Insulin: How Does It Work?

Medically Reviewed by Jabeen Begum, MD on April 24, 2024
Written by Rita Colorito, Stephanie Watson, Amy Gopal
11 min read

Insulin is a hormone your body makes that helps it control your blood sugar level and metabolism — the process that turns the food you eat into energy.

Your pancreas makes insulin and releases it into your bloodstream. Insulin helps your body use blood sugar, also called blood glucose, for the energy it needs. It also tells your liver to store the rest for later.

For some people, their pancreas can’t make enough insulin, or their body can’t use the insulin it makes the right way. When either happens, it can cause hyperglycemia, or high blood sugar levels, which leads to diabetes.

Insulin injections, or shots, help people with diabetes control their blood sugar levels. Insulin made in the U.S. comes from a laboratory.

After you eat, your intestines break down carbohydrates from food into glucose, a type of sugar. That glucose goes into your bloodstream, raising your blood sugar level.

Your pancreas is an organ that sits just behind your stomach. It releases insulin to control the level of glucose in your blood.

Your body makes and releases insulin in a feedback loop based on your blood sugar level. At its most basic level, it’s similar to your home's heating and cooling system, which releases cool or warm air as the temperatures rise or fall.

High blood sugar stimulates clusters of special cells, called beta cells, in your pancreas to release insulin. The more glucose you have in your blood, the more insulin your pancreas releases.

Insulin helps move glucose into cells. Your cells use glucose for energy. Your body stores any extra sugar in your liver, muscles, and fat cells. Once glucose moves into your cells, your blood sugar level goes back to normal.

Insulin and glucagon

Low blood sugar prompts a different cluster of cells in your pancreas to release another hormone called glucagon.

Glucagon makes your liver break down the stored sugar, known as glycogen, and release it into your bloodstream.

Insulin lowers blood sugar levels. Glucagon helps increase blood sugar levels. Insulin and glucagon alternate their release throughout the day to keep your blood sugar levels steady.

When you have high blood glucose, it can cause problems in your eyes, brain, heart, and kidneys. Over time, high blood sugar can lead to kidney disease and kidney failure.

Insulin is also a general term for many different types of injectable medications that can help control your body’s blood sugar levels and metabolism. Insulin types are grouped based on:

  • Onset, or how quickly they work
  • Peak time, or when they reach maximum strength and are most effective
  • Duration, or how long they last to lower blood sugar

The brand of insulin that your doctor prescribes may differ from another brand in terms of onset, peak, and duration. But in general, they follow the parameters below: 

Rapid-acting insulin

  • Onset: In about 15 minutes
  • Peak: At 1 hour
  • Duration: 2-4 hours

Most people take rapid-acting insulin before a meal and often with a longer-acting insulin.

Common types of rapid-acting insulin include:

  • Insulin aspart (Fiasp and NovoLog)
  • Insulin glulisine (Apidra)
  • Insulin lispro (Admelog, Humalog, Lyumjev)

There’s also a rapid-acting inhaled form of insulin, known as the Technosphere insulin-inhalation system (Afrezza). It starts to work within 10-15 minutes, has a peak time of 30 minutes, and lasts about 3 hours. Like the injectable version, it’s taken before a meal and often with a longer-acting injectable insulin. Adults with type 1 or type 2 diabetes can take the inhaled form. But they still need to take a longer-acting form to control their blood sugar.

Regular or short-acting insulin

  • Onset: 30 minutes
  • Peak: 2-3 hours
  • Duration: 3-6 hours

Human Regular (Humulin R, Novolin R, Velosulin R) is the only type of regular or short-acting insulin. It’s usually taken 30 minutes to 1 hour before a meal. 

Intermediate-acting insulin

  • Onset: 2-4 hours
  • Peak: 4-12 hours
  • Duration: 12-18 hours

NPH (Humulin N, Novolin N, ReliOn) is the only type of intermediate-acting insulin. Taking intermediate-acting insulin controls your blood sugar for half a day or overnight. It’s often taken with a rapid- or short-acting insulin.

Long-acting insulin

  • Onset: 2 hours
  • Peak: No peak
  • Duration: Up to 24 hours

Types of long-acting insulin include:

  • Degludec (Tresiba)
  • Detemir (Levemir)
  • Glargine (Basaglar, Lantus)

People often take these with rapid-acting insulin as needed.

