If you have diabetes, your doctor might suggest Humalog (insulin lispro) as a treatment option for your condition. Along with other questions you may have about the drug, you could be wondering about its side effects.

Humalog is a prescription medication that’s used to help manage blood sugar in people with diabetes. There are two types of Humalog product: Humalog and Humalog Mix. Both types can be used in adults with type 1 or type 2 diabetes. Humalog can also be used in certain children with type 1 diabetes.

Humalog is a rapid-acting insulin. You’ll receive it by injection under your skin. (You can give yourself injections after your doctor shows you how.) Healthcare professionals in a hospital may sometimes give Humalog by injection into a vein.

Humalog comes in these forms:

  • prefilled disposable injection pens (Humalog KwikPen, Junior KwikPen, and Tempo pen)
  • cartridges for use with reusable insulin pens
  • vials for use with insulin syringes or insulin pumps (devices that continuously inject insulin under your skin and can also give extra doses at mealtimes)

Humalog Mix is a premixed combination of rapid-acting and intermediate-acting insulin. You’ll receive Humalog Mix by injection under your skin. (You can give yourself injections after your doctor shows you how.) Humalog Mix comes in these forms:

  • prefilled disposable injection pens (Humalog Mix KwikPen)
  • vials for use with insulin syringes

For more information about Humalog, see this in-depth article on the drug.

Like other drugs, Humalog can cause mild or serious side effects. These are also referred to as adverse effects. Keep reading to learn more.

Some people may experience mild or serious side effects during their Humalog treatment. Examples of Humalog’s more commonly reported side effects may include:

* To learn more about this side effect, see “Side effects explained” below.

Examples of mild side effects that have been reported with Humalog include:

* To learn more about this side effect, see “Side effects explained” below.

In most cases, these side effects should be temporary. Some may be easily managed, too. But if you have any symptoms that are ongoing or that bother you, talk with your doctor or pharmacist. Don’t stop using Humalog unless your doctor recommends it.

Humalog may cause mild side effects other than the ones listed above. For details, see the patient information for the different forms: Humalog U-100, Humalog KwikPen U-200, Humalog Mix75/25, and Humalog Mix50/50.

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks side effects of the medication. If you’d like to notify the FDA about a side effect you’ve had with Humalog, visit MedWatch.

Serious side effects that have been reported with Humalog include:

* To learn more about this side effect, see “Side effects explained” below.

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

Here are the answers to some frequently asked questions about Humalog’s side effects.

Are the side effects of Humalog KwikPen different from those of other forms of Humalog?

No, the side effects of Humalog shouldn’t vary between its different forms.

There are two types of Humalog product: Humalog and Humalog Mix. Both come in different forms.

Humalog comes as:

  • prefilled disposable injection pens (Humalog KwikPen, Junior KwikPen, and Tempo pen)
  • cartridges for use with reusable insulin pens
  • vials for use with insulin syringes or insulin pumps (devices that continuously inject insulin under your skin and can also give extra doses at mealtimes)

Humalog Mix comes as:

  • prefilled disposable injection pens (Humalog Mix KwikPen)
  • vials for use with insulin syringes

For examples of possible side effects with all these forms of Humalog, see “What are the mild side effects of Humalog?” and “What are the serious side effects of Humalog?” above.

How do Humalog’s side effects compare with those of Lantus?

Humalog and Lantus are different types of insulin. Humalog contains insulin lispro, while Lantus contains insulin glargine. Both products can cause very similar side effects. For examples, see “What are the mild side effects of Humalog?” and “What are the serious side effects of Humalog?” above.

The main difference between Humalog and Lantus is that Humalog is rapid acting, while Lantus is long acting.

Humalog starts working in 15 minutes and lasts for about 4 hours. It’s used to help manage surges of blood sugar at mealtimes. Lantus doesn’t start working for a couple of hours, but it lasts for 24 hours or longer. It’s used to help provide steady background management of your blood sugar throughout the day.

Both products can cause low blood sugar levels. But because of their different profiles, this can happen at different times with each product.

For example, with Humalog you can have low blood sugar levels soon after taking a dose if you don’t eat a meal. With Lantus, it’s possible to have low blood sugar levels several hours after taking a dose. In this case, it could happen if you exercised more than you planned to.

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

What should I do if I notice thickening skin on my stomach where I inject Humalog?

Injecting Humalog (or any kind of insulin) can cause skin changes in the area where you have the injection.

These changes include thickening of the fatty tissue under your skin, which can cause lumps of fat to develop there. You may also have thinning of the fatty tissue, which can cause dents or pits to develop in your skin. To read more about these skin changes, see “Side effects explained” below.

