Leqembi show sits at the center of this dementia and brain health question.

Yes, patients with Alzheimer’s disease who receive long-term treatment with Leqembi (lecanemab) demonstrate measurable benefits in slowing cognitive decline, based on clinical trial data extending up to four years. The Clarity AD trial showed a 27% reduction in the rate of cognitive decline at 18 months, and subsequent analyses have confirmed these benefits persist when patients continue treatment—with three-year data showing a reduction of 0.95 points on the Clinical Dementia Rating Scale Sum of Boxes (CDR-SB), a standard measure of disease progression.

This means a patient who might have declined by 4 points over three years could instead decline by approximately 3 points, gaining roughly nine months of cognitive function compared to untreated disease progression. This article explores what the long-term data reveals about Leqembi’s effectiveness, including cognitive benefits, impacts on daily living abilities, safety profiles over extended treatment, and how new FDA-approved dosing options are making long-term treatment more practical for patients. Understanding these long-term benefits is important for patients with mild cognitive impairment or mild dementia, their families, and their healthcare providers who are deciding whether to pursue this disease-modifying treatment.

How Much Does Leqembi Slow Cognitive Decline in Alzheimer’s Disease?
What Do Three and Four Years of Continuous Leqembi Treatment Show?
How Does Leqembi Affect Daily Living and Overall Function?
Who Benefits Most from Long-Term Leqembi Treatment?
What Are the Real-World Safety Concerns Over Time?
How Has FDA Approval of New Dosing Options Changed Long-Term Treatment?
What Does the Real-World Treatment Persistence Data Tell Us?
Conclusion

How Much Does Leqembi Slow Cognitive Decline in Alzheimer’s Disease?

The primary evidence for Leqembi’s long-term benefit comes from the Clarity AD trial, which enrolled 1,795 patients with mild cognitive impairment or mild dementia due to Alzheimer’s disease. At the 18-month endpoint, lecanemab reduced cognitive decline by 27% compared to placebo—a statistically significant finding that became the basis for the drug’s FDA approval. On the CDR-SB scale, patients receiving lecanemab declined by an average of 1.21 points versus 1.66 points for placebo, a difference of 0.45 points over 18 months. What’s particularly important is that this benefit sustained over longer periods.

Three-year data from extended follow-up showed that patients treated continuously with lecanemab experienced cognitive decline of approximately 0.95 points per year, or roughly 2.85 points cumulatively over three years, compared to expected natural decline of 3.8 points or more in untreated populations. Four-year persistence data revealed a 1.75-point reduction in cognitive decline compared to benchmark controls, meaning patients remained in earlier disease stages longer. However, it’s essential to note that these benefits apply primarily to patients who start treatment in the early stages—those with mild cognitive impairment or mild dementia. Patients with moderate to advanced dementia have not been studied in these trials, so the benefits cannot be assumed for later-stage disease.

How Much Does Leqembi Slow Cognitive Decline in Alzheimer's Disease?

What Do Three and Four Years of Continuous Leqembi Treatment Show?

Extended treatment data indicates that cognitive benefits from Leqembi do not merely plateau but continue to accrue over multiple years of continuous administration. The three-year analyses demonstrated that the absolute difference in decline between lecanemab-treated and untreated groups grows over time, suggesting the medication creates a sustained “buffer” against progression. One particularly encouraging finding from long-term observation is that some patients treated early—those with mild cognitive impairment and low amyloid burden in the brain—may experience a potential delay in progression from MCI to mild dementia of up to 8.3 years. This projection is not guaranteed for every patient but represents the therapeutic potential when treatment begins as early as possible.

The limitation here is that four-year data is still relatively new, and we lack the 10, 15, or 20-year datasets that would show whether the protective effect continues indefinitely, diminishes over time, or eventually plateaus. Additionally, the four-year cohort represents a subset of the original trial population—only those who continued treatment for the full duration. This means the data does not fully capture the experiences of patients who discontinued early due to side effects, tolerability concerns, or other reasons. Some patients may decide to stop treatment, which would interrupt the accumulated benefit, making consistency and adherence over many years a practical requirement for achieving the full potential of long-term therapy.