Ultra-long-acting insulin

  • Onset: 6 hours
  • Peak: No peak
  • Duration: 36 hours or more

Glargine U-300 (Toujeo) is the only type of ultra-long-acting insulin.

Premixed insulin

  • Onset: 5 minutes to 1 hour
  • Peak: Varies
  • Duration: 10-16 hours

This type combines short-term and intermediate insulin. If you have trouble reading the dosage directions on your insulin, or if you have trouble drawing insulin from two bottles, premixed insulin is a good option given you can maintain your blood sugar using it. People who use premixed insulin take it 10-30 minutes before breakfast and dinner.

Choosing insulin

Insulin also comes in different strengths. The most common insulin strength is U-100. Your doctor will decide what type, brand, and strength you need based on:

  • Foods you eat
  • How well you can manage your blood glucose
  • How long it takes for your body to absorb insulin and how long insulin stays active in your body
  • Your age
  • Your physical activity
  • Your insurance coverage

You’ll also want to choose how you inject your insulin. Ways to inject insulin include:

Syringes. With syringes, you inject the insulin through a needle, similar to what your doctor uses to draw blood. They come in different sizes, depending on your insulin dosage.

Pens. In this form, you use needle cartridges to inject the insulin. Pens can be easier to use for children or people who have needle anxiety. They come prefilled with a dial to show when the pen is empty.

Insulin pump. This device automatically delivers rapid- or short-acting insulin per hour. It’s about the size of a deck of cards. The device is placed semi-permanently under your skin, either on your belly or the back of your upper arm.

There is no such thing as a normal insulin level. That’s because different people have different insulin needs. How much insulin you need can also change throughout the day, even from hour to hour.

Things that affect your insulin levels include:

  • How often, when, how much, and what you eat, especially carbohydrates
  • If you’re stressed or sick
  • If you’re asleep or awake
  • Other hormones
  • Medications you take, including corticosteroids
  • Your physical activity

But there are normal blood sugar levels. Blood glucose tests, also called diabetes tests, are often part of regular checkups. For people at normal risk of diabetes, doctors may use fasting blood glucose tests. For this, you have to stop eating after midnight before you have your blood drawn. Here’s what the measurements mean:

  • A fasting blood sugar level of 99 milligrams per deciliter (mg/dL) or lower means you have normal blood sugar.
  • A measure between 100-125 mg/dL means you have prediabetes.
  • A measure of 126 mg/dL or higher means you have diabetes.

Your body’s control of blood sugar levels works well when you have a healthy pancreas, but it can break down if you get diabetes. There are two main types of diabetes:

Type 1 diabetes

Type 1 diabetes is an autoimmune disease that often starts in childhood. Your immune system attacks and destroys beta cells in the pancreas that make insulin. Because your pancreas stops making insulin, you need daily insulin injections to help your body use glucose from the food you eat and to keep blood sugar levels steady.

Until the early 20th century, the only way to treat type 1 diabetes was with a strict low-carbohydrate, low-calorie diet. In 1921, Canadian surgeon Frederick Banting and his assistant Charles Best discovered insulin. The introduction of insulin shots as a treatment changed the outlook for people with this disease.

Type 2 diabetes

Type 2 diabetes can affect adults or children. It’s a progressive disease, meaning that it happens over time. Your pancreas can still make insulin but will develop problems releasing insulin. Eventually, this form of diabetes will also make it harder for your cells to use insulin, which is called insulin resistance. Type 2 diabetes is more common in people who are overweight or have obesity.

Some people with type 2 diabetes also need to take insulin shots every day. They can also use diabetes pills to manage their blood sugar levels. Diabetes pills don't contain insulin. That’s because if you were to take insulin in pill form, your body would break it down during digestion. It wouldn’t do anything to control your blood sugar levels.

In both types of diabetes, blood sugar rises. Having high blood sugar for a long period can damage blood vessels and organs such as your eyes, heart, and kidneys. Because your body can't use glucose properly for energy, diabetes can make you feel very tired. Getting your blood sugar levels back to normal helps prevent diabetes complications.

When your cells are starved of insulin for a long period, you can develop a life-threatening complication called diabetic ketoacidosis (DKA).

