Metformin causes diarrhea mainly due to its effects on the gut environment. The medication can alter gut chemicals such as serotonin and histamine, and it changes the gut microbiome by affecting the bacteria and viruses naturally present in the intestines. These changes can disrupt normal gut function, leading to loose and more frequent bowel movements. Additionally, metformin leads to glucose elimination through the intestines and increases lactic acid production, which can irritate the gut and contribute to diarrhea. This side effect often occurs in the first few weeks of treatment and usually improves over time as the body adjusts to the medication. However, in some cases, metformin can cause chronic diarrhea which resolves once the drug is discontinued.