If you have diabetes, your doctor might suggest Fiasp as a treatment option for you. It’s a prescription drug used to help manage blood sugar levels in adults and children with:

The active ingredient in Fiasp is insulin aspart. (An active ingredient is what makes a drug work.) Fiasp is a biologic drug, which means it’s made from living cells.

Fiasp comes as a liquid solution that you inject under your skin. It’s available in the following forms:

  • vial for use with syringes or an insulin pump
  • prefilled FlexTouch disposable pen
  • prefilled PenFill cartridges for use with a reusable insulin pen

If you and your doctor determine that Fiasp is effective and safe for you, you’ll likely use it long term.

For more information about Fiasp, including details about its uses, see this in-depth article.

Like other drugs, Fiasp can cause mild to serious side effects. Keep reading to learn more.

Some people may experience mild to serious side effects during their Fiasp treatment. Examples of this drug’s commonly reported side effects include:

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

Mild side effects have been reported with Fiasp. These include:

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

In most cases, these side effects should be temporary. And some may be easily managed. But if you have any symptoms that are ongoing or bother you, talk with your doctor or pharmacist. And do not stop using Fiasp unless your doctor recommends it.

Fiasp may cause mild side effects other than the ones listed above. See the drug’s prescribing information for details.

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 Fiasp, visit MedWatch.

Many common side effects of Fiasp are mild and go away within a few days or weeks. But in some cases, Fiasp may cause serious side effects.

Serious side effects that have been reported with Fiasp include:

If you develop serious side effects during Fiasp treatment, 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.

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

Many side effects in children using Fiasp are similar to those of adults using this drug. But in Fiasp studies of children with type 1 diabetes, the following side effects were different than those seen in adults:

Below are answers to some frequently asked questions about Fiasp’s side effects.

How long does Fiasp last in the body, and how long can it cause side effects?

Fiasp is a rapid-acting insulin, which means it’s released quickly into your bloodstream after you inject it. In Fiasp studies, insulin aspart (the active ingredient* in Fiasp) was absorbed into the bloodstream about 2.5 minutes after injection.

You usually get the maximum effect of Fiasp about 15 minutes after injection. And your risk of side effects is higher during this time. But the drug stays in your system for about 4 to 6 hours, and you can have side effects during this time also.

Because Fiasp starts working quickly, it’s important to have a meal within 15 minutes of your Fiasp dose. This will reduce your risk of side effects, such as hypoglycemia (low blood sugar).

Talk with your doctor or pharmacist to learn more about long-term side effects of Fiasp.

* An active ingredient is what makes a drug work.

Do Fiasp’s side effects vary depending on whether I use the FlexTouch pen or PenFill cartridge?

Probably not. There aren’t any know differences in side effects between Fiasp FlexTouch pens and Fiasp PenFill cartridges. Both forms of the drug have the same active ingredient, which means their side effects should be the same.

The main difference between the Fiasp FlexTouch pen and the PenFill cartridges is that the FlexTouch pens are disposable, and the PenFill cartridges are made to be used with a reusable insulin pen. But the procedures and techniques for injecting your insulin should be similar with both forms.

Talk with your doctor if you have questions regarding Fiasp injection forms. They’ll help you decide which is the best option for you.

Are the side effects of Fiasp similar to the side effects of NovoLog?

Yes. Fiasp and NovoLog both contain the same active ingredient (insulin aspart). Because of this, the side effects of Fiasp and NovoLog are similar and include:

Keep in mind that Fiasp is designed to work faster than NovoLog. So while many side effects are the same for both drugs, some may occur faster with Fiasp. For example, hypoglycemia may occur more quickly with Fiasp.

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

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

Weight gain

Weight gain was one of the more common side effects reported in Fiasp studies. But this side effect can happen with any kind of insulin.

Hypoglycemia (low blood sugar) with Fiasp occurs because of the way insulin works to manage your blood sugar. Fiasp removes sugar from your blood for your body to use as energy. Some of the sugar is stored as fat in your cells for future use. This may cause some people to gain weight over time.

