Something fishy may be going on with a daily pill trusted by millions of Americans.

Across the country, older adults have embraced a popular supplement often touted as a way to support brain health and possibly even help ward off dementia.

But new research is turning that idea on its head, suggesting people who regularly take the capsules could actually face a higher risk of faster cognitive decline.

The supplements in question? Omega-3 fatty acids, better known as omega-3s.

The pills contain concentrated doses of healthy fats the body needs but can’t produce on its own. As a result, people get them through food or supplements, often marketed to support brain, heart and joint health.

One of the most popular forms — fish oil — is taken by roughly 1 in 5 Americans over age 60 every day, according to a 2023 report.

In the study, scientists in China tracked more than 800 older adults over five years using repeated cognitive tests and brain imaging scans.

The team split the participants into two groups: 273 people who took omega-3 supplements and 546 who did not.

About half of participants in both groups carried the APOE ε4 gene, which is linked to a higher risk of Alzheimer’s disease — the most common form of dementia.

Once researchers adjusted for age, sex, genetics and diagnosis, they found that the omega-3 users had steeper drops in cognitive performance across three tests over the study period.

That pattern held regardless of genetics, suggesting APOE ε4 alone couldn’t explain the difference.

That’s notable, because past research has suggested carriers of the gene may respond differently to omega-3 than non-carriers.

To understand what might be driving the faster decline, researchers turned to the participants’ brain scans.

Surprisingly, the decline among supplement users didn’t appear to be linked to a buildup of amyloid plaques or tangled tau proteins — the classic hallmarks of Alzheimer’s disease.

Instead, scans revealed a significant drop in brain glucose metabolism in the omega-3 group, imeaning their brain cells were less efficient at using sugar for energy.

Researchers suspect the supplements may interfere with that process, disrupting communication between synapses, the tiny gaps that allow cells to send signals to one another.

If brain cells can’t use fuel efficiently, that communication system may begin to break down, potentially speeding up cognitive decline before any physical damage shows up on scans.

“These findings challenge the prevailing view of omega-3 as uniformly beneficial and highlight the need for a cautious reassessment of its widespread use for cognitive protection,” the study authors wrote.

The researchers noted that the study was observational, meaning it cannot prove omega-3 supplements directly caused the faster decline. Still, the findings give scientists a new lead to investigate.

In general, the science on omega-3s and brain health has long been mixed. Some studies have linked the supplements to better cognitive outcomes, while others have found little to no benefit.

Some experts believe whole-food sources of omega-3s — such as salmon and other fatty fish — may work differently because they contain a broader mix of nutrients that function together in ways isolated supplements cannot.

The findings come as cognitive decline is becoming an increasingly urgent health crisis in the US.

Nearly one-third of Americans 65 and older already have some level of cognitive impairment affecting memory, thinking and daily decision-making. About 10% have dementia.

Experts warn those numbers are expected to climb sharply as the population ages and Americans live longer.

A 2025 study estimated the US could see 1 million new dementia cases every year by 2060 — roughly double today’s rate.