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A drug that helps your mind may turn your gut microbes—and waistline—against you.

In a series of experiments in mice, researchers found that a common drug used to treat psychiatric illnesses, including autism and bipolar disorder, alters the gut microbial community. Those changes caused the mice to burn fewer calories while resting and gain weight, researchers report in EBioMedicine. The finding, which lines up with weight gain seen in patients, suggests that drugs other than antibiotics can easily mess with a person’s microbes, which in turn profoundly influence metabolism, weight, and overall health.

In the study, researchers led by microbiologist John Kirby of the University of Iowa gave mice water laced with the psychiatric drug risperidone. This drug is well-known to cause significant weight-gain, insulin resistance, and metabolic problems in people. Kirby and colleagues had a hunch that the hefty side-effects were linked to changes in the gut microbiome, but they were unsure of the exact mechanism.

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