Leqembi (lecanemab-irmb) is a prescription drug used to treat Alzheimer’s disease. Leqembi can cause side effects that range from mild to serious. Examples include headache and infusion reactions.

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 article.

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

  • headache
  • cough
  • diarrhea
  • mild infusion reactions*
  • amyloid-related imaging abnormalities (ARIA)*

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

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

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.

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

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

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.

* 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 Leqembi, visit MedWatch.

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.*

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

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

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.

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

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 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
  • trouble walking
  • trouble speaking or writing, or understanding speech or written words
  • seizure

On rare occasions, symptoms of ARIA can be serious or even life threatening.

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.

Infusion reactions

Leqembi is given by IV infusion, and infusion reactions were a common side effect reported in the drug’s studies. Most infusion reactions are mild, but on rare occasions, they can be severe.

Symptoms of infusion reactions can include:

Infusion reactions are most likely to happen with the first dose of Leqembi.

What might help

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.

Allergic reaction

Like most drugs, Leqembi 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

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 Leqembi, they’ll decide if you should continue receiving 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 Leqembi, they may have you switch to a different treatment.

Keeping track of side effects

During your Leqembi 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 receiving 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 Leqembi affects you. They can then use this information to adjust your treatment plan if needed.

Below is important information you should consider before starting Leqembi treatment.

Boxed warning: Risk of amyloid-related imaging abnormalities (ARIA)

Leqembi has a boxed warning for the risk of ARIA. A boxed warning is the most serious warning from the FDA.

Leqembi can cause ARIA, which are changes in the brain such as swelling or bleeding that can be seen with an MRI scan. If you have a certain gene, you may have an increased risk of this side effect.

To learn more, see the “Side effects explained” section above.

Other warnings

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

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

  • pregnancy
  • breastfeeding
  • previous allergic reaction to Leqembi

Alcohol and Leqembi

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

Pregnancy and breastfeeding with Leqembi

There is limited information about the safety of Leqembi use during pregnancy or while breastfeeding.

Pregnancy

It’s not known whether it’s safe to receive Leqembi treatment during pregnancy. If you’re pregnant or planning to become pregnant, talk with your doctor before starting this medication.

Breastfeeding

It’s not known whether Leqembi treatment is safe while breastfeeding. If you’re breastfeeding or planning to do so, talk with your doctor before starting this medication.

Like most drugs, Leqembi can cause a number of 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 Leqembi can cause, talk with your doctor.

Examples of questions to help get you started include:

  • Do I need to tell you if I have a headache while receiving Leqembi?
  • How often will I need to have MRI scans during Leqembi treatment?
  • Is Leqembi safe for me if I have heart disease?

To learn more about Leqembi, see these articles:

To get information on different conditions and tips for improving your health, subscribe to any of Healthline’s newsletters. You may also want to check out the online communities at Bezzy. It’s a place where people with certain conditions can find support and connect with others.

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.