Humulin R (insulin human) is a brand-name prescription medication. The Food and Drug Administration (FDA) has approved it to manage blood sugar levels in adults and in children with either type 1 or type 2 diabetes.

Here are some fast facts about Humulin R:

  • Active ingredient: insulin human, which is a biologic
  • Drug class: short-acting insulin
  • Drug forms:
    • solution given as an IV infusion by a healthcare professional in a doctor’s office or hospital*
  • Drug strengths:
    • Humulin R U-100, which contains 100 units of insulin per milliliter (100 units/mL)
    • Humulin R U-500, which contains 500 units/mL

If you and your doctor agree Humulin R is safe and working well for you, you’ll typically use it long term.

As with other drugs, Humulin R U-100 and U-500 insulin can cause side effects (also known as adverse effects). Read on to learn about potential common, mild, and serious side effects. For a general overview of Humulin R, including details about its uses and forms, see this article.

* This form is typically given using the Humulin R U-100 strength.

Humulin R can cause certain side effects, some of which are more common than others. These side effects may be temporary, lasting a few days to weeks. However, if the side effects last longer than that, bother you, or become severe, be sure to talk with your doctor or pharmacist.

These are just a few of the more common side effects reported by people who took Humulin R U-100 and Humulin R U-500 in clinical trials:

  • lipodystrophy (skin pitting or thickening at the injection site)
  • edema (fluid buildup), especially in your arms and legs
  • hypoglycemia*
  • injection site reaction*
  • weight gain*

* For more information about this side effect, see “Side effect specifics” below.

Mild side effects can occur with Humulin R use. This list doesn’t include all possible mild side effects of the drug. For more details, you can refer to the prescribing information for Humulin R U-100 and Humulin R U-500.

Mild side effects that have been reported with Humulin R include:

  • lipodystrophy (skin pitting or thickening at the injection site)
  • edema (fluid buildup), especially in your arms and legs
  • mild hypoglycemia*
  • injection site reaction*
  • weight gain*
  • protein buildup underneath the skin where you inject Humulin R*
  • mild allergic reaction*

These side effects may be temporary, lasting a few days to weeks. However, if the side effects last longer than that, bother you, or become severe, be sure to talk with your doctor or pharmacist.

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks side effects of the medication. If you develop a side effect while taking Humulin R and want to tell the FDA about it, visit MedWatch.

* For more information about this side effect, see “Side effect specifics” below.

Humulin R may cause serious side effects, although they aren’t common. The list below may not include all possible serious side effects of the drug. For more details, you can refer to the prescribing information for Humulin R U-100 and Humulin R U-500.

If you develop serious side effects while taking Humulin R, call your doctor right away. If the side effects seem life threatening or you think you’re having a medical emergency, immediately call 911 or your local emergency number.

Serious side effects that have been reported with Humulin R include the following:

To learn more about these serious side effects, see the “Side effect specifics” section below.

Humulin R may cause several side effects. Here are some frequently asked questions about the drug’s side effects and their answers.

Are side effects of Humulin R U-500 different from those of Humulin R U-100?

No, the side effects of Humulin R U-500 are not expected to be different from those of Humulin R U-100.

Humulin R U-100 and Humulin R U-500 both contain the same active ingredient, insulin human. The difference is that they contain different strengths of the active ingredient.

Humulin R U-100 contains 100 units of insulin per milliliter (100 units/mL). On the other hand, Humulin R U-500 contains 500 units/mL.

For more information about Humulin R, including details about its forms, see this article. You can also talk with your doctor or pharmacist to learn more.

Are there medical conditions that increase my risk of side effects from Humulin R?

Yes, some medical conditions may increase your risk of side effects from Humulin R.

People with kidney or liver problems have an increased risk of hypoglycemia (low blood sugar). This is the most common side effect of Humulin R treatment. To learn more about hypoglycemia, see the “Side effect specifics” section below.

If you’re concerned about any conditions that may increase your risk of side effects from Humulin R, talk with your doctor.

Learn more about some of the side effects that Humulin R may cause. To find out how often side effects occurred in clinical trials, see the prescribing information for Humulin R U-100 and Humulin R U-500.

Hypoglycemia (low blood sugar)

Similar to all insulins, hypoglycemia is the most common side effect of Humulin R treatment. With hypoglycemia, your blood sugar levels drop too low.

