Humalog (insulin lispro) is a prescription brand-name medication. It’s approved by the Food and Drug Administration (FDA) to help manage blood sugar levels in people with type 1 or type 2 diabetes.

Humalog comes in two types: Humalog and Humalog Mix. Both types are approved for use in adults with either type 1 or type 2 diabetes. Humalog is also approved for use in children ages 3 years and older with type 1 diabetes.

Humalog may be used as either a short-term or long-term treatment. Humalog Mix is typically used as a long-term treatment, as long as you and your doctor determine that it’s effective for helping to manage your blood sugar.

Here are some fast facts on Humalog:

  • Active ingredients:
    • Humalog: insulin lispro
    • Humalog Mix: insulin lispro and insulin lispro protamine
  • Drug classes:
    • insulin lispro: rapid-acting insulin analog
    • insulin lispro protamine: intermediate-acting insulin
  • Drug forms:
    • Humalog Mix: liquid solution for subcutaneous injection

Like other drugs, Humalog can cause side effects. Read on to learn about potential common, mild, and serious side effects. For a general overview of Humalog, see this article.

These are just a few of the more common side effects reported by people who took Humalog in clinical studies. These side effects can vary depending on whether you use the drug to treat type 1 or type 2 diabetes.

More common side effects in people taking Humalog, Humalog Mix 50/50, and Humalog Mix 75/25 for diabetes include:

  • swelling in the hands or feet
  • hypoglycemia* (low blood sugar levels)
  • lipodystrophy* (skin thickening or forming “pits” at an injection site)
  • injection site reactions*
  • weight gain*

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

Mild side effects can occur with Humalog use. This list doesn’t include all possible mild side effects reported with the drug. For more information, you can refer to Humalog’s patient information for the particular form of the drug that you’re using:

Mild side effects of Humalog can include:

  • itching
  • rash
  • swelling in your hands or feet
  • weight gain*
  • injection site reactions*
  • hypoglycemia* (low blood sugar levels)
  • lipodystrophy* (skin thickening or forming “pits” at an injection site)

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

These side effects may be temporary, lasting a few days or weeks. But 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 the side effects of the medication. If you develop a side effect while taking Humalog and want to tell the FDA about it, visit MedWatch.

Humalog may cause serious side effects. The list below may not include all possible reported serious adverse effects of the drug. For more information, you can refer to Humalog’s package instructions.

If you develop serious side effects while taking Humalog, 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* can include:

* To learn more about the side effects in this list, see the “Side effect specifics” section below.

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

Do side effects vary based on the type of Humalog insulin?

No, side effects usually don’t vary based on the type of Humalog insulin you use.

Humalog comes in two types: Humalog and Humalog Mix. Both types can cause side effects such as weight gain and low blood sugar levels. See the “More common side effects of Humalog” above for more examples of possible side effects.

If you have more questions about the differences between the two types of Humalog insulin, talk with your doctor or pharmacist.

Can Humalog cause stomach-related side effects?

Taking Humalog isn’t likely to cause stomach-related side effects. Side effects that affected the stomach weren’t reported by people taking Humalog in clinical studies.

If you have concerns about stomach-related side effects while taking Humalog, talk with your doctor or pharmacist.

Does Humalog affect the heart?

Humalog may cause some side effects that could affect the heart.

Humalog may cause hypoglycemia (low blood sugar levels). If hypoglycemia becomes severe, it can affect your heart. Rarely, Humalog may also cause hypokalemia (low blood levels of potassium). This side effect can also affect your heart. Both hypoglycemia and hypokalemia are discussed in detail in the “Side effect specifics” section just below.

Also, taking Humalog with a type of diabetes drug called thiazolidinediones can cause heart failure. If you already have heart failure, taking these drugs together may make heart failure worse. Examples of thiazolidinedione drugs include Avandia (rosiglitazone) and Actos (pioglitazone).

Make sure to discuss all medications you take, including all of your diabetes medications, with your doctor and pharmacist before you begin taking Humalog. If your doctor decides that taking thiazolidinediones with Humalog is the best way to treat your diabetes, they’ll closely monitor your heart health. And, you should immediately talk with your doctor if you notice any of the following possible symptoms of heart failure.

If you have more questions about how taking Humalog may affect your heart, talk with your doctor.