Other types of diabetes

Women can also get diabetes when they’re pregnant. That’s called gestational diabetes.

You may also have heard of prediabetes and insulin resistance, which can lead to type 2 diabetes (though it’s not too late to stop that from happening through lifestyle changes).

Insulin resistance may make it difficult to lose weight. Your body can’t use the insulin it produces properly. So, there’s excess blood sugar in your bloodstream, and excess insulin tells your liver and muscles to keep storing blood sugar. Eventually, your liver and muscles can’t store any more blood sugar. When that happens, the liver sends the excess blood sugar to fat cells. There they get stored as body fat, contributing to weight gain.

Controlling insulin levels may help with weight loss in people who have diabetes.

Potassium is a mineral your body needs to maintain regular fluid levels inside your cells. But when your blood sugar is too high, and there’s too much glucose outside of the cells, potassium inside the cells moves out. This increases potassium levels in your blood.

To help balance out potassium levels, insulin moves glucose into your cells and moves potassium from the blood back to your cells, lowering potassium levels in your blood.

People with low potassium levels release less insulin. This, in turn, causes blood sugar levels to rise, increasing your risk of having type 2 diabetes.

Insulin shots are necessary to help people with diabetes control their blood sugar levels. But taking too much insulin can cause hypoglycemia, or low blood sugar. It’s the most common complication of insulin treatment. You have low blood sugar when it’s below 70 mg/dL.

Common symptoms of low blood sugar include:

  • Anxiety or nervousness
  • Confusion
  • Dizziness
  • Hunger
  • Irritability
  • Shaking
  • Sweating

Other things that can cause low blood sugar include:

  • Drinking alcohol to excess
  • Certain medications, including other diabetes medicines
  • Exercising too much or overexertion
  • Malnutrition or an eating disorder
  • Serious liver, kidney, or heart diseases
  • Hormone deficiencies
  • Insulin overproduction due to a tumor

If you have low blood sugar, call your doctor. It can be dangerous and needs to be treated as soon as possible.

While rare, you can also have an allergic reaction at the injection site or to the insulin itself. When this happens, you can have pain and burning at the injection site. Your skin may become swollen, discolored, and itchy, with symptoms lasting several hours.

If you get side effects from your insulin shot, talk to your doctor.

In very rare cases, you can develop insulin antibodies, which get in the way of how well the manufactured insulin works. When this happens, you may need to take larger doses of insulin to help control your blood sugar levels.

To get the benefits of insulin treatment, you need to inject insulin into the fat under your skin. This helps it get into your blood where it can help lower blood sugar levels. The best places to inject insulin are in your:

  • Belly, at least 2 inches away from your belly button
  • Back of your upper arms
  • Front or side of your thighs
  • Upper buttocks

But don’t inject your insulin in the same spot each time. Change the arm and the side of your belly you use for each injection. Injecting insulin in the same spot too often can cause hard lumps or fatty deposits to form in these areas. Not only do these lumps and bumps look bad, but they can also make the insulin shot less effective.

Insulin is a hormone made by your pancreas that helps control your blood sugar levels and metabolism -- the process that turns the food you eat into energy. People with diabetes often can’t make enough insulin or their body can’t use the insulin it makes the right way. If you have diabetes, you need manufactured insulin to help control your blood glucose. Everyone should have their levels checked as part of regular health checkups to avoid complications of low blood sugar (hypoglycemia) or high blood sugar (hyperglycemia).

What is the effect of insulin on the body? 

Insulin is a hormone your pancreas makes to help control blood sugar (glucose) levels in your body. When your blood sugar is high, the pancreas releases insulin. Then:

  • Insulin helps move glucose into your cells. Your cells use glucose for energy. Your body stores any extra sugar in your liver, muscles, and fat cells.
  • Once glucose moves into your cells, your blood sugar level goes back to normal.

In a person with diabetes, where their body can’t make or use insulin properly, manufactured insulin can help control their blood sugar levels.

Is insulin bad for your kidneys? 

No, taking insulin can actually help protect your kidney health if you have diabetes. High blood sugar from diabetes leads to kidney damage and disease. So, taking insulin as recommended by your doctor can help manage your blood glucose and stop further damage.