What Is Alzheimer's Disease?

What This Article Covers

Alzheimer's disease is the most common cause of dementia, a progressive brain disease that gradually destroys memory, thinking, and the ability to carry out daily activities. This article explains what's currently understood about its causes, how it typically progresses, and what medical science can currently offer — including two recently approved drugs that, for the first time, target the disease process itself rather than just its symptoms. It also gives an honest, concise look at where stem cell–based research for Alzheimer's actually stands today, which is still early and experimental, not an available treatment.

What Causes Alzheimer's Disease?

Alzheimer's involves changes in the brain that build up over years, often beginning long before symptoms appear. The two hallmark changes are amyloid plaques — sticky clumps of a protein fragment called beta-amyloid that accumulate between nerve cells — and tau tangles, twisted strands of a protein that collapses inside dying neurons, disrupting the internal transport system cells need to survive. Scientists don't yet know with certainty what initiates this cascade, but these plaques and tangles are considered central to the disease process. Age is the strongest known risk factor: risk roughly doubles every five years after 65. Genetics also play a role — certain “risk genes” like APOE-e4 raise likelihood without guaranteeing the disease, while rare “deterministic” genes cause less than 1% of cases, typically early-onset familial Alzheimer's. Cardiovascular factors — high blood pressure, diabetes, smoking, and poor sleep — are modifiable risks linked to higher dementia risk, which is part of why heart-healthy habits are commonly recommended for brain health.

Symptoms and Stages

Alzheimer's generally progresses through three overlapping stages. In the early (mild) stage, people often still live independently but experience memory lapses, such as trouble finding words or misplacing items, and difficulty planning. The middle (moderate) stage, usually the longest, brings greater memory loss, confusion about time or place, and behavioral changes like agitation or wandering, requiring more caregiving support. In the late (severe) stage, individuals need around-the-clock care as communication, mobility, and awareness of surroundings decline, with swallowing difficulties raising infection risk.

Standard Care and Recently Approved Treatments

For decades, available medications — cholinesterase inhibitors and memantine — could only modestly ease symptoms without altering the disease's underlying course. That changed with two new antibody drugs that target amyloid directly. Lecanemab (Leqembi) received full FDA approval in July 2023 after its CLARITY-AD trial showed a 27% slower decline on a standard dementia-severity scale over 18 months versus placebo in people with early Alzheimer's or mild cognitive impairment and confirmed amyloid buildup. Donanemab (Kisunla) followed with FDA approval in July 2024; its trial of 1,736 participants showed roughly 29–35% slower decline on key measures, depending on tau levels at baseline. These are genuinely significant milestones — the first FDA-approved drugs demonstrated to affect the biology of Alzheimer's rather than only its symptoms, after decades without one. But the effects are modest, not curative: they slow decline over roughly a year and a half rather than stopping or reversing it. Both carry boxed warnings for ARIA (amyloid-related imaging abnormalities) — brain swelling or small bleeds, usually symptomless but occasionally serious. In donanemab's trial, ARIA-related swelling occurred in about 24% of treated patients and bleeding-type ARIA in roughly 31%, with serious events in around 1.6%, including deaths attributed to ARIA. For lecanemab, ARIA risk was markedly higher in people carrying two copies of the APOE-e4 gene (about 45%, versus 13–19% in others), which is why genetic testing and regular MRI monitoring are recommended before and during treatment.

Where Stem Cell Research Currently Stands

No stem cell therapy is FDA-approved or clinically proven for Alzheimer's disease. Current research runs along two separate tracks. The more mature one uses induced pluripotent stem cells (iPSCs) to model the disease: researchers reprogram a patient's cells into neurons or brain organoids in the lab to study how amyloid and tau abnormalities form and to screen drug candidates — a valuable research tool, not a treatment. The second track, direct cell-based therapy, remains in early, mostly small-scale human safety trials, primarily using mesenchymal stem cells, alongside neural stem cell approaches still largely confined to animal studies with mixed results. Reviews of this literature are consistent: mechanisms are unclear, human efficacy has not been demonstrated, and much more research is needed before any stem cell approach could be considered an established treatment. (Readers wanting a deeper look at this research area can see our dedicated article on stem cell therapy for Alzheimer's disease.)

Bottom Line

Alzheimer's disease remains a serious, progressive condition without a cure, but the picture is not static. The approvals of lecanemab and donanemab mark a genuine turning point — the first drugs shown to act on the disease's underlying biology — though their benefits are measured in months of slowed decline, not reversal, and come with real risks like ARIA that require careful screening and monitoring. Stem cell–based approaches, meanwhile, are a promising but still-early research direction, useful today mainly for understanding the disease rather than treating it. Anyone considering any Alzheimer's treatment, approved or experimental, should discuss individual risks and benefits with a qualified neurologist.