How Does Leqembi Affect Daily Living and Overall Function?

Beyond cognitive decline measured on standardized scales, Leqembi also slowed decline in activities of daily living—the practical abilities that matter most to patients and families. The Clarity AD trial showed a 37% slowing of decline in instrumental and basic daily living abilities compared to placebo. Additionally, a 26% reduction in decline on secondary cognitive measures suggested benefits across multiple cognitive domains, not just memory. This translates to a patient maintaining the ability to manage finances, take medications independently, or perform household tasks for several additional months compared to untreated disease.

However, these functional benefits depend on cognitive decline being slowed first. If a patient’s cognitive function declines rapidly due to factors other than amyloid pathology—such as vascular disease, Lewy bodies, or tau pathology—Leqembi may not provide the same functional benefit. This is why biomarker testing, including amyloid PET scans or blood tests showing elevated phosphorylated tau, is important before starting treatment. For example, a patient with predominantly vascular dementia may have a normal amyloid biomarker result, and in such cases, Leqembi would not be recommended because it targets only one aspect of Alzheimer’s pathology. The drug is disease-modifying for amyloid-driven cognitive decline, but not for decline caused by other neuropathological processes.

How Does Leqembi Affect Daily Living and Overall Function?

Who Benefits Most from Long-Term Leqembi Treatment?

The most substantial benefits from long-term Leqembi treatment emerge in patients treated earliest in disease progression, specifically those with mild cognitive impairment or mild dementia stage. The earlier a patient begins treatment relative to symptom onset, the greater the potential benefit, because more cognitive reserve and brain function remain to be preserved. The data suggesting an 8.3-year potential delay in progression from MCI to mild dementia applies to a subset of patients with biomarker-confirmed amyloid positivity and those with lower amyloid burden who begin treatment when they are still cognitively normal or only mildly impaired. In contrast, patients with moderate dementia at baseline were excluded from the Clarity AD trial and subsequent long-term studies, so the benefit in more advanced disease is unknown.

This creates an important clinical decision point: starting Leqembi requires a diagnosis of MCI or mild dementia, which means patients and families must be comfortable with early diagnosis and the psychological implications of knowing they have amyloid pathology. Some patients may prefer not to know this information or may feel burdened by a diagnosis when symptoms are still minimal. Additionally, patients with certain contraindications—such as uncontrolled hypertension, recent stroke, or other conditions that increase risk of amyloid-related imaging abnormalities—may be advised against treatment despite having early-stage disease. The decision to pursue long-term treatment should involve discussion of who in the family might benefit and what the realistic expectations and limitations are.

What Are the Real-World Safety Concerns Over Time?

Four years of continuous Leqembi treatment have not revealed new safety findings beyond those observed in the initial Clarity AD trial. The primary safety concern with anti-amyloid monoclonal antibodies like lecanemab is amyloid-related imaging abnormalities (ARIA)—brain changes visible on MRI that include microhemorrhages (ARIA-microhemorrhages or ARIA-H) and brain swelling (ARIA-edema or ARIA-E). In the Clarity AD trial, 17.3% of lecanemab-treated patients experienced microhemorrhages compared to 9% receiving placebo, while ARIA-edema occurred in a minority of patients but was more common with the active drug. A reassuring long-term finding is that ARIA rates decreased after the initial 12 months of treatment and remained consistent through four years, suggesting that early monitoring and management of these imaging changes reduces ongoing risk.

However, overall adverse events occurred in 21.3% of lecanemab-treated patients versus 9.3% with placebo in the primary trial, indicating the treatment is not risk-free. Patients on Leqembi require regular MRI monitoring, typically every year, to detect asymptomatic ARIA early. This means patients must be willing and able to tolerate frequent MRI procedures, which some patients find claustrophobic or intolerable. Additionally, the cognitive and emotional impact of learning about brain imaging changes—even if asymptomatic—can create anxiety and may influence a patient’s willingness to continue long-term treatment.

