Creator: Help Dementia Editorial Team
Published: 2026-04-09T02:15:06

Reviewed by the Help Dementia Editorial Team — our editors review every article for accuracy against guidance from the National Institute on Aging, the Alzheimer’s Association, and peer-reviewed sources.

Post-market surveillance sits at the center of this dementia and brain health question.

Post-market surveillance provides the most complete picture of how Alzheimer’s medications perform once they’re being used by thousands of patients outside the controlled environment of clinical trials. When the FDA approved lecanemab (Leqembi) in 2023, the agency didn’t simply hand off the drug to the public. Instead, multiple surveillance systems sprang into action, collecting real-world safety data that now forms the foundation for confident recommendations about who should receive these treatments and how they should be monitored. This ongoing watchfulness has proven essential—not because the drugs are uniquely dangerous, but because brain-targeted medications affecting amyloid proteins work in complex ways that only become fully apparent across diverse patient populations.

The real-world safety picture for lecanemab demonstrates both the promise and the necessity of post-market vigilance. Within the first 18 months of availability, the FDA Adverse Event Reporting System logged 1,986 adverse events linked to lecanemab affecting 868 patients, including 203 serious adverse events and 22 deaths. While these numbers might sound alarming in isolation, they gain meaning only within the context of how many people are actually taking the drug, what alternative treatments are available, and whether reported events correlate with the drug or with the underlying disease. This is precisely what post-market surveillance is designed to answer.

How Does the FDA Monitor Alzheimer’s Drug Safety After Approval?
The Real-World Safety Profile of Lecanemab Revealed
When Clinical Trials and Real-World Experience Don’t Match
Why Monitoring Protocols Protect Patients Who Receive These Drugs
Detecting Safety Signals Before They Become Crises
What Increasing Reports Tell Us About Drug Safety Confidence
The Next Phase: FDA Decisions and Expanded Access
Conclusion

How Does the FDA Monitor Alzheimer’s Drug Safety After Approval?

The FDA operates multiple overlapping systems to track medication safety in the real world, with the Sentinel System representing the most comprehensive effort. This system monitors medication usage and medical data on over 300 million people across the United States, generating evidence about adverse drug reactions from actual medical records rather than relying solely on voluntary reports. For Alzheimer’s drugs specifically, the agency combines Sentinel data with the traditional Adverse Event Reporting System (FAERS), where healthcare providers, patients, and manufacturers can submit reports of side effects they observe during treatment. The distinction between these systems matters for how we interpret safety data.

FAERS reports are voluntary and can be submitted by anyone, which means they capture events people believe are important enough to report—but they may also include coincidental events that happened to occur while someone was taking a drug. The Sentinel System, by contrast, pulls from electronic health records and insurance claims, allowing FDA analysts to calculate rates of adverse events and compare them to control groups. When lecanemab adverse event reports jumped from 7.5 cases per month in early 2023 to 104 cases per month by mid-2024, both systems were signaling increased detection, heightened clinical awareness, and broader patient access. The question then became: were we discovering previously unrecognized risks, or simply monitoring more patients taking the drug?.

How Does the FDA Monitor Alzheimer's Drug Safety After Approval?

The Real-World Safety Profile of Lecanemab Revealed

The most common reported adverse events from lecanemab surveillance paint a picture of tolerability challenges rather than catastrophic danger. Headache topped the list with 193 reported cases, followed by chills (100 cases), fatigue (93 cases), and amyloid-related imaging abnormality-edema or ARIA-E (91 cases). These events range from inconvenient to serious—chills and fatigue might resolve with continued dosing, but brain swelling visible on MRI represents a medical concern requiring immediate clinical attention. One important limitation of these numbers: they reflect reported cases in a voluntary system, not the actual rate of events. If 10,000 people take lecanemab and 91 experience ARIA-E, that’s roughly a 1% rate. If 100,000 people take it and 91 experience ARIA-E, that’s 0.1%.

