The prescribing information for metformin lists congestive heart failure (CHF) as a contraindication to its use, because of concerns that CHF patients might be at increased risk for metformin-induced lactic acidosis. However, little evidence supports this position.
In this study, researchers analyzed data from the provincial health care database in Saskatchewan, Canada. They identified 1883 diabetic patients with incident CHF who used a sulfonylurea alone (42%), metformin alone (11%), or both drugs (47%) during a 6-year interval. In analyses adjusted for potentially confounding variables, all-cause mortality rates during the year following the diagnosis of CHF were significantly lower in the metformin group (hazard ratio, 0.66) and the combined-therapy group (HR, 0.54) than in the sulfonylurea group. A combined endpoint of all-cause mortality and hospitalization also was significantly lower in the metformin and combined-therapy groups. During longer follow-up (mean, 2.5 years), these findings persisted. No deaths were attributed to lactic acidosis.
These results challenge the prohibition against using metformin in CHF patients. However, the study has limitations: For example, we don't know how long these patients used metformin after the onset of CHF, and we don't know anything about renal function (another presumed determinant of susceptibility to metformin-induced lactic acidosis). Nevertheless, in a related article on the safety of diabetes drugs, a physician who works at the FDA suggests that "metformin should no longer be contraindicated in patients with CHF."
References:
1. Eurich DT et al. Improved clinical outcomes associated with metformin in patients with diabetes and heart failure. Diabetes Care 2005 Oct; 28:2345-51.
2. Misbin RI. Evaluating the safety of diabetes drugs: Perspective of a Food and Drug Administration insider. Diabetes Care 2005 Oct; 28:2573-6.