What are the Side Effects of Taking Insulin?

At least 7.4 million Americans with diabetes use one or more formulations of insulin. Men and women with type 1 diabetes need to take insulin every day in order to manage their condition.

If you, or someone you know, need insulin, it’s useful to learn everything you can about it. That also includes potential side effects. Adverse reactions of insulin are the focus of this post. Scroll down to learn more about insulin side effects and more.

What is insulin?

Insulin is a hormone essential for life. The pancreas, a gland behind your stomach, produces insulin. Islets (more specifically, beta-cell), clusters of cells in the pancreas, produce insulin and determine the right amount based on blood sugar levels in the body.

The main function of this hormone is to allow the body to use blood glucose for energy. Glucose is a form of sugar present in carbohydrates. 

Insulin allows cells in muscles and other tissues to absorb glucose from the blood. Without insulin, blood sugar can’t enter these cells. 

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How does it work?

Well, after a meal, your digestive tract breaks down carbohydrates. The carbs are converted into glucose. Then the absorption of glucose into the bloodstream ensues through the lining in the small intestine. When glucose is in the blood, insulin signals cells to absorb it for energy.

We can easily compare insulin to a ticket without which you can’t go in and see a movie or a concert. Problems with insulin, such as insulin resistance, don’t allow glucose to enter cells. This leads to high blood sugar, diabetes, and other problems.

Additionally, insulin secretion promotes the balance of blood glucose. When there’s excess blood sugar, insulin starts signaling the body to store surplus glucose in the liver. Once stored, glucose is not released until the blood sugar level in the body declines. This usually happens between meals or when your energy levels are low. 

This hormone acts on other metabolic processes too. For instance, insulin participates in the breakdown of fat and protein.

Unfortunately, many people experience problems with insulin production and sensitivity. Diabetes is a common condition. There are two different types of diabetes:

  • Type 1 – an autoimmune disease. In this case, the body can’t produce insulin on its own. This happens because the immune system destroys insulin-producing cells in your pancreas.

  • Type 2 – occurs when the body becomes resistant to insulin’s effects. In other words, the body requires more insulin to achieve the same effects. To meet the body’s needs, the pancreas starts overproducing insulin. Eventually, insulin-producing cells may burn out.

Balance of insulin is crucial for our health and wellbeing. Changes in insulin levels, whether low or high, can cause various symptoms. If these problems persist, a person is at a higher risk of developing serious health problems.

What is insulin used for?

Interestingly, the first type of insulin administered to humans was animal insulin.

Insulin is primarily prescribed to people with type 1 diabetes because their body doesn’t produce this hormone. However, sometimes a doctor may recommend a person with type 2 diabetes to take insulin as well. That happens because their pancreas doesn’t produce enough insulin. 

Interestingly, 40% of people with type 2 diabetes initially turn down their doctor’s recommendation to start insulin therapy. A study from Diabetic Medicine found individuals with uncontrolled type 2 diabetes who declined insulin subsequently had worse glycemic control. Most people start with medications first, including insulin secretagogues. 

Insulin medication is also used for diabetic ketoacidosis, a serious and life-threatening problem that affects people with diabetes. The problem occurs when the body starts breaking down fat too fast.

Why do we need insulin therapy?

Insulin therapy can be lifesaving. After all, it is vital to get blood sugar levels under control. High blood glucose can lead to excessive urination, fatigue, thirst, and other problems.

In men and women with diabetes mellitus, persistently high blood sugar can damage nerves and cause diabetic neuropathy. It can also damage kidneys, vision, blood vessels and trigger kidney failure, stroke, and a heart attack.

Also, some diabetes medications can lead to lactic acidosis, a serious problem where a liver is unable to remove excess acid from the body.

Human insulin belongs to the class of medications we call hormones. The main goal of insulin treatment is to make sure the insulin you inject takes the place of insulin produced by the body. Then, insulin works to move sugar from the blood into other tissues. Additionally, insulin also stops the liver from producing too much sugar.

Three main types of insulin therapy

What most people don’t know is that there are several types of insulin therapy. A patient can take different forms of insulin depending on how long they need the hormone’s effects to last.

