New research has found that people suffering from manic depression – otherwise known as bipolar disorder – who are hospitalized for bouts of mania are more than five times as likely to be on antibiotics than the general population. Though this doesn’t necessarily mean that antibiotics themselves cause manic episodes, it does add to the growing body of scientific evidence that both the immune system and the microbial populations inhabiting a person’s body can directly influence their brain function.

Writing in the journal Bipolar Disorders, the study authors describe how they looked through the records of 234 psychiatric patients suffering from a range of disorders such as schizophrenia, depression, and bipolar, who had been admitted to hospital for mania. Prescription records revealed that a total of 18 of these patients, or 7.7 percent, were receiving antibiotics for a bacterial infection at the time of their episode.

In contrast, a survey of 555 healthy people with no diagnosis of mental disorders indicated that just 1.3 percent of these were on antibiotics. The fact that the prescription rate was more than five times higher for people experiencing mania than for healthy individuals is, at this stage, merely observational, and doesn’t suggest a causal connection between antibiotics and mental illness.

However, the researchers believe this observation is likely to be more than coincidental, and suggest a number of possible explanations for this phenomenon. For instance, it is possible that the administration of antibiotics alters a person’s microbiome, leading to the proliferation of certain bacteria that release neurotoxins that alter brain activity.

^{The link between gut bacteria and brain function is already well established. T. L. Furrer/Shutterstock}

Alternatively, the extraordinarily high correlation between mania and bacterial infections could be indicative of certain immune disorders that might influence brain function. For instance, it may be that these people have some sort of fault in their immune system, which not only makes them more susceptible to bacterial infections but also somehow disrupts their cognition. On the other hand, it may be that these people are no more prone to infection than the general population, but that a particular quirk in their immune response to invading bacteria may somehow activate manic brain activity.

When looking at the types of infections associated with mania, the researchers found that urinary tract infections were the most common in women, while respiratory tract infections were the most common in men.

In a statement, study co-author Robert Yolken explained that “more research is needed, but ours suggests that if we can prevent infections and minimize antibiotic treatment in people with mental illness, then we might be able to prevent the occurrence of manic episodes.” As such, he suggests that “good-quality health care and infection prevention methods” should be a priority for those suffering from mental illness.