The Link Between Gum Disease And Alzheimer’s: What Researchers Have Found So Far

Spit after brushing, see a streak of pink in the sink, and most people think about their next dental cleaning, not their memory. Over the past several years, though, researchers have tied gum disease to dementia, traced one gum bacterium to the brains of people who died with Alzheimer’s, and tested a drug built to block that germ’s toxic enzymes in people with the disease.

The research is real, but it does not add up to proof. Observational studies have found links, though a pooled analysis found a clearer signal for dementia overall than for Alzheimer’s disease specifically. Lab work has shown how the bacterium could harm the brain, mostly in mice. The two tests that come closest to settling the question of cause, a genetic analysis and a clinical trial, did not show that gum disease drives Alzheimer’s. Here is what each line of evidence shows, and what it means for your next trip to the dentist.

Why Do Scientists Suspect A Connection?

Because several human studies have found links between gum disease and dementia, though the evidence is mixed and comes from observational research.

The largest came from the National Institute on Aging. Researchers used dental exams and stored blood samples from more than 6,000 U.S. adults ages 45 and older who took part in the third National Health and Nutrition Examination Survey (1988 to 1994). They then followed participants through Medicare and death records for up to 26 years. The blood tests measured antibodies, the immune system’s record of past run-ins with 19 kinds of mouth bacteria.

Among people who were 65 or older at the start, higher levels of antibodies against two gum-disease bacteria, Porphyromonas gingivalis and Campylobacter rectus, were linked to a greater chance of a later Alzheimer’s diagnosis. In the Journal of Alzheimer’s Disease report, each step up on a combined score for those two antibodies (one standard deviation) was tied to about a 22% higher risk.

The study had real limits. Antibodies reflect past exposure rather than active gum infection, and diagnoses came from Medicare billing records that can miss cases. The authors also noted they could not rule out other influences, such as genetic risk.

A much smaller New York University study, published in Alzheimer’s & Dementia: Diagnosis, Assessment & Disease Monitoring, looked at 48 cognitively normal adults ages 65 and older. Researchers sampled bacteria from under the gumline and collected spinal fluid. People whose gum bacteria tilted toward disease-linked species and away from health-linked ones had lower levels of amyloid-beta 42 in their spinal fluid.

That sounds backward, but lower levels in the fluid generally mean more of the protein is collecting in the brain as plaques, a hallmark of Alzheimer’s. The bacterial imbalance was not linked to tau, the disease’s other hallmark protein. Because everything was measured once, the study could not show which change came first or whether any participant would go on to develop dementia.

When a 2022 review and meta-analysis in the International Journal of Molecular Sciences pooled results from seven studies, gum disease was tied to about 39% higher odds of dementia overall. In statistical terms, that is an odds ratio of 1.39, with a 95% confidence interval of 1.02 to 1.88. For Alzheimer’s specifically, the pooled odds ratio was 1.03, with a range of 0.98 to 1.07, which includes the possibility of no difference at all. The authors concluded that the risk of bias in the underlying studies kept them from calling gum disease a risk factor.

How Could A Gum Bacterium Affect The Brain?

Potentially through P. gingivalis, its toxic protein-cutting enzymes called gingipains, and the inflammation they trigger. A 2019 Science Advances paper reported P. gingivalis DNA and gingipains in postmortem Alzheimer’s brain samples. Gingipain levels rose along with tau and with ubiquitin, a protein tag that marks damaged proteins and builds up in Alzheimer’s brains.

In mice infected by mouth, the bacterium reached the brain and increased production of amyloid-beta 42. Experimental drugs that block gingipains reduced the bacterial load in the brain, stopped the extra amyloid-beta 42 production, lowered brain inflammation and protected neurons in the hippocampus, a region central to memory.

These findings have limits. Postmortem tissue cannot show when bacteria arrived, and mice deliberately infected in a lab differ from people with ordinary gum disease. The work was also funded by Cortexyme, the company developing the inhibitors, and several authors worked there.

What Evidence Pushes Back On Causation?

Weak spots in the human studies, a genetic analysis that found no causal link and a drug trial that missed its main goals.

A 2023 review in Heliyon examined 18 observational studies of gum disease and Alzheimer’s. Seventeen were case-control or cross-sectional studies. These designs look at people at a single point, or compare those who already have Alzheimer’s with those who don’t, so they cannot show which condition came first. The reviewers rated most studies at low risk of bias overall.

Even so, half did not account for confounders, which are outside factors that could affect both conditions. Adjustment methods varied, 62% of studies did not mention bias, and only four used the current 2017 system for classifying gum disease. The authors urged caution in interpreting the link.

A 2024 study in Brain and Behavior used Mendelian randomization, a method that treats gene variants people are born with as a natural experiment. If genes that raise the odds of gum disease also raised Alzheimer’s risk, that would point toward cause. The researchers used gum-disease data from FinnGen, a large Finnish genetic project, and two separate Alzheimer’s genetic databases. They found no evidence of a causal link.

The method has blind spots: it tests an inherited tendency toward gum disease, so it may miss effects tied to a specific bacterium, the timing of an infection or how severe it becomes.

The strongest test so far was the phase 2/3 GAIN trial of atuzaginstat, a Cortexyme drug designed to block gingipains. It randomized 643 adults ages 55 to 80 with mild-to-moderate Alzheimer’s to a placebo or 40 or 80 milligrams of the drug twice daily for 48 weeks. Across the full group, the drug did not meet its two main goals, measures of thinking and of daily functioning.

Cortexyme reported slower decline among participants with signs of P. gingivalis infection. Subgroup results from a trial that missed its main goals are considered exploratory, though. The trial also treated people who already had Alzheimer’s, so it says nothing about prevention.

What’s The Practical Takeaway?

There is no established evidence that treating gum disease prevents Alzheimer’s. The National Institute of Dental and Craniofacial Research recommends:

• brushing twice a day with fluoride toothpaste
• cleaning between teeth regularly
• getting routine dental checkups and cleanings
• quitting all forms of tobacco

Red, swollen, tender or bleeding gums, persistent bad breath, pain while chewing and loose or sensitive teeth are reasons to see a dentist.

The mouth-brain idea remains credible enough to test. Until randomized studies show that preventing or treating gum disease changes who gets Alzheimer’s or how fast it progresses, a bleeding gumline is a reason to see a dental professional, not a forecast of what will happen to the brain.