Memory Digest

Lecanemab (Leqembi)

How the anti-amyloid drug lecanemab works, who it's for, how well it works, side effects including ARIA, and what treatment involves.

Last reviewed: October 2026

Lecanemab (brand name Leqembi) is a medicine for early Alzheimer's disease. It was the first drug of its kind to receive full (traditional) FDA approval, in July 2023, after showing that it can modestly slow the disease. It is an important step forward—but it isn't a cure, and it isn't right for everyone.

How it works

Lecanemab is a monoclonal antibody—a lab-made immune protein. It attaches to forms of beta-amyloid, the sticky protein that builds up in the brains of people with Alzheimer's Disease, and helps the immune system clear it. Brain scans show amyloid levels fall substantially during treatment.

Who it's for

Lecanemab is approved for people with:

• Mild Cognitive Impairment (MCI) or mild dementia due to Alzheimer's (the stages studied in trials), and
• Confirmed amyloid in the brain, shown by an amyloid PET scan or spinal fluid test (and increasingly supported by blood tests).

It is not approved for moderate or severe Alzheimer's, or for other types of dementia. Specialists also generally advise against it for people who take blood thinners (anticoagulants), have many small brain bleeds or significant other changes on MRI, or have certain other medical conditions.

How well does it work?

In the 18-month CLARITY AD trial of about 1,800 people, published in the New England Journal of Medicine in 2023, lecanemab slowed decline on a standard scale of memory and daily function (CDR-SB) by about 27% compared with placebo. In practical terms, the difference over 18 months was modest—about half a point on an 18-point scale. People on lecanemab still declined, just more slowly. Researchers continue to follow participants to see whether the benefit grows over longer periods, and longer-term data presented by the manufacturer suggest the difference persists, though such extension data are less certain than randomized results.

Whether this benefit is meaningful is a personal decision. Some families value more time in the early stage; others feel the burden and risk outweigh a modest effect.

Side effects: ARIA

The most important risk is ARIA (amyloid-related imaging abnormalities):

• ARIA-E – temporary swelling in the brain. Occurred in about 13% of people in CLARITY AD.
• ARIA-H – small spots of bleeding (microhemorrhages) or iron deposits. Occurred in about 17%.

Most ARIA causes no symptoms and is found on routine MRI scans. Some people have headache, confusion, dizziness, vision changes, nausea or seizures. Rarely, ARIA is serious or life-threatening; deaths have been reported. The risk is highest in people with two copies of the APOE4 gene, so genetic testing is recommended before treatment, and the FDA label carries a boxed warning about ARIA.

Other side effects include infusion reactions (fever, chills, flu-like symptoms, nausea), usually with the first doses, and headache.

If someone on lecanemab develops sudden headache, confusion, vision changes or seizures, seek medical care promptly and tell the team they are on an anti-amyloid drug—some clot-busting stroke treatments may be dangerous in this situation.

What treatment involves

• Before starting: cognitive testing, amyloid confirmation, a baseline brain MRI, and usually APOE genetic testing with counseling.
• Dosing: an IV infusion every two weeks, lasting about an hour. An under-the-skin autoinjector form (Leqembi Iqlik) was approved in 2025 for weekly at-home maintenance dosing after the initial IV phase.
• Monitoring: repeated MRI scans, especially in the first months of treatment, to check for ARIA.
• Time and travel: regular visits to an infusion center can be a significant commitment for families.

Cost and coverage

The list price is high (tens of thousands of dollars per year before insurance). Medicare covers lecanemab for eligible people when the prescribing clinician participates in a registry; PET scans, MRIs and infusion costs add to the total, and out-of-pocket costs depend on the plan. Check with your insurer and the treating center.

Questions to ask

• Am I in the early stage where this drug was studied?
• Do I have confirmed amyloid? What is my APOE status and ARIA risk?
• What would my schedule of infusions and MRIs look like?
• What will it cost me?
• How will we decide whether to continue?
• How does it compare to Donanemab (Kisunla) for someone like me?

The bottom line

Lecanemab offers a modest slowing of early Alzheimer's for carefully selected people, with real risks and a significant time commitment. It's a decision to make with a specialist, weighing benefits, risks and personal values. See also Dementia Treatments: An Overview.

Sources

1. van Dyck CH et al. Lecanemab in Early Alzheimer's Disease (CLARITY AD). New England Journal of Medicine, 2023
2. FDA: FDA Converts Novel Alzheimer's Disease Treatment to Traditional Approval (July 2023)
3. Alzheimer's Association: Leqembi (lecanemab)
4. Cummings J et al. Lecanemab: Appropriate Use Recommendations. J Prev Alzheimers Dis, 2023

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

Related articles

• Donanemab (Kisunla)
• Dementia Treatments: An Overview
• Alzheimer's Disease
• Getting a Diagnosis
• Cholinesterase Inhibitors and Memantine