Humulin R is prescribed to manage blood sugar levels in people with diabetes. Diabetes can cause blood sugar levels to become too high. However, medications used to treat diabetes can cause blood sugar levels to become too low. This includes Humulin R.

Symptoms of mild hypoglycemia can include:

  • feeling lightheaded or dizzy
  • irritability
  • sweating
  • headache
  • blurry vision
  • hunger

In most cases, hypoglycemia caused by Humulin R is mild. In rare cases, insulins, including Humulin R, can cause severe hypoglycemia. Severe hypoglycemia can be life threatening. It can require hospitalization if it is not treated right away.

Symptoms of severe hypoglycemia can include:

What you can do

Be sure to watch for symptoms of hypoglycemia while taking Humulin R. If you notice any symptoms, check your blood sugar right away to make sure it isn’t too low.

If you notice symptoms of severe hypoglycemia, it must be treated right away. Your doctor may prescribe an emergency glucagon kit, such as GlucaGen, to use along with Humulin R. This kit allows others to give you a glucagon injection if your blood sugar levels become severely low and you’re unable to take sugar by mouth.

If your symptoms feel life threatening, you should immediately call 911 or seek emergency medical attention. Do not drive yourself to the hospital if you have symptoms of severe hypoglycemia.

If you have mild hypoglycemia, you can treat it by drinking or eating glucose (sugar) right away. Try to take at least 15 grams (g) of glucose. Some examples include:

  • three to four glucose tablets, which you can buy at a pharmacy
  • one tablespoon of honey or sugar
  • half a cup of fruit juice, such as apple or orange juice
  • half a cup of non-diet soda (the soda must contain at least 15 g of sugar, so diet or sugar-free soda won’t work)

Injection site reactions

Humulin R can cause reactions at the site where you inject the drug. These injection site reactions can occur when the drug is given by subcutaneous injection. (To learn more about how Humulin R is given, see this article.) Injection site reactions are among the more common side effects reported in clinical trials of Humulin R U-100 and Humulin R U-500.

Injection site reactions reported with Humulin R include:

  • bleeding
  • hematoma (pooling of blood under the skin)
  • itchiness
  • pain
  • skin rash, discoloration, redness, or warmth
  • skin nodule or mass
  • swelling

Injection site reactions are typically mild and most go away on their own within a few hours or days.

What you can do

To lower your risk of injection site reactions, be sure to rotate your Humulin R injection sites. These include your upper arm, abdomen, or thigh.

If you have injection site reactions that do not go away or bother you, talk with your doctor. They may suggest switching to a different medication for your diabetes. They may also examine your reaction to determine if something else is causing it.

Low blood potassium

Rarely, Humulin R can cause hypokalemia (low blood potassium level). This is a known side effect of all insulins. The way insulins work to lower blood sugar levels can cause hypokalemia.

Symptoms of low blood potassium can include:

What you can do

Talk with your doctor if you notice symptoms of hypokalemia while you’re using Humulin R. Your doctor may recommend you take a potassium supplement. Or they may prescribe medications to treat low blood potassium levels.

If you have severe hypokalemia, your doctor may give you potassium through an IV infusion.

Weight gain

Weight gain is a known side effect of all insulins, including Humulin R. It’s not known exactly how often this occurs with Humulin R U-100 or Humulin R U-500.

Fluid buildup, a mild side effect of Humulin R,* can also cause minor weight gain. Fluid buildup can occur in the hands, lower legs, or feet.

Taking certain diabetes medications with Humulin R can also cause fluid buildup or weight gain. And if you have heart failure, these drug combinations can worsen your heart failure in rare cases. To learn more, see the “Humulin R precautions” section below.

What you can do

Talk with your doctor if you’re concerned about weight gain and Humulin R. Your doctor can work with you to develop ways to maintain a weight that’s healthy for you.

* To learn more about the mild side effects of Humulin R, see the “Mild side effects of Humulin R” section above.

Protein buildup underneath the skin where you inject Humulin R

Humulin R can cause localized cutaneous amyloidosis at injection sites in some people.* With this condition, there’s a buildup of amyloid protein underneath the skin. This is different from amyloidosis.

Symptoms can include lumps under the skin if you use the same injection site for your insulin.

Injecting Humulin R into skin that has this type of protein buildup may prevent the drug from working correctly. This could affect how the drug manages your blood sugar levels.

What you can do

To avoid protein buildup underneath your skin, be sure to rotate your injection site with each dose of Humulin R. Injection sites include your upper arm, abdomen, or thigh.

