Fasenra (benralizumab) is a prescription drug used to help prevent severe asthma. Fasenra can cause side effects that range from mild to serious. Examples include headache and sore throat.

Fasenra is prescribed for adults and children ages 12 years and older to help prevent asthma flare-ups caused by eosinophils. Fasenra is not a fast-acting emergency breathing medication. It’s not used to stop sudden breathing problems. It’s best to have a rescue inhaler on hand to treat sudden shortness of breath or breathing problems that worsen.

The active ingredient in Fasenra is benralizumab. (An active ingredient is what makes a drug work.) It’s a biologic medication that belongs to a group of drugs called monoclonal antibodies. Fasenra comes as a liquid solution that’s injected under the skin using a prefilled auto-injector pen or syringe.

Keep reading to learn about the common, mild, and serious side effects Fasenra can cause. See this article for a general overview of the drug, including details about its uses.

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

  • sore throat
  • headache*

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

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

In most cases, these side effects should be temporary. And some may be easily managed. But if you have ongoing or bothersome symptoms, talk with your doctor or pharmacist. And do not stop treatment with Fasenra unless your doctor recommends it.

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

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

The only serious side effect reported in studies of Fasenra was severe allergic reaction.*

If you develop serious side effects during Fasenra 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.

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

Get answers to some frequently asked questions about Fasenra’s side effects.

Can Fasenra cause cancer?

Fasenra hasn’t been studied to determine whether it can cause cancer in humans. All medications similar to Fasenra (monoclonal antibodies) can affect the immune system, and some people worry these drugs might cause cancer. However, there were no indications in Fasenra’s studies that this would occur.

Talk with your pharmacist or doctor if you have questions about your cancer risk from Fasenra.

Does Fasenra cause long-term side effects?

Long-term side effects weren’t reported in early studies of Fasenra. Since its approval by the Food and Drug Administration (FDA) in 2017, research on extended safety was published in 2021. This study found people safely used Fasenra for up to 5 years. Scientists said no new safety concerns were found, and the drug worked as well in reducing flare-ups as it did in earlier studies.

If you’re concerned about possible long-term side effects from Fasenra, talk with your doctor or pharmacist.

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


Headaches were the most common side effect reported in studies of Fasenra. Headache may cause symptoms such as:

  • a dull ache on the top of the head, at the eyebrows, or ears
  • a tight band feeling around the head
  • mild neck or shoulder pain
  • tiredness
  • irritability

Factors that can increase the risk of headache with Fasenra include dehydration (low fluid level), hunger, and lack of sleep.

What might help

If you get a headache during Fasenra treatment, it may help to drink more water, eat a healthy snack, and get some rest.

You can also try over-the-counter (OTC) acetaminophen to help relieve your symptoms. Avoid aspirin, ibuprofen, and naproxen, as these OTC products may worsen asthma symptoms. Follow package instructions and ask your pharmacist for help when choosing a product.

If your headaches become frequent or severe, talk with your doctor.

Injection site reaction

Injection site reactions were an occasional side effect reported in studies of Fasenra. This side effect may cause the following symptoms at or near your Fasenra injection site:

  • stinging or aching pain
  • discolored or darkened skin
  • swelling
  • itchiness
  • bruising
  • raised rash

What might help

Fasenra video instructions recommend setting the injection pen at room temperature for 30 minutes before using it. If the medication in the pen is too cold, the risk of injection site reactions may increase. It may also help to review the self-injection video instructions occasionally, as having the correct injection technique may help reduce site reactions.

Avoid using the same spot on your arm, stomach, or thigh for each Fasenra injection. Instead, keep a calendar or list of sites used (upper-right thigh, lower-left stomach, etc.) and choose a new spot each time. This may help reduce injection site reactions.

If you get an injection site reaction during Fasenra treatment, you can apply a cool compress to the area as frequently as needed.

If the injection site becomes swollen, painful, and hot to the touch (signs of infection) or other symptoms worsen, contact your doctor or pharmacist.

Allergic reaction

Like most drugs, Fasenra can cause an allergic reaction in some people. In Fasenra’s studies, most allergic reactions happened within a couple hours of an injection. But some reactions were delayed and occurred days afterward.

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

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 oral antihistamine, 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 Fasenra, they’ll decide whether you should continue treatment with 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 Fasenra, they may have you switch to a different treatment.

Keeping track of side effects

During your Fasenra 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 a new drug 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 your symptoms 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 Fasenra affects you. They can then use this information to adjust your treatment plan if needed.

Below is important information you should consider before starting Fasenra.


Fasenra can sometimes cause harmful effects in people with certain conditions. This is known as a drug-condition interaction. Other factors may also affect whether Fasenra is a good treatment option for you.

Talk with your doctor about your health history before starting this medication. Be sure to tell them if any of the following factors apply to you:

Alcohol and Fasenra

There are no known interactions between alcohol and Fasenra. If you have questions about consuming alcohol during Fasenra treatment, talk with your doctor or pharmacist.

Pregnancy and breastfeeding with Fasenra

There is limited information about the safety of Fasenra treatment during pregnancy and while breastfeeding.


There isn’t enough information from studies to know whether Fasenra treatment is safe during pregnancy. Monoclonal antibodies such as Fasenra can reach a fetus in the last 3 months of pregnancy. So a fetus may be more at risk of harm from the medication during this time.

Pregnant people with unmanaged asthma may be at risk of developing preeclampsia (pregnancy-induced high blood pressure). In these cases, your doctor may decide the benefit of Fasenra treatment outweighs the risk of having asthma leading to preeclampsia.

If you’re pregnant or planning to become pregnant, talk with your doctor before starting Fasenra. If you’re able to become pregnant, your doctor may recommend using a form of birth control during your Fasenra treatment.

If you use Fasenra while pregnant, consider signing up for the drug’s pregnancy registry by visiting the website or by calling 877-311-8972. A pregnancy registry collects information about the safety of certain drugs when used during pregnancy.


It isn’t known whether Fasenra treatment is safe while breastfeeding. Scientists have not studied the presence of Fasenra in breast milk. And they don’t know the effects the drug has on milk production or on children who are breastfed.

If you’re breastfeeding or planning to do so, talk with your doctor before starting this medication.

Like most drugs, Fasenra can cause side effects that range from mild to serious. But most are temporary and go away after a few days to weeks. If you have questions about side effects Fasenra can cause, talk with your doctor. Examples of questions to help get you started include:

  • How can I manage the side effects of Fasenra?
  • What is my risk of a severe allergic reaction to Fasenra?
  • How likely is Fasenra to weaken my immune system?

To learn more about Fasenra, see these articles:

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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.