Mixed

Long-term metformin treatment (up to 2,000 mg/day) significantly alters gut microbiota composition by increasing Escherichia coli and Ruminococcus torques while decreasing Intestinibacter bartlettii and Roseburia species, effects that persist for at least 12 months.

If you take metformin, expect your gut bacteria to change in a specific way: more E. coli and Ruminococcus torques, less Intestinibacter bartlettii and Roseburia. These changes don't just happen briefly; they stick around for at least a year. Importantly, these changes happen regardless of how much weight you lose, meaning the drug itself is driving this shift. While this might sound concerning because E. coli can be harmful, in this context, it is linked to increased production of short-chain fatty acids like acetate, which may help improve insulin sensitivity. If you experience stomach upset, talk to your doctor about slow dose titration.