What might help

Talk with your doctor if you’re concerned about weight changes while using Fiasp. They can recommend ways to adjust your diet and physical activity level to help you reach or maintain a weight that’s healthy for you.


Hypoglycemia (low blood sugar) was the most common side effect reported in Fiasp studies. It’s also a common side effect with all kinds of insulin.

Symptoms of hypoglycemia can be mild at first. But, if not treated, hypoglycemia can quickly become severe and sometimes be life threatening. For this reason, it’s important to treat hypoglycemia as soon as you notice symptoms.

Symptoms of mild hypoglycemia can include:

Symptoms of more severe hypoglycemia can include:

What might help

It’s important to test your blood sugar regularly with a portable blood glucose meter while using Fiasp. This can help prevent hypoglycemia. Your doctor will likely talk with you about healthy ranges of blood sugar levels.

If you have symptoms of low blood sugar or your meter readings fall below the range your doctor recommends, eat or drink something containing carbohydrates. This will help increase your blood sugar level quickly. Examples of foods and drinks that work fast to raise your blood sugar include:

  • hard candy
  • honey
  • glucose tablets
  • sugar
  • fruit juice or other sugary drinks, but not diet or sugar-free drinks

Talk with your doctor if you have frequent episodes of mild hypoglycemia. They may adjust your insulin dosages. But if you have symptoms of severe hypoglycemia, seek emergency medical care.


Lipodystrophy was a common side effect in Fiasp studies. It’s also a common side effect of other kinds of insulin.

With lipodystrophy, your skin may become thickened or dimpled near the areas where you inject Fiasp. This is due to repeated injections into the same site. Injecting Fiasp into places with lipodystrophy can cause the insulin not to work as it should, which may lead to hyperglycemia (high blood sugar).

What might help

Injecting Fiasp into a different site each time you inject a dose can help prevent lipodystrophy. If you use an insulin pump, you can also rotate the infusion sites.

If you notice changes in skin thickness or appearance at an injection site, let your doctor know right away. They can talk with you about recommended injection areas, techniques, and how to rotate sites.

Injection or infusion site reaction

You may have reactions at the site where you inject Fiasp or place your pump. This was a common side effect in Fiasp studies.

Examples of injection site reactions include:

In many cases, injection or infusion site reactions are mild and go away within a few days or weeks.

What might help

Rotating the site where you inject each time you use Fiasp can help prevent injection site reactions. If you use a pump, you can also rotate your infusion site.

If injection site reactions occur, applying an ice pack to the area might help relieve pain or swelling. But avoid rubbing the site after injecting Fiasp. Rubbing the area could further irritate it.

Talk with your doctor if you have very painful or bothersome injection or infusion site reactions. They can discuss injection techniques with you to help prevent reactions.

Your doctor may also recommend that you take over-the-counter (OTC) pain relievers, such as ibuprofen (Advil, Motrin) or acetaminophen (Tylenol). Be sure to talk with your doctor or pharmacist before taking any OTC drugs. They’ll let you know if they’re safe to take with Fiasp or diabetes.

Allergic reaction

Like most drugs, Fiasp can cause an allergic reaction in some people. Symptoms can be mild to serious and can include:

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

What might help

If you have mild symptoms of an allergic reaction, such as a mild rash, call your doctor right away. They may suggest a treatment to manage your symptoms. Examples include:

  • an antihistamine you take by mouth, such as Benadryl (diphenhydramine)
  • a product you apply to your skin, such as hydrocortisone cream

If your doctor confirms you’ve had a mild allergic reaction to Fiasp, they’ll decide if you should continue 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’ve had a serious allergic reaction to Fiasp, they may have you switch to a different treatment.

Keeping track of side effects

During your Fiasp treatment, consider taking notes on any side effects you’re having. You can then share this information with your doctor. This is especially helpful 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 the drug you were taking when you had the side effect
  • how soon you had the side effect after starting that dose
  • what your symptoms were
  • how it affected your daily activities
  • what other medications you were taking
  • any other information you feel is important

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

Fiasp may not be right for you if you have certain medical conditions. (This is known as a drug-condition interaction.) Other factors may also affect whether Fiasp is a good treatment option for you. Talk with your doctor about your health history before starting Fiasp. Below are some factors to consider.