If you have symptoms that don’t go away or bother you, talk with your doctor. They may suggest switching to a different medication for your diabetes. Your doctor may also want to examine your injection site to determine if something else is causing it.

* Humulin R is given by subcutaneous injection. To learn more, see this article.

Allergic reaction

As with most drugs, Humulin R can cause an allergic reaction in some people. However, it’s not clear how often this happened in clinical trials of Humulin R U-100 and Humulin R U-500.

Symptoms can be mild or serious and can include:

  • skin rash
  • itching
  • flushing
  • swelling under your skin, typically in your lips, eyelids, hands, or feet
  • swelling of your mouth, tongue, or throat, which can make it hard to breathe

What you can do

For mild symptoms of an allergic reaction, call your doctor right away. They may recommend ways to ease your symptoms and determine whether you should keep taking Humulin R. However, if your symptoms are serious and you think you’re having a medical emergency, immediately call 911 or your local emergency number.

Be sure to talk with your doctor about your health history before you take Humulin R. This drug may not be the right treatment for you if you have certain medical conditions or other factors that affect your health. These are considered drug-condition or drug-factor interactions. The conditions and factors to consider include:

Kidney or liver problems. If you have a kidney or liver problem, you may have an increased risk of hypoglycemia from using Humulin R. Examples of these problems include chronic kidney disease or alcohol-related liver disease. Your doctor may closely monitor your blood sugar levels during Humulin R treatment if you have kidney or liver problems.

Heart failure. If you have heart failure and take Humulin R with thiazolidinediones (a type of diabetes medication), your heart failure symptoms can worsen. Examples of thiazolidinediones include Avandia (rosiglitazone) and Actos (pioglitazone). For this reason, it’s important to tell your doctor if you have heart failure before you begin Humulin R treatment. You should also tell your doctor and pharmacist about all medications you take. If you take a thiazolidinedione and Humulin R together, and your heart failure symptoms worsen, your doctor may advise you to stop taking one drug or another.

Low blood potassium level. Similar to other insulins, Humulin R can cause low blood potassium levels. If your potassium levels are already low, using Humulin R may lower it further. Your doctor may want to treat your low potassium level before prescribing you Humulin R.

Allergic reaction. If you’ve had an allergic reaction to Humulin R or any of its ingredients, your doctor will likely not prescribe Humulin R. Ask your doctor about other medications that may be better options for you.

Alcohol with Humulin R

It’s possible for Humulin R to interact with alcohol. This is because alcohol can increase or decrease how well insulin works. (Humulin R is a type of insulin.)

Also, both Humulin R and alcohol can lower blood sugar levels. So, taking the two together may raise your risk of low blood sugar.

If you consume alcohol, talk with your doctor to find out whether it’s safe to do so while using Humulin R. They can also advise how much is safe.

Pregnancy and breastfeeding while taking Humulin R

It’s likely safe to use Humulin R during pregnancy.

Data from decades of insulin use have not linked insulin to any possible harms to pregnancy.* On the other hand, high blood sugar levels are known to raise the risk of pregnancy complications.

The American Diabetes Association recommends insulin as the preferred treatment for managing type 1 or type 2 diabetes in pregnant people.

While Humulin R is known to pass into human breast milk, it’s not known to cause side effects in a breastfed child.

Whether or not you breastfeed during Humulin R treatment depends on what you and your doctor decide is best for you. Talk with your doctor to learn more about feeding options for your child while you’re using Humulin R.

* For more details, you can refer to the prescribing information for Humulin R U-100 and Humulin R U-500.

Humulin R can cause mild and severe side effects. Similar to all insulins, it can cause hypoglycemia (low blood sugar), with most cases being mild. And as with other insulins, Humulin R commonly causes weight gain and fluid buildup as side effects.

If you’d like to learn more about Humulin R, talk with your doctor or pharmacist. They can help answer any questions you have about side effects from taking the drug.

Besides talking with your doctor, you can do some research on your own. These articles might help:

  • More information about Humulin R. For details about other aspects of Humulin R, refer to this article.
  • Drug comparison. Learn how Humulin R compares with Humulin N and Novolin R.
  • Dosage. For information about the dosage of Humulin R, view this article.
  • A look at diabetes. For details about Humulin R U-500 for diabetes, see this article. To learn about type 1 or type 2 diabetes in general, see our diabetes hub. You can also view this list of diabetes articles.

Disclaimer: Medical News Today has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.