Learn more about some of the side effects that Humalog may cause.

Weight gain

Humalog can cause weight gain, like all insulins. This can occur because of how insulin works in your body.

Your cells use insulin to remove excess sugar from your blood. Some of this excess sugar gets stored as fat to be used for your body’s future energy needs. Over time, this can lead to weight gain. And weight gain has been reported in people using Humalog to treat either type 1 or type 2 diabetes.

Note: If you’re taking Humalog with a type of diabetes drug called a thiazolidinedione, these drugs can interact in a way that can lead to sudden weight gain. This can occur due to fluid retention (excess fluid in the body), which can lead to heart failure. Although rare, heart failure can occur when taking Humalog with a thiazolidinedione drug. See the “FAQs about the side effects of Humalog” section above for more information.

What you can do

When you start taking Humalog, talk with your doctor about ways you can maintain a healthy weight. Humalog is used with a healthy diet and regular exercise to help lower blood sugar in people with diabetes. A healthy diet and regular exercise can also help you maintain a weight that’s healthy for you.

If you have questions about weight gain with Humalog or how to manage and maintain your weight, talk with your doctor. And, make sure to talk with your doctor about all of the drugs you take before starting Humalog.

Hypoglycemia

Humalog can cause hypoglycemia (low blood sugar levels), just like other insulins.

Insulin helps remove excess sugar from your blood. Sometimes too much sugar is removed, causing your blood sugar levels to become low. This can cause symptoms including:

If left untreated, hypoglycemia can quickly become serious. Extremely low blood sugar levels can cause coma or seizures, and very rarely, death. Symptoms of severe hypoglycemia can include:

  • slurred speech
  • coordination problems, such as trouble walking
  • confusion
  • trouble concentrating
  • weakness

It’s important to note that many factors can affect blood sugar levels, and they vary throughout the day. For example, skipping meals or being more active than usual can cause low blood sugar levels. So it’s hard to say how often hypoglycemia may occur as a side effect of Humalog.

What you can do

If you develop symptoms of hypoglycemia, immediately eat or drink something that contains glucose (sugar) that your body can quickly absorb. Examples include a glass of juice, regular soda, a glucose tablet, or a piece of candy. Sugar-free candy or gum or diet soda won’t work to treat hypoglycemia.

Your doctor will discuss with you how to manage any episodes of low blood sugar while you’re taking Humalog.

Hypokalemia

Hypokalemia (low blood levels of potassium) can occur with Humalog use. Potassium is an electrolyte (mineral) that your body needs to function normally. It’s especially important for heart muscle cell function.

Mild hypokalemia doesn’t often cause symptoms. Although it isn’t common, hypokalemia can be severe and cause symptoms such as:

If you’re at higher risk for hypokalemia, your doctor may monitor your blood potassium levels more closely than usual during Humalog. For example, you may be at higher risk for hypokalemia if you take another medication that can also lower your blood potassium. These include certain diuretic drugs used for blood pressure management, such as furosemide (Lasix).

What you can do

Call your doctor right away if you notice any of the above symptoms while taking Humalog. There are medications that can manage mild cases of hypokalemia and help keep symptoms from becoming severe. Or, your doctor may have you switch to a medication other than Humalog to help manage your blood sugar levels.

Lipodystrophy and injection site reactions

Lipodystrophy and injection site reactions are two possible side effects of Humalog injections.

With lipodystrophy, your skin thickens or forms “pits” in the area where you inject Humalog. And an injection site reaction refers to symptoms such as pain, swelling, redness, or itchiness at your injection site.

Both of these side effects are most common in people taking Humalog by subcutaneous injection. Lipodystrophy is less likely to occur when you receive Humalog via intravenous injection or infusion.

What you can do

Each time you inject Humalog, use a different injection site. This will reduce your risk for lipodystrophy. It may also reduce your risk for injection site reactions.

Avoid injecting Humalog into any skin that’s thickened or has formed lumps or “pits.” You should also avoid giving injections in areas of skin that are bruised, tender, scaly, hard, or damaged in any way.

If you experience lipodystrophy or injection site reactions that bother you or concern you, talk with your doctor. They can discuss other treatment options for your diabetes.