Low potassium level. Fiasp and other insulins can cause a low potassium level. If you already have low potassium or take a drug to help lower it, Fiasp can worsen this condition. Untreated low potassium levels can lead to serious complications that can sometimes be life threatening. Be sure to talk with your doctor about your health history, including all the medications you take. If needed, they may check your potassium levels regularly during your Fiasp treatment.

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

Low blood sugar. Fiasp helps lower your blood sugar. If you already have low blood sugar, Fiasp can cause severe hypoglycemia (very low blood sugar). In some cases, severe hypoglycemia can be life threatening. Talk with your doctor if you have symptoms of hypoglycemia while using Fiasp.

Heart failure. If you have heart failure, using Fiasp with certain drugs called thiazolidinediones can worsen your heart condition. Examples of these drugs include Avandia (rosiglitazone) and Actos (pioglitazone). Due to this risk, your doctor will likely monitor you for signs of heart failure more often if you take a thiazolidinedione while using Fiasp.

Kidney problems. If you have kidney problems, you may have an increased risk of developing hypoglycemia (low blood sugar) with Fiasp. Due to this risk, your doctor may recommend monitoring your blood sugar levels more often if you have kidney problems. They may adjust your Fiasp dosage to help prevent hypoglycemia.

Liver problems. If you have liver problems, you may have an increased risk of developing hypoglycemia with Fiasp. Because of this, your doctor may recommend monitoring your blood sugar levels more often if you have a liver condition. They may need to adjust your Fiasp dosage to help prevent hypoglycemia.

Hyperglycemia and ketoacidosis due to insulin pump defect or failure. Fiasp vials may be used with an insulin pump. A defect or failure in your pump while you’re using Fiasp can cause hyperglycemia (high blood sugar) or ketoacidosis (high blood ketones). In some cases, these conditions can be severe or life threatening. Let your doctor know right away if your insulin pump doesn’t work correctly. They’ll likely have you inject Fiasp with a syringe and needle while your pump is repaired.

Alcohol and Fiasp

You should not drink alcohol or take over-the-counter (OTC) medications that contain alcohol while using Fiasp.

Alcohol can affect the way Fiasp works to lower your blood sugar. It may cause your blood sugar to become too high or too low.

Talk with your doctor if you have questions about alcohol consumption during your Fiasp treatment. They can recommend how much alcohol, if any, is safe to drink with this drug.

Pregnancy and breastfeeding while using Fiasp

It’s not known if Fiasp is safe to use during pregnancy. There haven’t been any Fiasp studies of pregnant people to determine the drug’s safety during pregnancy.

But studies of pregnant people using insulin aspart* during their second trimester of pregnancy did not show an increased risk of problems with fetal development (commonly known as birth defects).

Having untreated diabetes during pregnancy can cause harm to a fetus. Due to this risk, the American Diabetes Association recommends diabetes treatment with insulin, such as Fiasp, during pregnancy.

It’s not known whether Fiasp passes into breast milk or what its effect would be on a breastfed child if it did.

If you’re pregnant, planning a pregnancy, breastfeeding, or planning to breastfeed, talk with your doctor before starting Fiasp. They’ll recommend a treatment plan that’s right for you.

* Insulin aspart is the active ingredient in Fiasp. (An active ingredient is what makes a drug work.)

Many common Fiasp side effects are mild and go away within a few days to weeks. But some can be severe and require medical attention. Talk with your doctor before starting Fiasp to learn if it’s right for you. Here are some examples of questions you can ask:

  • Is the risk of side effects from Fiasp higher with type 1 diabetes than with type 2 diabetes?
  • Can Fiasp cause weight loss?
  • Is my risk of hypoglycemia (low blood sugar) higher when I first start treatment?
  • Does the Fiasp FlexTouch pen have fewer side effects than the Fiasp vial?
  • Will other drugs I take increase my risk of side effects with Fiasp?

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Disclaimer: Healthline 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.