Metformin Disease Interactions

The use of metformin is contraindicated in patients with renal dysfunction (serum creatinine >= 1.5 mg/dL in males and 1.4 mg/dL in females, or above the upper limit of normal for age); congestive heart failure requiring pharmacologic treatment (especially unstable or acute CHF where there is risk of hypoperfusion and hypoxemia); and any condition associated with hypoxemia (e.g., severe anemia, myocardial infarction, asphyxia, shock), dehydration (e.g., severe diarrhea or vomiting), or sepsis. Patients with these conditions may be at increased risk for the development of lactic acidosis, which is a rare but serious metabolic complication associated with metformin accumulation in plasma usually at levels exceeding 5 mcg/mL. Metformin should also not be administered to patients with acute or chronic metabolic acidosis. In addition, metformin should generally be avoided in alcoholics and patients with clinical or laboratory evidence of hepatic disease, since alcohol potentiates the effects of metformin on lactate metabolism and impaired hepatic function may significantly limit the ability to clear lactate. All patients treated with metformin should have renal function monitored regularly (at least annually or more frequently if necessary) and be advised of the significance of nonspecific symptoms such as malaise, myalgias, respiratory distress, increasing somnolence, and gastrointestinal disturbances that arise after stabilization of metformin dosage. More marked acidosis may be associated with hypothermia, hypotension, and resistant bradyarrhythmias. Immediate medical attention is necessary if these symptoms occur, and metformin therapy withheld until the situation can be clarified. If lactic acidosis is diagnosed, prompt supportive measures and hemodialysis are recommended.

The use of oral hypoglycemic agents may be associated with an increased risk of cardiovascular mortality compared to treatment with diet alone or diet with insulin. This warning is based on the University Group Diabetes Program (UGDP) study, a long-term prospective clinical trial designed to evaluate the effectiveness of glucose-lowering drugs in preventing or delaying vascular complications in patients with non-insulin-dependent diabetes. Patients treated with diet plus a fixed dosage of either tolbutamide (a sulfonylurea) or phenformin (a biguanide) for 5 to 8 years had a cardiovascular mortality rate approximately 2.5 times that of patients treated with diet alone, resulting in discontinuation of both these treatments in the study. Despite controversy regarding interpretation of these results, clinicians and patients should be aware of the potential risk when making treatment decisions for diabetes, particularly in the presence of underlying cardiovascular disease. Data are not available for other sulfonylureas or biguanides, nor for hypoglycemic agents belonging to other classes. However, given the similarities in chemical structure and/or mode of action, the same caution should be applied.

Hypoglycemia may commonly occur during treatment with insulin and/or oral hypoglycemic agents. Care should be taken in patients who may be particularly susceptible to the development of hypoglycemic episodes during the use of these drugs, including those who are debilitated or malnourished, those with defective counterregulatory mechanisms (e.g., autonomic neuropathy and adrenal or pituitary insufficiency), and those receiving beta-adrenergic blocking agents.

Metformin may interfere with vitamin B12 (cyanocobalamin) absorption from the B12-intrinsic factor complex. A decrease to subnormal levels of previously normal serum B12 levels has been reported in approximately 7% of patients treated with metformin during controlled clinical trials. Although the decrease is generally well-tolerated and rarely associated with clinical manifestations such as megaloblastic anemia, caution may be warranted when metformin therapy is administered in patients with preexisting B12 deficiency. Vitamin B12 supplementation as well as annual measurements of hematologic parameters may be appropriate.