Published June 2, 2026 1 min read

# Why APOE genotype now matters for new Alzheimer’s drugs

## The short version

If you are considering lecanemab or donanemab for early Alzheimer's, your APOE genotype changes brain-swelling risk, monitoring, and sometimes eligibility. In the lecanemab trial, about a third of people with two copies had swelling on MRI, which is why genotyping comes first. These drugs treat confirmed early disease, not healthy carriers.

By the OutliveAPOE4 editorial team. [How we research & source](/methodology).

Here is the tension in one sentence: the new anti-amyloid drugs (**lecanemab** and **donanemab**) treat a disease APOE4 carriers are at higher risk for, and their main side effect is also more common in carriers. The benefit is modest: lecanemab slowed decline by about **27%** over 18 months (roughly a few months’ worth of difference), not a reversal.

ARIA is an MRI finding (magnetic resonance imaging, a scan that uses magnets and radio waves rather than X-rays), not a synonym for symptoms; the [full anti-amyloid evidence review](/topics/anti-amyloid-drugs-and-apoe4) explains the mechanism and genotype-specific rates. If this affects a real treatment decision, first review [how APOE testing should be confirmed clinically](/topics/how-to-get-tested-for-apoe4) and bring the monitoring questions to the treating specialist.

The side effect is **ARIA** (amyloid-related imaging abnormalities: brain swelling or small bleeds on MRI), and the genotype gradient is steep. In the lecanemab trial, ARIA-E occurred in about **5%** of non-carriers, **11%** of one-copy carriers, and **33%** of ε4/ε4 homozygotes. That is why APOE genotyping is now done before starting, and why eligibility can hinge on your genotype: the EU, for instance, approved lecanemab only for non-carriers and heterozygotes, excluding ε4/ε4 homozygotes.

The practical upshot: these are treatments for early, confirmed disease, not a general carrier intervention, and genotype affects both eligibility and the risk-and-benefit calculation. We unpack it in the deep dive on [anti-amyloid drugs and APOE4](/topics/anti-amyloid-drugs-and-apoe4).

## Source & references

1.  [Alzheimer’s Association: Lecanemab (Leqembi)](https://www.alz.org/alzheimers-dementia/treatments/lecanemab-leqembi)

## Related deep dives

-   [Anti-amyloid drugs (lecanemab, donanemab) and what they mean for carriers A new class of Alzheimer’s drugs can modestly slow decline, but APOE4 carriers, especially homozygotes, face higher rates of a key side effect. How they work and what to weigh.](/topics/anti-amyloid-drugs-and-apoe4)
-   [APOE4 and Alzheimer’s risk: what the numbers actually mean Relative risk, absolute risk, and age of onset: how to read the scary statistics about APOE4 and Alzheimer’s, with real ranges and the hopeful part that gets buried.](/topics/apoe4-and-alzheimers-risk)
-   [Blood-based biomarkers for Alzheimer’s: the coming shift For years, confirming Alzheimer’s biology meant a spinal tap or a PET scan. Blood tests are starting to change that. What they measure, where they stand, and the real caveats.](/topics/blood-based-biomarkers-alzheimers)