Without denominator data, the raw case counts can mislead. The trajectory of adverse event reporting deserves particular attention. The sixfold increase in reports between early 2023 and mid-2024—from 45 cases in the first half of 2023 to 625 in the first half of 2024—reflects a combination of factors. More patients were accessing the drug as it became established in clinical practice. Clinicians were becoming more familiar with which side effects to watch for and more likely to report them. Some events that appeared in early reports might have been captured again as the same patients continued treatment. Rather than indicating that lecanemab suddenly became more dangerous, this reporting increase reflects the normal pattern when a new medication enters widespread use and healthcare providers attune themselves to its side effect profile.

When Clinical Trials and Real-World Experience Don’t Match

The controlled environment of clinical trials and the chaotic reality of medical practice often produce different safety signals. In lecanemab’s pivotal clinical trial, 17.3% of participants showed brain swelling or microhemorrhages on imaging, compared to 9% in the placebo group—a finding that raised concerns but was ultimately deemed acceptable given the drug’s cognitive benefit. Real-world surveillance has continued to detect amyloid-related imaging abnormalities, but with a broader population including older patients, those with cardiovascular disease, and people taking anticoagulants who were largely excluded from trials. This explains some of the discrepancy: trials are conducted in relatively healthy people willing to commit to frequent clinic visits and MRI scans, while real-world patients are messier, more varied, and more representative of who actually has Alzheimer’s disease.

Donanemab, the second-generation monoclonal antibody that’s undergone similar post-market scrutiny, demonstrated even higher rates of brain imaging abnormalities in trials—up to 30.5% of participants showed amyloid-related changes compared to just 0.8% to 7.2% in placebo groups. More troubling, three deaths were attributed to donanemab-related ARIA, though causality remained difficult to establish in patients with advanced dementia. These findings prompted the FDA and manufacturers to implement mandatory MRI monitoring protocols: surveillance scans are now required before the 2nd, 3rd, 4th, and 7th infusions of donanemab, plus additional imaging before the 12th dose in higher-risk patients and whenever cognitive decline or symptoms suggest ARIA. This represents post-market surveillance driving real changes in how the drug is administered—not a failure of the approval process, but evidence that it’s working as intended.

When Clinical Trials and Real-World Experience Don't Match

Why Monitoring Protocols Protect Patients Who Receive These Drugs

The MRI requirements for donanemab illustrate how post-market surveillance translates into clinical practice changes that benefit patients. Rather than receiving infusions blindly, patients now get brain imaging before certain doses, allowing clinicians to catch asymptomatic swelling or microhemorrhages before they cause symptoms. This monitoring is resource-intensive—not every patient can easily access MRI machines, and the scans add time and cost to treatment. For patients and their families, this creates a tradeoff: the drug may slow cognitive decline by several months, but only if you’re willing to undergo repeated MRI scans, accept the risk of incidental findings, and manage the anxiety of waiting for results. Post-market surveillance makes these risks visible enough that informed consent becomes possible.

Lecanemab patients face similar monitoring expectations despite less formal requirements. Clinicians experienced with the drug recommend baseline MRI and cognitive assessment before starting treatment, with repeat imaging if patients develop symptoms suggesting amyloid-related changes—confusion, headaches that worsen, or personality changes. For patients in early symptomatic stages of Alzheimer’s disease, this level of surveillance represents manageable overhead. But for patients approaching end-stage disease or those with multiple medical complications, the monitoring burden may outweigh benefits. Post-market surveillance has revealed these practical realities, allowing doctors and patients to make treatment decisions based on actual experience rather than trial data alone.

Detecting Safety Signals Before They Become Crises

One of the more remarkable findings from lecanemab surveillance was the detection of a confusional state signal—a specific pattern of acute confusion distinct from the cognitive decline of Alzheimer’s disease itself. Real-world analysis found this adverse event showing a 15-fold increased reporting risk associated with lecanemab compared to baseline. This type of signal detection represents post-market surveillance at its most valuable: identifying a side effect that might not have appeared prominently in trials because trial populations are younger, more cognitively intact, and more closely monitored. In the real world, acute confusion superimposed on existing dementia can be catastrophic, leading to falls, hospital admissions, and accelerated decline.

