In a patient started on metformin for type 2 diabetes, what is the most common side effect?

• A. Diarrhea
• B. Hypoglycemia
• C. Lactic acidosis
• D. Vitamin B12 deficiency

Answer: (A)

Starting metformin commonly produces GI intolerance, most notably diarrhea. This gut-related side effect is dose-dependent and often appears soon after therapy begins or when the dose is increased; it tends to improve over time as the body adapts and as dosing is titrated upward, especially when taken with meals. That makes diarrhea the most frequent adverse effect you’ll encounter with metformin.

Lactic acidosis is a rare, serious risk, mainly in patients with significant kidney or liver disease, or situations causing hypoxia. Vitamin B12 deficiency can occur with long-term use but is less common and accumulates over years. Hypoglycemia is uncommon with metformin alone because it does not directly raise insulin levels.