In a clinical trial of 286 subjects, 53.2% of the 141 given immediate-release metformin (as opposed to placebo) reported diarrhea, versus 11.7% for placebo, and 25.5% reported nausea/vomiting, versus 8.3% for those on placebo [REF].
Long-term use of metformin has been associated with increased homocysteine levels[REF] and malabsorption of vitamin B12 [REF].