The mechanisms of diarrhoea under metformin are poorly known. Recent data indicate that a change in gut flora might be responsible for this intestinal disorder. The effect of metformin on the gut flora has been extensively described. It has been shown that the therapeutic effect of metformin depends on the microbiota. In agreement with these data, a recent publication has shown that metformin's main site of action in humans was the intestine. In light of these results, it now seems plausible that metformin's effect on the gut flora is responsible not only for its therapeutic effect but also for its undesirable digestive effects. In this respect, Lactobacillus rhamnosus has shown anti-diarrhoeal effects (approximately 50% reduction in diarrhoeas) in the contexts of infection-caused dysbiosis and post-antibiotic dysbiosis.