Injecting Humalog into areas with skin thickening or lumps can make the problem worse. More importantly, the insulin won’t be absorbed into your body correctly. This means it won’t manage your blood sugar as well as usual.

If you notice thickening skin on your stomach, stop injecting Humalog in that particular area of your stomach. You can still inject into other areas on your stomach where there isn’t skin thickening. Be sure to change the exact injection site each time to help prevent more skin thickening.

If there aren’t any places on your stomach without skin thickening, you can inject Humalog in your thighs, buttocks, and upper arms. But be aware that Humalog is absorbed into your body more slowly from these places, so it won’t work as quickly. Talk with a healthcare professional if you’re concerned about injecting Humalog in a place other than your stomach.

Skin thickening and lumps tend to go away on their own with time if you avoid injecting in the area. But if these changes are bothersome or don’t go away, talk with your doctor.

What side effects might occur if I switch from Humalog to Novolog?

Humalog and Novolog are both rapid-acting insulins, but they contain different types of insulin. Humalog contains insulin lispro, and Novolog contains insulin aspart. Both products can cause very similar side effects. For examples of these, see “What are the mild side effects of Humalog?” and “What are the serious side effects of Humalog?” above.

Switching from one insulin product to another could affect your blood sugar management for a time. For example, it could cause you to have high or low blood sugar levels.

If your doctor recommends switching from Humalog to Novolog, you’ll need to monitor your blood sugar more frequently for a while after the change. Your doctor will help you adjust your new insulin regimen based on your blood sugar levels.

Are the side effects of Humalog, Humalog Mix75/25, and Humalog Mix50/50 different?

No, the side effects you may have with Humalog aren’t different from those you may have with Humalog Mix. For examples of possible side effects with both forms of Humalog, see “What are the mild side effects of Humalog?” and “What are the serious side effects of Humalog?” above.

Humalog doesn’t last as long as Humalog Mix. Humalog is a rapid-acting insulin that starts working in about 15 minutes and lasts about 4 hours. Humalog Mix is a premixed combination of rapid-acting and intermediate-acting insulin. It starts working in about 15 minutes and lasts about 22 hours.

Both products can cause low blood sugar levels. But because Humalog Mix lasts longer, it’s possible to have low blood sugar levels several hours after taking a dose. For example, this could happen if you exercised more than you planned to.

If you have any questions about the differences between these products, talk with your doctor or pharmacist.

Can Humalog cause heart-related side effects?

Yes, Humalog can sometimes cause heart-related side effects.

If you have low blood sugar levels with Humalog, this can cause heart palpitations. This may feel like your heart is pounding, racing, fluttering, or skipping beats. To read more about low blood sugar levels with Humalog, see “Side effects explained” below.

More rarely, Humalog can cause hypokalemia, which is a low potassium level in your blood. Hypokalemia can cause an irregular heartbeat. If your heart feels like it’s beating irregularly while you’re receiving Humalog, contact your doctor. You may need a blood test to check your potassium level.

If you use Humalog with a type of diabetes drug called thiazolidinediones, this might in rare cases cause or worsen heart failure. Examples of thiazolidinediones include Avandia (rosiglitazone) and Actos (pioglitazone). If you use Humalog with one of these drugs, contact your doctor right away if you have symptoms of heart failure. These may include:

  • swelling of your ankles or feet
  • sudden weight gain
  • shortness of breath

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

Can Humalog cause diarrhea?

Yes. As with other insulins, some people who use Humalog may have diarrhea. It’s worth noting that diarrhea is common in people with diabetes, but it’s not clear what the reason for this is.

Contact your doctor if you experience frequent diarrhea while using Humalog. They can investigate what might be causing this side effect and suggest ways to manage it.

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

Swelling in your hands and feet

Humalog can sometimes cause peripheral edema, which is swelling in your hands, feet, or ankles caused by fluid retention. This type of swelling is one of the more common side effects of all insulin products, including Humalog.

Swelling in your feet and ankles can also be caused by problems related to diabetes, such as problems with blood circulation, your kidneys, or your heart.

What might help

If you have swelling of your feet or ankles, the following tips may help:

  • Raise your feet up on a cushion while you’re sitting or lying down.
  • Wear compression socks to help improve blood flow.
  • Exercise regularly.
  • Avoid sitting for long periods of time.
  • Drink plenty of fluids to help prevent your body from holding onto fluid.

If these tips don’t help ease your swelling, or if it becomes worse or more widespread, talk with your doctor.

