What the label lists and what reports show
Lecanemab side effects
Lecanemab's FDA label: “Most common adverse reactions (at approximately 10% and higher incidence compared to placebo): infusion-related reactions, amyloid related imaging abnormality-microhemorrhages, amyloid related imaging abnormality-edema/effusion, and headache.” FAERS holds 3,591 reports naming it through July 30, 2026; the reaction reported most often, leaving out the conditions it is used for, product-use terms and outcomes, is amyloid related imaging abnormality-oedema/effusion (491 reports).
- No generic listed
- Boxed warning
Check it at the source: the lecanemab FDA label on DailyMed (effective July 21, 2026); the FDA FAERS public dashboard.
What does the patient leaflet say about side effects?
From the patient leaflet: What are the possible side effects of LEQEMBI?LEQEMBI can cause serious side effects, including: see “What is the most important information I should know about LEQEMBI?” serious allergic reactions. Swelling of the face, lips, mouth, or tongue, hives, or difficulty breathing have happened during treatment with LEQEMBI. Tell your healthcare provider and get medical help right away if you have any symptoms of a serious allergic reaction during or after treatment with LEQEMBI . infusion-related reactions. Infusion-related reactions are a common side effect with LEQEMBI injection into a vein (intravenously), which can be serious. You will receive your infusion at a healthcare facility. If a serious infusion reaction happens while you are receiving LEQEMBI, your infusion may be stopped. Serious infusion reactions can also occur after completion of the infusion. […]
Source: FDA drug label, Eisai Inc., effective July 21, 2026 (SPL set 9d1ff786-e577-410a-a273-c4d7d0e4e975), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsMost common adverse reactions (at approximately 10% and higher incidence compared to placebo): infusion-related reactions, amyloid related imaging abnormality-microhemorrhages, amyloid related imaging abnormality-edema/effusion, and headache.
Table 7: Adverse Reactions Reported in at Least 5% of Patients Treated with LEQEMBI 10 mg/kg Every Two Weeks and at least 2% Higher than Placebo in Study 2
| Adverse Reaction | LEQEMBI 10 mg/kg Every Two Weeks N= 898 % | Placebo N= 897 % |
|---|---|---|
| Infusion-related reactions | 26 | 7 |
| ARIA-H | 14 | 8 |
| ARIA-E | 13 | 2 |
| Headache | 11 | 8 |
| Superficial siderosis of central nervous system | 6 | 3 |
| Rash 1 | 6 | 4 |
| Nausea/Vomiting | 6 | 4 |
Rows and numbers as the label prints them, under its own column headings (the first rows of a longer table where it runs on). A reaction in a trial table is not necessarily caused by the medicine, and trials differ from everyday use.
From the label: adverse reactions, continuedThe following clinically significant adverse reactions are described elsewhere in the labeling: Amyloid Related Imaging Abnormalities [see Warnings and Precautions ( 5.1 )] Hypersensitivity Reactions [see Warnings and Precautions ( 5.2 )] Infusion-Related Reactions [see Warnings and Precautions ( 5.3 )] Most common adverse reactions (at approximately 10% and higher incidence compared to placebo): infusion-related reactions, amyloid related imaging abnormality-microhemorrhages, amyloid related imaging abnormality-edema/effusion, and headache. Additionally, injection-related reactions occurred in patients receiving subcutaneously administered LEQEMBI. ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Eisai Inc. at 1-888-274-2378 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch. 6.1 Clinical Trials Experience Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared to rates in the clinical trials of another drug and may not reflect the rates observed in practice. Clinical Trials with Intravenous Administration The safety of LEQEMBI has been evaluated in 2090 patients who received at least one dose of LEQEMBI by intravenous infusion. …
Source: FDA drug label, Eisai Inc., effective July 21, 2026 (SPL set 9d1ff786-e577-410a-a273-c4d7d0e4e975), via openFDA; the full label on DailyMed
What do FAERS reports show?
3,591 reports list lecanemab among the medicines taken, each a report of something that happened, not proof the medicine caused it; 154 are coded as a death and 898 as a hospital stay. Counts follow how widely a medicine is used and watched, and they are not a measure of how risky it is. Reports rose sharply in 2024 (1,410, against a median of 0 a year): FAERS counts follow reporting, which news, litigation and label changes can drive, not how often a reaction happens. Of the 25 reactions below, 12 are named by the same term (in US spelling) in the label's adverse reactions, warnings or patient leaflet; a reaction the label does not name may still be there under another word, and a report does not show that the medicine caused it.
Reports received each year
The 25 most reported reactions
Source: FDA Adverse Event Reporting System (FAERS) via openFDA, reports through July 30, 2026; reactions as MedDRA preferred terms
What next?
- Lecanemab: the full record
- label, generics, price, shortages, recalls
- Report a side effect to the FDA
- MedWatch
- Side effects hub
- the most reported medicines and reactions