  • Fast-acting insulin – absorbed quickly. People use it primarily to correct hyperglycemia (high blood sugar). Rapid-acting insulin can also help prevent spikes in blood glucose after meals. This type of insulin includes rapid-acting insulin analogs and the regular insulin. Rapid-acting insulin analogs take five to 15 minutes to work. The onset of the regular hormone insulin is from about 30 minutes to an hour, and its effects may last up to eight hours. Humulin R is a well-known short-acting insulin.

  • Intermediate-acting insulin – enters the bloodstream at a slower rate than fast-acting insulin, but its effects last longer. It is particularly effective for managing blood glucose levels overnight or between meals. Options for this type of insulin therapy include NPH human insulin and pre-mixed insulin. The NPH insulin takes between an hour and two hours to work. It reaches its peak in four to six hours. But, its effects may last over 12 hours. Pre-mixed insulin is a combination of NPH and fast-acting insulin. Therefore, its effects are a mixture of fast- and intermediate-acting insulins.

  • Long-acting insulin – reaches the bloodstream slowly and has a relatively low peak. However, long-acting insulin has a stabilizing plateau effect on blood glucose. This effect can last for most of the day. This type of insulin therapy is beneficial for fasts, between meals, and overnight. The only available type of this therapy is long-acting insulin analogs such as Lantus and Semglee. It takes between an hour and a half and two hours for them to start working. Their effects may last 12 to 24 hours, depending on the brand.

Other options for insulin therapy

In most cases, insulin therapy involves injecting the hormone with a syringe. Other options are also available, besides insulin syringe, such as:

  • Insulin pen – looks like a large writing pen, prefilled, prevents overdosing.

  • Insulin pumps – attached to a thin tube implanted under the skin. The pump is computerized or motorized and delivers insulin before each meal along with small amounts throughout the day. In the United States, about 60% of people with diabetes use some form of insulin pump.

  • Jet injection – holds several doses of insulin. A patient places the jet injection against the skin presses the button, and insulin is pushed through.

  • Inhaled insulin – comes in a premeasured inhaler. The short-acting insulin therapy of this kind is usually not covered by insurance.

What are the side effects of insulin?

Any medication or therapy we take carries a certain risk of side effects. Insulin is not an exception. The most important thing to do is to adhere to the doctor’s recommendations. Regardless of the form of therapy (injection, pen, or pump), a healthcare professional will show you how to administer it. 

When it comes to injections, you can administer insulin in several body parts such as:

  • Upper arms

  • Thighs

  • Abdomen

  • Buttocks

Although the abdomen is an easy injection site, you should avoid injecting insulin within two inches of your belly button because the body can’t absorb it that well.

Also, you should vary the locations of insulin injections to prevent thickening of the skin.

The use of insulin varies according to a patient’s blood sugar levels and diabetes management. For example, your doctor may recommend administering insulin 60 minutes before a meal or right before eating. The amount of insulin you need during the day depends on several factors:

  • Diet

  • Physical activity levels

  • Severity of diabetes

Some diabetic patients may need a single insulin shot per day, while others may need three or four.

The more you adhere to the doctor’s recommendations, the safer you’ll be. But, keep in mind insulin may induce some adverse reactions. You may also experience weight loss before you start with this therapy.

More common side effects

The most common adverse reactions occurring with insulin regular human use include:

  • Weight gain

  • Swelling of arms and legs

  • Injection site reactions such as redness, swelling, and itching

  • Skin changes at the injection site (lipodystrophy) such as shrinking or thickening at the injection sites

  • Low blood sugar (hypoglycemia) indicated by symptoms such as sweating, shakiness, dizziness and/or lightheadedness, hunger, rapid heart rate, slurred speech, blurred vision, anxiety, irritability, mood changes, and tingling in hands, feet, tongue, or lips

Serious side effects

Serious side effects of insulin are:

  • Swelling of hands and feet

  • Severe hypoglycemia or low blood sugar levels indicated by the following symptoms:
    • Confusion (including delirium)
    • Mood changes (irritability, impatience, sadness, stubbornness, anger)
    • Loss of consciousness
    • Seizures
    • Nightmares
    • Impaired coordination 
    • Impaired or blurred vision
    • Weakness and/or fatigue
    • Headaches
    • Sleepiness
    • Tingling or numbness in lips and tongue

  • Hypokalemia (low blood potassium levels), characterized by:
    • Weakness
    • Tiredness
    • Constipation
    • Muscle cramps
    • Heart rhythm problems
    • Breathing problems

  • Allergic reaction with the following symptoms:
    • Feeling faint
    • Rash throughout the body
    • Sweating
    • Rapid heart rate
    • Difficulty breathing 

  • Heart failure indicated by:
    • Shortness of breath
    • Sudden weight gain
    • Swelling of feet and ankles 

Insulin interaction with other medications

Insulin can interact with several medications. Every drug interaction may have its unique effects. In other words, insulin interacting with different classes of drugs may not have the same side effects. For example, some interactions interfere with how well a drug works. Others can amplify adverse reactions to the drug.

Before you start with insulin therapy, you should inform your doctor about the medications you’re taking. 

What can insulin interact with?

  • Other diabetic drugs such as thiazolidinediones (pioglitazone and rosiglitazone). Interaction with these drugs can cause fluid retention and heart failure. Insulin can also interact with pramlintide and cause low blood sugar.

  • Antidepressants such as fluoxetine and monoamine oxidase inhibitors (MAOIs), low blood sugar may occur.

  • Drugs for high blood pressure such as enalapril, lisinopril, captopril, losartan, valsartan, propranolol, metoprolol, all of which may lead to low blood pressure. On the flip side, insulin can also interact with diuretics. In this case, high blood sugar may occur.

  • Drugs for heart rate disorders such as disopyramide – may cause low blood sugar.

  • Drugs to treat cholesterol e.g., niacin which may cause high blood sugar levels

What else can insulin interact with?

  • Drugs for pain such as salicylates

  • Blood thinners such as pentoxifylline

  • Drugs for asthma or allergies e.g., corticosteroids, sympathomimetic agents

  • Hormones in birth control such as estrogens and progesterone

  • Drugs used for HIV treatment such as protease inhibitors (ritonavir and saquinavir)

  • Medications for psychiatric disorders (olanzapine, clozapine, phenothiazines)

  • Drugs for tuberculosis (isoniazid)

  • Some antibiotics (pentamidine, sulfonamide antibiotics)

  • Drugs for heart disorders (beta-blockers, clonidine, reserpine, guanethidine)

  • Drugs for hormone disorders (glucagon, danazol, somatropin, thyroid hormones)

When to see a doctor

If the common side effects of insulin are mild, they may go away in a few days or up to a few weeks. However, if the adverse reactions are more severe, you should consult your healthcare provider.

When it comes to severe side effects of insulin (listed above), you should call your doctor immediately, i.e., as soon as you notice them. If you feel your symptoms are life-threatening or have a medical emergency, you need to call 911.


Insulin therapy is recommended to patients with type 1 diabetes, but in some cases to those with type 2 diabetes mellitus as well.

Side effects of insulin are generally mild and tend to go away on their own. Some people may have serious side effects that require medical attention. Stick to the doctor’s instructions and inform them about the medications you’re taking.

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Cefalu WT, Dawes DE, Gavlak G, et al. (2018). Insulin access and affordability working group: Conclusions and recommendations. Diabetes Care, 41(6):1299-1311. https://doi.org/10.2337/dci18-0019
What is insulin? Hormone Health Network https://www.hormone.org/your-health-and-hormones/glands-and-hormones-a-to-z/hormones/insulin
Turchin A, Hosomura N, Zhang H, et al. (2020). Predictors and consequences of declining insulin therapy by individuals with type 2 diabetes. Diabetic Medicine, 37(5):814-821. https://doi.org/10.1111/dme.14260
What is insulin? Endocrine Web. https://www.endocrineweb.com/conditions/type-1-diabetes/what-insulin

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