If you have swollen ankles or feet and you’re using Humalog with a type of diabetes drug called thiazolidinediones, contact your doctor immediately. Examples of these drugs include Avandia (rosiglitazone) and Actos (pioglitazone). Swollen ankles, shortness of breath, or sudden weight gain could be symptoms of heart failure. This condition can be a problem with this combination of drugs.

Low blood sugar levels

Humalog, like all insulin products, can sometimes cause low blood sugar levels. The medical term for this is hypoglycemia. It’s one of the most common side effects of Humalog.

Low blood sugar levels can happen for several reasons while you’re using Humalog, such as:

  • skipping meals
  • eating less than usual or less than planned
  • exercising more than usual or more than planned
  • using too much Humalog

Low blood sugar levels can quickly become serious or life threatening if not treated, so it’s important to know the symptoms. Note that symptoms of hypoglycemia may vary between people, and your own symptoms may change over time.

Early symptoms of hypoglycemia may include:

Symptoms of severe low blood sugar levels can make a person appear drunk. They may include:

Very low blood sugar levels can lead to seizures or coma, and even death.

What might help

If you have symptoms of low blood sugar levels, you need to eat or drink something containing sugar that your body can quickly absorb as soon as possible. Examples include:

  • glucose tablets or gel
  • fruit juice or soda (not diet or sugar-free versions)
  • candies or jellybeans

Talk with your doctor about ways to avoid and treat low blood sugar levels during Humalog treatment.

Injection site reactions and lipodystrophy

You might have a mild injection site reaction after receiving Humalog. For example, you may have tenderness, redness, itchiness, or swelling in the area where you have the injection. These symptoms usually go away quickly.

Occasionally, you might also notice a bit of bruising after your injection. This can sometimes happen if you accidentally catch a tiny blood vessel under the skin with the injection. It’s usually nothing to worry about.

As with other insulin products, long-term use of Humalog injections can also commonly cause skin changes called lipodystrophy. With lipodystrophy, you may have thickening of the fatty tissue under your skin. These areas may feel hard, rubbery, or lumpy. You may also have thinning of this fatty tissue, which can lead to dents or pits developing in your skin.

Injecting Humalog can sometimes cause protein to build up under your skin in the injection area. This is called cutaneous amyloidosis. It can also cause lumps under your skin.

If you inject Humalog into areas with hardening, thickening, or lumps, the medication might not be correctly absorbed into your body. This can make Humalog less effective than usual at managing your blood sugar levels. On the other hand, if you inject Humalog into areas with pitting or dents, the medication can be more easily absorbed into your body. And this can raise your risk for low blood sugar levels (see “Low blood sugar levels” above).

What might help

To help avoid injection site reactions and lipodystrophy, it’s important to use a different injection site each time you use Humalog. This is called rotating injection sites.

If you notice any thickening, hardening, lumps, pits, or dents developing in your skin, don’t inject Humalog in those areas. Also don’t inject into areas that are tender, bruised, scaly, scarred, or damaged.

Lipodystrophy tends to get better with time if you avoid injecting in the affected area. But it may take several months.

If you have skin changes that are bothersome or don’t go away, talk with your doctor.

Weight gain

It’s possible to gain weight while using any insulin product, including Humalog. That’s because insulin causes some of the excess sugar in your blood to be stored as fat in your body.

What might help

Talk with your doctor about ways to maintain a moderate weight during Humalog treatment. Humalog is meant to be used alongside a balanced diet and regular exercise. Your doctor can help you establish an eating and exercise plan that works for you. They can also help you with managing your weight.

If you suddenly gain a lot of weight and you’re using Humalog with a type of diabetes drug called thiazolidinediones, contact your doctor right away. Examples of this type of drug include Avandia (rosiglitazone) and Actos (pioglitazone). Sudden weight gain, swollen ankles, or shortness of breath could be symptoms of heart failure. This condition can be a problem with this combination of drugs.

Allergic reaction

Like most drugs, Humalog can cause an allergic reaction in some people. An allergic reaction is an adverse reaction that’s caused by your immune system.

Symptoms can be mild or serious and can include:

  • skin rash
  • itchiness
  • flushing (temporary warmth, redness, or deepening of skin color)
  • swelling under your skin, typically in your eyelids, lips, hands, or feet
  • swelling of your mouth, tongue, or throat, which can make it hard to breathe

What might help

If you have mild symptoms of an allergic reaction, such as a mild rash, call your doctor right away. To manage your symptoms, they may suggest an over-the-counter antihistamine you take by mouth, such as Benadryl (diphenhydramine). They may also recommend a product you apply to your skin, such as hydrocortisone cream.

