Quick answer: Leqembi can cause infusion-related reactions, ARIA (brain swelling or tiny bleeds), headaches, nausea, and a handful of less-common issues. Most side effects are mild and manageable, but serious cases do happen.
Bottom line: Knowing how often these reactions occur, spotting the early warning signs, and having a clear plan for what to do next can keep you safe while you're on the treatment.
What is Leqembi?
Leqembi (lecanemab-irmb) is an intravenous antibody approved by the FDA for people with mild-to-moderate Alzheimer's disease. It works by targeting amyloid plaques in the brain, aiming to slow cognitive decline. The drug is given as a monthly infusion for up to 18 months, after which doctors reassess the need for further treatment.
Why side effects matter? Because any medication that touches the brain can tip the balance either wayenhancing benefits while also opening the door to unwanted reactions. Understanding that balance is the first step toward informed, shared decision-making.
Common side effects
Infusion-related reactions
These are the most frequently reported issues. You might feel a sudden warmth, chills, fever, nausea, or a pounding headache during or shortly after the infusion.
Studies show that roughly 3040% of patients experience at least one of these symptoms.
Quick-care checklist
- Stay well-hydrated the day before your appointment.
- Tell the infusion nurse if you've had a fever over 38C recently.
- Ask about pre-medicating with an antihistamine or acetaminophen.
ARIA (Amyloid-Related Imaging Abnormalities)
ARIA is a brain-specific reaction that shows up on MRIs as swelling (ARIA-E) or tiny bleeds (ARIA-H). It's the second most common side effect, affecting about 27% of patients, with serious cases in roughly 3%.
Typical symptoms include confusion, visual disturbances, persistent headache, or nausea. If you notice any of these, call your neurologist right away.
Monitoring protocol
| Week of treatment | MRI check | Action if ARIA-positive |
|---|---|---|
| Baseline (Week 0) | Proceed with first infusion | |
| Weeks 36 | Hold infusion if moderatesevere ARIA; consider dose reduction | |
| Week 12+ | Continue with monitoring; adjust schedule if needed |
Headaches & mild neurological complaints
About 1520% of patients report headaches, occasional dizziness, or a feeling of brain fog. These are usually short-lived. If a headache lasts more than 48 hours or comes with visual changes, it's a red flag.
Home-relief suggestions
- Take an OTC pain reliever like acetaminophen.
- Prioritize sleep and stay hydrated.
- Avoid caffeine spikes on infusion days.
Less common / severe side effects
Gastrointestinal upset
Diarrhea, nausea, and occasional vomiting are reported by 510% of patients. Most settle with dietary tweaks or a short course of antiemetics. If you experience persistent nausea, it's worth discussing strategies with your care teamsometimes, switching the timing or environment of your infusion can help. For more on managing drug-related side effects, see our guide on serious drug side effects.
Vision changes & dizziness
A small handful of users notice blurred vision or sudden dizziness. Sudden loss of vision should trigger an immediate stop of the infusion and a call to emergency services.
Cardiovascular warnings
Rarely, patients report chest tightness or blood-pressure swings during infusion. If you have a history of heart disease, let your infusion team know ahead of time so they can monitor vitals closely.
How often do side effects occur?
Below is a quick visual breakdown of side-effect frequencies based on pooled Phase 3 data:
| Side effect | Incidence (%) | Severity |
|---|---|---|
| Infusion reactions | 3040 | Mildmoderate |
| ARIA (any) | 27 | Mostly mild |
| Headaches | 1520 | Mild |
| Gastrointestinal | 510 | Mildmoderate |
| Vision changes | 24 | Raremoderate |
| Cardiovascular | <1 | Raresevere |
These percentages are drawn from the FDA label, the CLARITY-AD trial, and independent analyses posted on reputable medical sites.
Real-world experiences
What Reddit users say
Scrolling through the Leqembi subreddit, three themes keep popping up: anxiety about the first infusion, surprise at ARIA findings on MRI, and the constant weighing of benefit versus side-effect load.
One user wrote, "I was terrified when my first scan showed mild ARIA, but the team held my infusion, gave me steroids, and the swelling cleared in a month. I'm still on the drug and feeling steadier." This kind of candid sharing gives a human face to the numbers.
Patient handout (PDF)
Many memory-center clinics offer a downloadable handout that summarizes what to watch for after each infusion. It's a handy cheat-sheet you can keep on the fridge.
Managing & mitigating side effects
Pre-infusion preparation
Start with a light meal, avoid alcohol, and double-check any new medications with your neurologist. A baseline lab panel and an MRI are standard before the first dose.
During infusion
The infusion nurse will check blood pressure, temperature, and heart rate every 15 minutes. Emergency medslike epinephrine and antihistaminesare kept on standby just in case.
Post-infusion care
Keep a symptom diary for the next 48 hours. Note any fever, headache, visual change, or unusual fatigue. If anything feels off, call the clinic within the same day.
Sample symptom diary
| Date | Time | Symptom | Severity (15) | Action taken |
|---|---|---|---|---|
| 2025-09-01 | 09:30 | Mild headache | 2 | Took acetaminophen |
| 2025-09-01 | 14:00 | Warm flush | 3 | Called nurse, no further action |
Contraindications & drug interactions
Absolute contraindications include active brain hemorrhage, uncontrolled hypertension, and a recent stroke. Relative contraindications are ongoing anticoagulant therapy, severe renal impairment, or a history of severe ARIA.
Leqembi can interact with strong CYP-enzyme inhibitors and some anticoagulants; always run a medication review with your doctor before starting.
Bottom lineweighing benefits and risks
The promise of Leqembi is real: slowing the march of Alzheimer's in many patients. But that promise comes with a side-effect profile you need to understand. Infusion reactions are common but usually brief; ARIA is rarer and often manageable with close MRI monitoring. Severe cardiovascular events are extremely uncommon.
The best approach is open communication with your care team, diligent monitoring, and a willingness to pause the treatment if a serious reaction appears. Remember, you're not alonethousands of families are walking this path and sharing tips on forums, Reddit, and support groups.
Leqembi side effects range from the everyday (a warm sensation during infusion) to the more serious (ARIA on MRI). For additional information on the most serious type of drug warnings, see our resource on black box warning labels and how they relate to patient safety.
Final thoughts
Knowing the percentages, recognizing early signs, and having a concrete action plan empowers you to stay in control of your health journey. If you've experienced any of these reactions, or if you're about to start treatment, consider downloading the symptom diary, scheduling your next MRI, and joining an online community for support. Your questions and experiences mattershare them in the comments below, and let's help each other navigate this new frontier together.
FAQs
What are the most common Leqembi side effects?
The most frequently reported effects are infusion‑related reactions (warmth, chills, headache) occurring in 30‑40 % of patients, and ARIA (brain swelling or micro‑bleeds) seen in about 27 %.
How is ARIA detected and what should I do if it appears?
ARIA is identified through scheduled MRIs before and during treatment. If you experience confusion, persistent headache, visual changes, or nausea, contact your neurologist immediately for evaluation.
Can I take anything to prevent infusion reactions?
Many clinics pre‑medicate with acetaminophen and an antihistamine. Staying well‑hydrated and avoiding alcohol before the appointment also helps reduce symptoms.
Are there any serious cardiovascular risks with Leqembi?
Severe heart‑related events are rare (<1 %). Patients with existing heart disease should inform their infusion team so vitals can be closely monitored during the infusion.
What should I do if I develop mild gastrointestinal upset?
Typical remedies include light meals, staying hydrated, and over‑the‑counter anti‑emetics. If symptoms persist beyond a few days, discuss medication adjustments with your doctor.
