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A 50-year-old African-American woman returns to the clinic for a follow-up appointment for diabetes treatment. At her last visit, she was screened for diabetes because both of her parents and one of her two sisters have diabetes. After being found to have repeatedly elevated fasting blood glucose levels, the patient was given diabetic education and tried on a course of diet and exercise. While she has no specific complaints today and is pleased that she has lost 4 lb over the last 6 months, her fasting blood glucose is 130 mg/dL and a hemoglobin A1C level is 7.7%. After a lengthy discussion with the patient, it is decided that she should start therapy with metformin as well as continue her dietary controls and exercise regimen. Which of the following is the most common side effect for which she is at risk when starting this therapy?
A. Anemia, megaloblastic
B. Gastrointestinal distress
C. Lactic acidosis
D. Liver function abnormalities
E. Weight gain
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Weight loss > GI upset >> lactic acidosis
contraindicated in > 80yr and renal disease
D/C if lactic acidosis occurs or 24hrs before an IV contrast study
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The correct answer is B. Metformin is one of the most common oral agents used to treat noninsulin diabetes mellitus, necessitating an understanding of its side-effect profile. The most common side effect is moderate to severe gastrointestinal distress, which can cause patients to discontinue therapy. This can be avoided or minimized by starting at a low dose of metformin, taking the drug with meals, and titrating the dose slowly.
Metformin can reduce B12 absorption, though this effect is almost never clinically significant. It is extremely rare for metformin to cause a megaloblastic anemia (choice A).
Lactic acidosis (choice C) is a rare but serious side effect of metformin, which usually occurs only if predisposing factors are present. Common predisposing factors include renal insufficiency, heart failure, serious acute illness, or any hypoxic state. It is recommended that metformin be held for any imaging study with iodinated dye, to reduce the risk of having metformin present if contrast induced nephropathy occurs.
Liver function abnormalities (choice D) are uncommon and do not need to be monitored for drug intolerance. Other oral agents, such as the thiazolidinediones (œglitazones) require monitoring for hepatic complications.
Metformin will often result in a small weight loss, not a weight gain (choice E), unlike most of the other medications used to treat diabetes.