Allergic reaction

As with most drugs, Humalog can cause an allergic reaction in some people.

Symptoms can be mild or severe and can include:

  • rash
  • itching
  • flushing (warmth or redness/deepening of skin color for a brief time)
  • swelling under your skin, typically in your lips, eyelids, feet, or hands
  • 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 Humalog. But 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 Humalog. This drug may not be the right treatment for you if you have certain medical conditions or other factors that affect your health. The conditions and factors to consider include:

Hypoglycemia. Using Humalog while you’re currently experiencing hypoglycemia can lead to severely low blood sugar levels, which can be life threatening. For this reason, you shouldn’t take Humalog if your blood sugar levels are already low. (See the “Side effect specifics” section above to learn more.)

Hypokalemia. Using Humalog can make existing hypokalemia worse. This is because Humalog can cause low blood potassium levels. If you have hypokalemia or are at risk for this condition, talk with your doctor. They may closely monitor your potassium levels while you’re taking Humalog.

You may be at higher risk for hypokalemia if you take another medication that can lower your blood potassium. These include certain diuretic drugs used for blood pressure management, such as furosemide (Lasix).

Heart failure. If you have heart failure and take certain diabetes drugs with Humalog, your heart failure may get worse. These diabetes drugs are called thiazolidinediones. Examples include Avandia (rosiglitazone) and Actos (pioglitazone).

Make sure your doctor knows about all drugs you take before you start Humalog treatment, including any thiazolidinedione drugs. If your heart failure symptoms get worse while you’re taking Humalog, you may need to stop taking your other medication.

Liver or kidney problems. You’re more likely to have low blood sugar levels while taking Humalog if you have liver or kidney problems. Examples of these problems include kidney failure and liver failure. Your doctor can discuss ways to help prevent low blood sugar levels while taking Humalog.

Allergic reaction. You shouldn’t take Humalog If you’ve ever had an allergic reaction to Humalog or any of its ingredients. Talk with your doctor about which other treatments are better choices for you.

Alcohol use with Humalog

Alcohol and insulins such as Humalog can lower your blood sugar levels. So if you drink alcohol while you’re using Humalog, you’re more likely to develop hypoglycemia (low blood sugar levels).

The American Diabetes Association recommends talking with your doctor about whether it’s safe for you to drink alcohol with your diabetes treatment plan. However, it’s important to note that the manufacturer of Humalog recommends avoiding alcohol while taking the medication.

If you drink alcohol, it’s important to talk with your doctor about how much, if any, is safe for you to drink while you’re taking Humalog. Your doctor may have you monitor your blood sugar levels more closely than usual around the time when you’re drinking alcohol. This is due to the risk of hypoglycemia.

Pregnancy and breastfeeding while taking Humalog

Below is information on pregnancy and breastfeeding during Humalog treatment.

Pregnancy. It isn’t known for certain whether Humalog is safe to use during pregnancy. However, the American Diabetes Association recommends insulin for managing blood sugar levels during pregnancy. If you’re pregnant or planning to become pregnant, talk with your doctor about the risks and benefits of using Humalog.

Breastfeeding. In general, Humalog is considered safe to use while breastfeeding. It isn’t known whether Humalog may pass into breast milk. But, the body doesn’t absorb insulin taken by mouth. So, even if Humalog does pass into breast milk, a breastfed child can’t absorb the drug.

It’s important to note that your insulin needs may change while you’re breastfeeding. If you’re breastfeeding while taking Humalog, make sure to talk with your doctor about potential changes to your dosage.

Most side effects from Humalog aren’t common. When side effects do occur, they’re usually mild and go away without medical attention.

The most common side effects of Humalog are hypoglycemia (low blood sugar), injection site reactions, and lipodystrophy. Lipodystrophy refers to skin thickening or forming “pits” at your injection site. It’s important to note that there are ways to help prevent these side effects. See the “Side effect specifics” section above for more information. Your doctor or pharmacist can also explain more and help recommend ways to reduce your risk for side effects from Humalog.

Although rare, Humalog can cause serious side effects. You should talk with your doctor if you experience symptoms of:

You should also talk with your doctor if you become pregnant while taking Humalog.

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

Note: For more information on type 1 and type 2 diabetes, see our diabetes content hub.

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 other 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.