The challenge with signal detection is distinguishing true drug effects from background noise. When thousands of elderly people with cognitive decline take a medication, some will develop confusion coincidentally—from infections, medication interactions, metabolic imbalances, or disease progression alone. The statistical methods FDA uses attempt to separate these background events from genuine drug-related risks, but uncertainty always remains. The 15-fold signal for lecanemab-associated confusion represents a strong signal, but researchers continue to investigate whether this reflects a true pharmacological effect, a detection bias (clinicians are more likely to attribute confusion to the drug), or a clinical phenomenon specific to how lecanemab affects certain patients’ cognition acutely before any long-term benefit accrues.

Detecting Safety Signals Before They Become Crises

What Increasing Reports Tell Us About Drug Safety Confidence

The dramatic increase in adverse event reports over the first 18 months of lecanemab availability—from 7.5 reports per month to 104 per month—might be interpreted as bad news. In reality, this pattern resembles the normal trajectory of safety surveillance for any newly approved medication. Early reports tend to come from specialists and academic centers most likely to recognize and report drug reactions. As the drug diffuses into community practice, more generalist physicians prescribe it, and their reporting patterns differ. Patient advocacy groups begin alerting their communities about potential side effects, prompting existing patients to file reports about events that occurred months earlier.

This “catch-up” reporting creates an apparent spike in adverse events that simply reflects increased surveillance intensity rather than increasing danger. For Alzheimer’s medications specifically, this reporting pattern provides reassurance. If lecanemab’s safety profile had fundamentally worsened over time, we would expect to see changing patterns in the types of events reported or demographic shifts toward older or sicker patients suffering greater harm. Instead, the adverse event profiles have remained consistent: the same complications appearing at similar rates, suggesting that initial trial data reasonably predicted real-world experience. The monitoring systems are working exactly as designed—catching edge cases and unusual events, documenting demographic variations in risk, and providing the FDA with sufficient evidence to make informed decisions about appropriate use.

The Next Phase: FDA Decisions and Expanded Access

The post-market surveillance systems tracking lecanemab and donanemab continue to inform regulatory decisions about how these drugs should be used. The FDA is expected to decide in May 2026 on whether to approve at-home initial starter doses of Leqembi (lecanemab), a convenience that has generated considerable excitement among patients and clinicians. This decision will rest substantially on post-market safety data accumulated over the previous three years. If surveillance data shows that home-based initial dosing carries unacceptable risks—perhaps because patients lack immediate access to medical support if they experience acute confusion or allergic reactions—the FDA can decline the application or impose restrictions.

Conversely, if the data show that carefully selected patients with appropriate home support can safely begin treatment outside a clinic, home dosing becomes possible. This forward-looking application of post-market surveillance illustrates why confidence in these drugs’ safety profiles matters. Public confidence, built on transparent reporting of side effects and rigorous analysis of their frequency and severity, allows the FDA to make bold decisions like expanding access when evidence supports it. Without ongoing surveillance, regulators face a choice: either deny patients access to potentially beneficial treatments out of abundance of caution, or approve new access patterns without adequate safety data. Post-market surveillance breaks this false binary by continuously collecting evidence about how drugs behave in the real world.

Conclusion

Post-market surveillance demonstrates that confidence in Alzheimer’s drug safety is not naïve optimism—it’s earned through systematic collection and analysis of real-world experience. The 1,986 adverse events reported for lecanemab, the mandatory MRI protocols for donanemab, and the emerging signals of confusion all represent the system working correctly. These findings don’t prove the drugs are unsafe; rather, they document that the medications carry identifiable risks that can be managed through appropriate patient selection, monitoring protocols, and informed consent conversations between doctors and patients.

For families considering Alzheimer’s medications, post-market surveillance provides the foundation for realistic decision-making. These drugs may modestly slow cognitive decline, but they carry documented risks of brain imaging abnormalities, infusion reactions, and confusion that demand honest discussion. The FDA’s ongoing monitoring ensures that these risks are identified, quantified, and communicated to healthcare providers as experience accumulates. Your doctor’s recommendation about whether lecanemab or donanemab is appropriate for a specific patient should rest on this post-market evidence, not on trial data alone.