you are taking. Side effects may include:2

  • Lumps, redness, swelling, or irritation at the injection site
  • Low blood sugar (hypoglycemia)
  • Weight gain

These are not all the possible side effects of insulin. Talk to your doctor about what to expect when taking insulin. You also should call your doctor if you have any changes that concern you when taking insulin.

Other things to know

The number and frequency of insulin injections will vary depending on a person's needs. For some, insulin treatment may involve multiple daily injections. For others, a treatment approach involving fewer daily injections may be more effective. Some may need a continuous supply of insulin and need to use an insulin pump.1-3

Talk with your doctor, registered dietitian, or a diabetes care and education specialist about the best insulin regimen for you. They will teach you how to take your insulin.

Before beginning treatment for T2D, tell your doctor about all your health conditions and any other drugs, vitamins, or supplements you take. This includes over-the-counter drugs. For more advice and support, reach out to the American Diabetes Association at 1-800-342-2383 or,3

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