If any of these occur, check your blood sugar closely and tell your doctor right away. Metformin may commonly cause stomach upset, indigestion, nausea, vomiting, or diarrhea at the beginning of treatment. If you develop unusual or unexpected stomach problems, or if you develop stomach problems later during treatment, contact your doctor at once. This click at this page may be a sign of lactic acidosis. Lab tests, including kidney function, fasting blood glucose, hemoglobin a 1c. And blood counts, may be performed while you use metformin. These tests may be used to monitor your condition or check for side effects. Be sure to keep all doctor and lab appointments. Use metformin with caution in the elderly; They may be more sensitive to its effects.