Glyphosate: A common weedkiller may induce anxiety by disrupting gut bacteria

Everyday exposure to the popular weedkiller glyphosate at levels deemed safe by government regulators causes heightened anxiety and alters the gut microbiome in animal models. The researchers observed that rats drinking the chemical developed hypervigilance toward harmless objects and sounds. This altered behavior was accompanied by abnormal changes in a specific dread-inducing region of the brain. The new research was published in the journal Frontiers in Toxicology.

Glyphosate is the active ingredient in many commercial weedkillers used around the globe. It destroys weeds by blocking a specific chemical pathway found in plants and bacteria. Because human and animal cells lack this specific biological machinery, regulators previously concluded that the chemical was entirely harmless to mammals. The Environmental Protection Agency currently sets the safe daily limit for long-term human exposure at 2.0 milligrams per kilogram of body weight.

In recent years, medical professionals have noticed a correlation between heavy weedkiller use and human mental health conditions. Past laboratory experiments have also shown that rodents exposed to the chemical display excessive fear. But those prior experiments exposed animals to enormously high daily amounts, reaching up to 500 milligrams per kilogram of body weight. The health results of long-term exposure at the much lower levels considered safe by the government remained largely untested.

A team of scientists wanted to see if chronic exposure to this government-approved dosage could alter emotional processing. Researchers Mauricio Cáceres-Chacón, Demetrio Sierra-Mercado, and their colleagues at the University of Puerto Rico School of Medicine led the investigation. They suspected that low but continuous doses of the chemical could incrementally influence brain networks responsible for fear. They also theorized that the chemical might disrupt the healthy balance of bacteria in the digestive tract, an area of the body that heavily influences human and animal mental health.

To test these ideas, the research team added the weedkiller to the drinking water of adult male rats. The team controlled the concentration so that the animals consistently consumed the exact threshold dose of 2.0 milligrams per kilogram of body weight each day. A separate, matched control group of rats received regular filtered water. The total exposure period lasted for 16 weeks, during which the scientists ran a variety of psychological tests to gauge the animals’ emotional states.

At the four-week mark, the team placed the rats in an open space to measure general comfort levels. At this early stage of the experiment, the scientists saw no clear behavioral differences between the two animal groups. Both groups explored the center of the testing area for roughly the same amount of time. The lack of an early effect suggests that the chemical might need prolonged time to accumulate in the body before altering brain function.

By the tenth week of the experiment, an unnatural shift in behavior emerged. The researchers tested the rats in a maze elevated high off the ground. The maze featured some enclosed, sheltered paths and some paths that were completely open and exposed to the room. The animals drinking the weedkiller spent far less time exploring the open paths than the control animals did, which is a standard indicator of elevated anxiety in rodents.

Four weeks later, the team examined how the animals reacted to new experiences. They introduced a completely harmless, novel object into the rodents’ testing environment. Rats usually investigate new items out of natural curiosity and a desire to forage. The exposed animals spent much less time exploring the unfamiliar object and spent more time frozen in place or hugging the perimeter of the testing space.

When the researchers introduced a new, unfamiliar rat into the space, the weedkiller-exposed rodents interacted with their living peer at completely normal levels. This specific detail showed that the chemical did not completely suppress social behavior. Instead, it seems the animals incorrectly processed the inanimate object as a potential threat. Avoiding harmless novelties is an action frequently associated with trauma and hypervigilance disorders in humans.

During the final week of testing, the scientists played brief, neutral audio tones for the animals. The exposed rats froze in place much more often than the control rats did when hearing these harmless sounds. But when the researchers paired the tones with a mild foot shock, both groups reacted with identical fear levels. The experiment revealed that the chemical caused the animals to misinterpret neutral stimuli as dangerous, without altering their normal response to actual physical threats.

To figure out how the herbicide caused this state of dread, the scientists examined the rats’ brain tissue. They looked for cellular activity in several regions known to process fear and emotion, including the amygdala and parts of the prefrontal cortex. The results from these brain regions were not statistically significant. This means the chemical did not appear to overstimulate or damage their baseline function.

However, the team did find heightened cellular activity in a specific area called the bed nucleus of the stria terminalis. This deep-brain structure is heavily involved in threat interpretation and the promotion of generalized anxiety. Overactivity in this region perfectly aligned with the behavioral observation that the rats were acting overly cautious. The animals were fundamentally misinterpreting safe environments as risky due to physical changes in this brain region.

Beyond the brain, the scientists also analyzed the animals’ digestive tracts. Using genetic sequencing on fecal samples, they found that the exposed rats experienced an unnatural shift in their gut microbiome. Specifically, the animals lost a large portion of a bacterial group known as Lactobacillus. This bacterial family relies heavily on manganese, a nutrient that the weedkiller tends to bind to and remove from circulation.

The loss of Lactobacillus provides a strong biological explanation for the rodents’ altered moods. These specific gut bacteria are essential for processing dietary proteins into the chemical messenger serotonin. Serotonin regulates mood, and in a healthy brain, it acts to keep the anxiety-promoting regions calm. Without enough friendly bacteria, serotonin production drops, leaving the brain susceptible to unwarranted fear responses.

The researchers noted a few limitations in their experimental design. The study only observed male rats, which leaves an open question regarding how female physiology might respond to the same chemical exposure. Male and female rodents often display different behavioral coping strategies when dealing with fear or stress. Additionally, their microbiomes can react differently to environmental toxins, meaning future investigations will need to include both sexes to build a complete picture of the chemical’s risks.

Additionally, the scientists only measured the gut bacteria located in the animals’ waste at the very end of the 16-week period. This specific measurement method does not capture the populations of bacteria that live deep within the protective mucus lining of the intestines. Checking the digestive tract makeup at multiple points throughout the experiment could help researchers see exactly when the bacterial die-off occurs. Following the timeline more closely would confirm whether the gut changes happen before or after the behavioral shifts.

Despite these boundaries, the experiment demonstrates a clear biological mechanism by which everyday chemical exposures alter mental states. Regulators based their original safety limits on the assumption that animal biology was immune to the weedkiller’s mechanism of action. Because the chemical targets the bacteria living inside the intestinal tract, it indirectly shapes brain health. The resulting loss of mood-stabilizing brain chemicals shows that environmental safety thresholds may need a thorough reevaluation.

The study, “Exposure to the herbicide glyphosate leads to inappropriate threat responses and alters gut microbial composition,” was authored by Mauricio Cáceres-Chacón, Osmarie Martínez-Guzmán, Héctor A. Haddock-Martínez, Alexdiel Figueroa-Pérez, Sian Rodríguez-Rosado, Jaleniz Suárez-Pérez, Raúl Y. Ramos-Sánchez, Filipa Godoy-Vitorino and Demetrio Sierra-Mercado.