Antibiotic Use Increases Risk of Cognitive Decline

Antibiotic Use Increases Risk of Cognitive Decline

Antibiotics have long been used to treat or prevent bacterial infections and can be considered the most important medications ever developed. While highly effective and potentially lifesaving in the treatment of bacterial infections, antibiotics have been increasingly prescribed for conditions, in particular viral infections for which they are completely ineffective and where they cause unanticipated serious consequences. The Center for Disease Control and Prevention (CDC) estimates that at least 30% of outpatient antibiotic prescriptions in the United States are unnecessary.

“Increasing prescription of antibiotics has led to an increase in antibiotic resistant bacteria.”

According to the first study to provide longitudinal estimates for human antibiotic consumption covering 204 countries, published in Lancet Planetary Health by the Global Research on Antimicrobial Resistance (GRAM) Project, global antibiotic consumption increased by 46% from 2000 to 2018. This increase has been associated with an increase in the resistance of bacteria to the effects of antibiotics, a phenomenon called antibiotic resistance. Antibiotic resistance has become one of the principal public health problems worldwide. In 2015, the White House released The National Action Plan for Combating Antibiotic-Resistant Bacteria (CARB), which set a goal of reducing inappropriate outpatient antibiotic use by at least half by 2020.

But the development of antibiotic resistance is not the only collateral damage resulting from our war on microbes. With gut health and gut microbiome science becoming such popular topics in the last decade, the science is starting to reveal that “nuking” our gut microbes so frequently, and particularly during childhood with these drugs isn’t a great idea.
It has been estimated that 2.7 courses of antibiotics are administered to infants by age 2, and 10.9 courses by the age of 10. Prematurely born babies in the neonatal intensive care units receive even higher doses. This massive onslaught on the gut microbial ecosystem happens at a time when this ecosystem is still developing to its adult form, and when the interactions with the gut immune system play a prominent role in the programming of immune responses throughout life. This early interference with the shaping of normal immune system responses has been implicated in the progressive increase in autoimmune diseases and various allergies.

“…these negative effects were comparable to those caused by 3-4 years of aging.”

But a recent study for the first time revealed that antibiotic use and its effect on the gut microbiome can have serious health consequences in old age as well. In a recent study from Harvard School of Public Health, published on March 23, 2022, in PLOS ONE, Chan et. al 2022 reported that the prolonged usage of antibiotics in midlife was significantly correlated with later cognitive decline. The study used the Nurses’ Health Study II, an ongoing US-nationwide prospective cohort study, which began in 1989 with the enrollment of 116,430 female nurses aged 25-42 years. This study looked at self-reported antibiotic use of 14,000 middle-aged nurses and looked at outcomes on neuropsychological tests conducted 7 years later.

The results showed that those who took antibiotics for at least 2 months during a period of 4 years had lower scores on tasks involving memory, learning, attention, and psychomotor speed. Overall, these negative effects were comparable to those caused by 3-4 years of aging.

Andrew Chan, MD, MPH, a gastroenterologist at Harvard Medical School who co-led the study, hypothesized from these unexpected results that the negative effect of antibiotics on cognitive function was not a direct effect of the drugs on the brain, but were mediated by an indirect effect of antibiotics on the gut microbiome. Such a negative effect of the gut microbes could change the signaling within the brain gut microbiome system, leading to neuroinflammation and neurodegeneration of regions in the brain concerned with memory and cognitive function.

“Studies have shown a link between a reduction in gut microbial diversity and richness and our current epidemic of chronic non-communicable diseases.”

Previous studies have shown that antibiotic induced dysbiosis, e.g. changes in gut microbiota, and be associated with cognitive decline in animals. A number of population-based studies strongly suggest a role for altered gut microbiome including alterations in gut microbial neuroactive metabolites and inflammatory mediators in cognitive decline. Studies have also found a reduction in gut microbial diversity and richness in the chronic diseases making up our current epidemic of chronic non-communicable diseases, including but not limited to Crohn’s disease, Autism Spectrum Disorder, Multiple Sclerosis, and Parkinson’s disease.

The Harvard study is an important reminder of the need to pay more attention to the health of our gut microbial ecosystem and become aware of the grave health consequences resulting from a lack of such attention. The consequences of widespread, inappropriate use antibiotics to treat viral diseases and to prevent bacterial diseases can be seen throughout the lifespan, from the time of delivery, throughout infancy and as the Harvard study shows all the way into advanced age.

As we have often pointed out in this newsletter, and in Dr. Mayer’s book, The Gut Immune Connection, the health of the gut microbial ecosystem is not only threatened by our inappropriate use of antibiotics, but also by our dietary habits, in particular the Standard American Diet (SAD).

With this new information at your disposal, the next time you are prescribed antibiotics, discuss with your doctor the potential negative effects of such a treatment on the health of your gut microbiome and the risk for accelerating cognitive decline.


E. Dylan Mayer is a graduate from the University of Colorado at Boulder with both a major in Neuroscience and minor in Business. He is fascinated by the interactions of brain, gut and microbiome, and the role of nutrition in influencing the health of our microbiome, as well as our own well-being.