ARIA: Side Effects of Anti-Amyloid Drugs

ARIA (amyloid-related imaging abnormalities) is the main side effect of anti-amyloid Alzheimer's drugs such as lecanemab and donanemab; learn what it is, who is at higher risk, how it is monitored, and which symptoms need urgent care.

Last reviewed: October 2026

The newest Alzheimer's medicines—lecanemab (Leqembi) and donanemab (Kisunla)—work by clearing a sticky protein called beta-amyloid from the brain. They can modestly slow decline in early Alzheimer's. But they also carry an important risk with an unusual name: ARIA, short for amyloid-related imaging abnormalities.

If you or someone you love is considering one of these drugs, understanding ARIA is one of the most important parts of the decision. This page explains it in plain language.

What is ARIA?

ARIA is a change in the brain that shows up on an MRI scan. The term was introduced in 2010 to describe findings seen in people receiving anti-amyloid antibody treatments, according to a review in the American Journal of Roentgenology (AJR). Experts think it happens because as the drug pulls amyloid away from brain tissue and blood vessel walls, the vessels can become temporarily "leaky."

There are two main types:

ARIA-E (edema/effusion): swelling or a buildup of fluid in part of the brain. The FDA describes it simply as "brain swelling or fluid buildup."
ARIA-H (hemosiderin): tiny spots of bleeding (microhemorrhages) or small deposits of iron on the brain's surface (superficial siderosis) left behind by leaked blood.

The two types often happen together. Doctors grade each as mild, moderate or severe based on how large or how many the changes are on MRI.

How common is it?

ARIA is common with these drugs, though most cases cause no symptoms. In the major clinical trials:

Lecanemab: In the 18-month CLARITY AD trial of 1,795 people, published in the New England Journal of Medicine, ARIA-E occurred in 12.6% of people taking lecanemab.
Donanemab: In the TRAILBLAZER-ALZ 2 trial, published in JAMA, ARIA-E occurred in 24.0% of people taking donanemab (52 of the 205 cases had symptoms), compared with 2.1% of people on placebo.

Rates in everyday practice can differ from trial results, and dosing schedules have been updated since these trials. See Lecanemab (Leqembi) and Donanemab (Kisunla) for more on each drug.

Who is at higher risk?

The AJR review and FDA materials point to several risk factors:

The APOE4 gene. This is the biggest known risk factor. People who carry the APOE ε4 gene—especially those with two copies—have higher rates of ARIA. For this reason, the AJR review notes that genetic testing before treatment may be warranted, and it is something to discuss with the specialist. See Genetic Testing and APOE4.
Existing microbleeds. People who already have small bleeds on their baseline MRI are at higher risk of more.
Blood thinners. The AJR review notes that using antithrombotic medicines (blood thinners, including aspirin) along with these drugs is linked to a higher risk of ARIA-H. Always tell the care team about every medicine you take.
Early treatment. ARIA usually appears in the first months of treatment, especially as the dose is being increased.
Higher doses. ARIA-E is more common at higher doses.

What are the symptoms?

Most people with ARIA have no symptoms, and it is found only on a routine scan. When symptoms do occur, the FDA lists:

Headache
Confusion
Dizziness
Vision changes
Nausea
Trouble walking
Trouble speaking or finding words (aphasia)
Weakness
Seizures

These signs can be easy to confuse with "a bad day" or with Alzheimer's itself. Caregivers play an important role: if something seems new or different in the days after an infusion, speak up.

Can ARIA be serious?

Usually ARIA is mild and goes away on its own, often after the drug is paused. But serious and life-threatening events can happen. In its August 2025 safety communication, the FDA reported that it had reviewed 101 cases of serious ARIA-E with lecanemab, including six deaths that occurred early in treatment. In the donanemab trial, three deaths in the drug group were considered treatment-related.

Keeping this in perspective matters: most people who develop ARIA recover. But because serious cases do occur, careful monitoring is essential.

How is ARIA monitored?

Because ARIA is usually silent, the main safety tool is scheduled MRI scans:

A baseline MRI before treatment starts, so doctors have something to compare against.
Repeat MRIs at set points during the first months of treatment. For lecanemab, the FDA now recommends MRIs before the 3rd, 5th, 7th and 14th infusions—the 3rd-infusion scan was added in 2025 after the FDA found that serious cases could appear early.
An urgent MRI any time symptoms suggest ARIA.

Donanemab has its own MRI schedule in its prescribing information. Ask your care team exactly when your scans are due, and keep those appointments. See Brain Imaging: MRI, CT and PET for what an MRI involves.

What happens if ARIA is found?

It depends on how severe it looks on the scan and whether there are symptoms. According to the AJR review and FDA guidance, the care team may:

Continue treatment with closer monitoring if ARIA is mild and causes no symptoms.
Pause the drug until the swelling clears or the changes stabilize.
Stop treatment permanently if ARIA is severe or keeps coming back.
Treat symptoms if needed; the AJR review notes that steroids have sometimes been used for symptomatic ARIA-E.

ARIA-E is usually temporary, and the AJR review notes that symptoms tend to resolve when treatment is paused or stopped. Small bleeds (ARIA-H) may remain on later scans; doctors watch to make sure they stay stable.

Questions to ask the doctor

What is my APOE status, and what does it mean for my ARIA risk?
Do I have any microbleeds on my baseline MRI?
Should any of my medicines, such as blood thinners, be reviewed?
When are my MRI scans scheduled?
Who do I call—day or night—if symptoms appear?
If I need emergency care, what should I tell the hospital about this treatment?

It can also help to carry a wallet card or phone note saying you are taking an anti-amyloid drug, so that any emergency or hospital team knows right away.

When to see a doctor

Call your care team or go to the nearest emergency room right away if someone taking lecanemab or donanemab has a new headache, confusion, dizziness, vision changes, nausea, trouble walking or speaking, weakness or a seizure. The FDA advises that if the person cannot reach out themselves, the caregiver should do so for them.

This page is general information, not medical advice. Decisions about starting, pausing or stopping an anti-amyloid drug should always be made with a specialist who knows the person's full health history. For an overview of all options, see Dementia Treatments: An Overview.

Sources

Not medical advice. This article is general information, not a diagnosis or treatment plan for any individual. Talk with a doctor about your situation.

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