What Are the Real-World Safety Concerns Over Time?

How Has FDA Approval of New Dosing Options Changed Long-Term Treatment?

When Leqembi was initially approved, the standard regimen was a 45-minute intravenous infusion once every two weeks, requiring frequent clinic visits and tying up healthcare resources. In January 2025, the FDA approved a new maintenance dosing schedule of once every four weeks, reducing treatment burden by half. More significantly, in August 2025, the FDA approved a weekly subcutaneous (under-the-skin) injection that patients can self-administer at home, fundamentally changing the practical feasibility of long-term treatment. This shift to at-home subcutaneous weekly dosing removes a major barrier to treatment continuation.

In the previous example where 78.4% of patients continued treatment at 18 months, 71.7% at 20 months, and 67.3% at 24 months (based on real-world U.S. data), clinic visit burden and infusion center availability were contributing factors to discontinuation. With weekly self-injection, patients who are motivated and physically capable can maintain treatment without frequent travel to healthcare facilities. However, this also requires patient education about self-injection technique and responsibility for maintaining adherence—missing weekly injections would interrupt the continuous benefit. For elderly patients with cognitive impairment, arthritis, or vision problems, even subcutaneous self-injection may remain challenging, and these patients may require caregiver assistance or might prefer the structure of scheduled clinic infusions.

What Does the Real-World Treatment Persistence Data Tell Us?

Real-world treatment persistence data from the United States, presented at the Alzheimer’s Prevention and Dementia Trials (ADPD) Conference in 2026, provides a sobering look at long-term adherence beyond controlled clinical trial settings. Among patients who initiated Leqembi, 78.4% were still receiving the drug at 18 months, 71.7% at 20 months, and 67.3% at 24 months. This means that approximately one-third of patients who began Leqembi discontinued treatment within two years.

Reasons for discontinuation include patient preference, development of side effects, logistical challenges, cost and insurance barriers, and in some cases, disease progression or medical complications requiring treatment cessation. These real-world numbers underscore that the long-term cognitive benefits observed in clinical trials depend on actual, sustained treatment adherence. A patient who receives Leqembi for six months but then stops cannot achieve the 27% reduction in decline or the potential 8.3-year delay in progression; they experience only partial benefit during the treatment window. As new at-home dosing options become available, persistence rates may improve, but they also place greater responsibility on patients to self-manage treatment and maintain adherence without the accountability of scheduled clinic visits.

Conclusion

Long-term data spanning up to four years demonstrates that Leqembi provides measurable, sustained benefits in slowing Alzheimer’s disease progression, with cognitive decline reduced by 27% in controlled trials and functional abilities declining 37% more slowly in treated patients compared to placebo. These benefits accumulate over time and are most pronounced in patients who begin treatment early, when they have mild cognitive impairment or mild dementia, with potential for delaying progression to more severe disease stages by years. The drug’s safety profile remains stable over four years of continuous treatment, though amyloid-related imaging abnormalities require regular MRI monitoring.

For patients and families considering long-term Leqembi treatment, the next steps involve discussions with a neurology or memory care specialist about biomarker testing (amyloid PET or blood-based amyloid and tau markers), baseline cognitive assessment, MRI screening for contraindications, and realistic expectations about benefits and risks. The new at-home weekly subcutaneous dosing approved in 2025 significantly improves treatment practicality compared to twice-weekly clinic infusions. However, treatment success depends on consistent adherence over years, and patients should prepare for the long-term commitment of weekly injections or visits while monitoring for imaging changes and side effects. Starting early, staying informed, and maintaining regular communication with healthcare providers offer the best chance of realizing the long-term cognitive benefits that Leqembi can provide.