If your doctor confirms you had a mild allergic reaction to Humalog, they’ll decide if you should keep using it.

If you have symptoms of a severe allergic reaction, such as swelling or trouble breathing, call 911 or your local emergency number right away. These symptoms could be life threatening and require immediate medical care.

If your doctor confirms you had a serious allergic reaction to Humalog, they may have you switch to a different treatment.

Keeping track of side effects

During your Humalog treatment, consider keeping notes on any side effects you’re having. Then, you can share this information with your doctor. This is especially helpful to do when you first start taking new drugs or using a combination of treatments.

Your side effect notes can include things such as:

  • what dose of drug you were using when you had the side effect
  • how soon after starting that dose you had the side effect
  • what your symptoms were from the side effect
  • how it affected your daily activities
  • what other medications you were also using
  • any other information you feel is important

Keeping notes and sharing them with your doctor will help your doctor learn more about how Humalog affects you. Your doctor can use this information to adjust your treatment plan if needed.

Humalog may not be right for you if you have certain medical conditions or other factors that affect your health. Talk with your doctor about your health history before you use Humalog. The list below includes factors to consider.

Allergic reaction. If you’ve had an allergic reaction to Humalog or any of its ingredients, you shouldn’t use Humalog. Ask your doctor what other medications are better options for you.

Low blood sugar levels. Humalog can cause and worsen low blood sugar levels. If you frequently have low blood sugar levels, discuss this with your doctor before you start receiving Humalog. If you’re having an episode of low blood sugar, don’t use Humalog because the drug will make it worse. Severe low blood sugar levels could be life threatening. To learn more, see “Side effects explained” above.

Kidney or liver problems. If you have a kidney or liver problem, you’re more likely to have low blood sugar levels with Humalog. Talk with your doctor about ways to avoid this.

Low potassium level. If you have a low level of potassium in your blood, using Humalog could make this worse. If you have low potassium, or you’re at risk for this problem, your doctor may monitor your potassium level while you receive Humalog. You might be at risk for low potassium if you take certain other medications with Humalog. Examples include corticosteroids and certain diuretics.

Heart failure. If you have heart failure and you take a type of diabetes drug called thiazolidinediones, using Humalog as well could make your heart failure worse. Ask your doctor if Humalog is right for you. Examples of thiazolidinediones include Actos (pioglitazone) and Avandia (rosiglitazone).

Alcohol use and Humalog

Drinking alcohol with Humalog can raise your risk for low blood sugar levels. If you drink alcohol, talk with your doctor about how much you can safely drink while using Humalog. Your doctor may suggest that you check your blood sugar more often if you drink alcohol with Humalog.

Pregnancy and breastfeeding while using Humalog

There haven’t been enough studies to know for sure if Humalog is safe to use during pregnancy. But studies do show that it doesn’t seem to increase the risk of pregnancy loss or developmental problems.

If diabetes isn’t managed well in pregnancy, this can have serious risks for both the mother and her baby. The American Diabetes Association recommends managing diabetes with insulin during pregnancy.

If you’re pregnant or planning to become pregnant, talk with your doctor about the best way to manage your diabetes.

Insulin, including Humalog, is generally considered to be safe to use during breastfeeding. This is because insulin can’t be absorbed when it’s taken by mouth. So even if insulin gets into breast milk, it can’t be absorbed by a child who is breastfed.

While all insulin products, including Humalog, may cause very similar side effects, there aren’t many that they can cause. And although they can be common, they’re usually mild.

Serious side effects such as low blood sugar levels and lipodystrophy can usually be avoided by carefully managing your diabetes. This involves having a clear plan for doses, as well as diet and exercise, and regularly rotating your injection sites.

It’s important to remember that Humalog is used to help manage mealtime surges of blood sugar. You’ll typically use it with a longer acting insulin that provides background control of your blood sugar over the day.

If you’re starting insulin treatment and your doctor recommends Humalog, you may have more questions about side effects. You may also have questions about side effects if your doctor recommends switching to Humalog from a different type of insulin.

Here are some examples of questions you might want to ask your doctor:

  • Do Humalog injections hurt?
  • Is there a risk of infection when injecting Humalog?
  • Am I more likely to have low blood sugar levels, injection site reactions, or lipodystrophy if I use Humalog with an insulin pump?
  • Do my other diabetes treatments raise my risk for side effects with Humalog?
  • Can other conditions raise my risk for side effects with Humalog?

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