Leqembi can cause side effects that range from mild to serious. More common side effects include headache and infusion reactions. If side effects from Leqembi become difficult to tolerate, talk with your doctor about coping tips or other treatment options.

Leqembi (lecanemab-irmb) is used in adults to treat Alzheimer’s disease.

Keep reading to learn about the common, mild, and serious side effects Leqembi can cause. For a general overview of the drug, including details about its uses, see this Leqembi overview.

Leqembi has a boxed warning. A boxed warning is the most serious warning from the Food and Drug Administration (FDA). For details, see the “Boxed warning for Leqembi” section.

Leqembi can cause certain side effects, some more common than others. These side effects may be temporary, lasting a few days to weeks. Talk with your doctor if side effects do not go away, get worse, or become too difficult for you to tolerate.

Leqembi’s commonly reported side effects include:

  • headache
  • cough
  • diarrhea
  • mild infusion reactions
  • amyloid-related imaging abnormalities (ARIA) (see “Boxed warning for Leqembi”)

For more information about some of these side effects, see the “Tips for managing side effects” section.

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

  • headache
  • cough
  • diarrhea
  • skin rash
  • nausea and vomiting
  • mild infusion reactions

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

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

For more information about some of these side effects, see the “Tips for managing side effects” section.

Leqembi may cause serious side effects. The following list may not include all possible serious side effects of the drug. For more information, you can refer to Leqembi’s prescribing information.

If you develop serious side effects during Leqembi 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.

Serious side effects have been reported including:

  • decreased levels of infection-fighting white blood cells called lymphocytes
  • a type of irregular heartbeat called atrial fibrillation
  • severe infusion reactions (see “Tips for managing side effects”)
  • risk of ARIA (see “Boxed warning for Leqembi”)

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks and reviews side effects of the medication. Sharing your experience helps make medication safer for everyone by giving doctors and researchers more information about how the drug works in real life. If you develop a side effect while taking X drug and want to tell the FDA about it, visit MedWatch or call 800-FDA-1088.

Allergic reaction

For some people, Leqembi can cause an allergic reaction.

In general, symptoms of an allergic reaction can be mild or serious.

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 and can determine whether you should keep taking Leqembi.

If you have symptoms of a severe allergic reaction, such as swelling or difficulty 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 Leqembi, they may have you switch to a different treatment.

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

How do side effects of Leqembi compare with those of Aricept?

Leqembi and Aricept (donepezil) are both used to treat Alzheimer’s disease. However, they come in different forms, work in different ways, and tend to cause different side effects.

Leqembi comes as an intravenous (IV) infusion (an injection into a vein given over time). More common side effects include headache and infusion reactions. (For more information about these side effects, see the “Tips for managing side effects” section.)

Aricept comes as an oral tablet. More common side effects include nausea, vomiting, diarrhea, and insomnia (difficulty sleeping).

These two drugs can also cause different serious side effects. For example, Leqembi has a boxed warning for brain changes, called ARIA, which involve swelling or bleeding in the brain. (For more information, see the “Boxed warning for Leqembi” section.) Aricept can cause a slow heart rate, urinary incontinence, and an increased risk of stomach ulcers.

To learn more about how these drugs compare, talk with your doctor. They can help determine whether one of these drugs is a good option for you.

Does Leqembi cause long-term side effects?

It’s unlikely. Long-term side effects weren’t reported in studies of Leqembi. In these studies, people received treatment with Leqembi for up to 18 months.

However, Leqembi is a relatively new drug. (It was approved by the FDA in 2023.) So information about its long-term safety is still limited.

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

Leqmebi has a boxed warning. A boxed warning is the most serious warning from the Food and Drug Administration (FDA).

Risk of amyloid-related imaging abnormalities (ARIA)

ARIA was a common side effect reported in studies of Leqembi. ARIA refers to changes in the brain that can be seen with the help of an MRI scan. They involve areas of swelling or bleeding in the brain.

ARIA doesn’t cause symptoms in most people, but in rare cases, the following symptoms have been reported:

  • headache
  • confusion
  • vision changes
  • nausea
  • dizziness
  • weakness
  • difficulty walking
  • difficulty speaking or writing, or understanding speech or written words
  • seizure

On rare occasions, symptoms of ARIA can be serious or even life threatening. ARIA can be fatal in some cases.

ARIA is most common in the first few months of Leqembi treatment but can happen at any time. These changes usually go away over time, but it’s possible to have several episodes of ARIA.

Factors that can increase the risk of ARIA when receiving this drug include having two copies of a gene called APOE ε4.

Certain other factors can increase the risk of bleeding in your brain with Leqembi. These include:

What might help

Before you start treatment with Leqembi, your doctor may recommend genetic testing. This is to determine whether you have an increased risk of ARIA. If you have a higher risk of this side effect, talk with your doctor about whether this treatment is right for you.

Your doctor will order MRI scans of your brain before you start Leqembi and regularly during your treatment. These scans help them monitor your brain for any changes. If you have symptoms of ARIA during Leqembi treatment, tell your doctor right away.

If you have symptoms of ARIA, or your MRIs show these changes in your brain, your doctor may recommend pausing your Leqembi treatment.

While taking Leqembi, you may be able to take steps to make side effects easier to manage.

If you have problems with side effects, they do not go away, or they become worse, talk with your doctor or pharmacist. They can discuss options for managing side effects or other available treatment options.

Infusion reactions

Leqembi is given by IV infusion, and infusion reactions were a common side effect reported in the drug’s studies. Infusion reactions are most likely to happen with the first dose of Leqembi.

Most infusion reactions are mild, but on rare occasions, they can be severe. Symptoms of infusion reactions can include:

If you have symptoms of an infusion reaction, tell the healthcare professional administering the infusion right away. They may slow down or pause your infusion until your symptoms resolve. They may also give you medication to treat your symptoms.

If you have an infusion reaction, you may be given medication before future Leqembi infusions to help prevent the reaction from happening again. For example, your doctor may prescribe antihistamines, acetaminophen (Tylenol), nonsteroidal anti-inflammatory drugs (NSAIDs), or corticosteroids.

Headache

Headache was a common side effect reported in studies of Leqembi. Steps you can take to manage symptoms associated with this side effect include:

  • taking over-the-counter pain medication, such as acetaminophen (Tylenol)
  • drinking plenty of water
  • applying a cold compress
  • drinking caffeinated tea or coffee

Consider the following important information before starting Leqembi.

Leqembi has a boxed warning. A boxed warning is the most serious warning from the Food and Drug Administration (FDA). For details, see the “Boxed warning for Leqembi” section.

Other precautions

Before taking Leqembi, discuss your health history with your doctor. Leqembi may not be right for you if you have certain medical conditions or other factors affecting your health. Be sure to talk with your doctor if any of the following apply to you:

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.