Is Metformin Good or Bad For Patients With Diabetes and Heart Failure?

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Improved clinical outcomes associated with metformin in patients with diabetes and heart failure, by D.T. Eurich and colleagues. Diabetes Care 28:2345–2351, 2005.

What is the problem and what is known about it so far?

Heart failure is common in people with type 2 diabetes, and having high blood glucose levels can increase the risk of getting and dying from diabetes-related heart problems. Some studies suggest that metformin, which is often prescribed to patients with type 2 diabetes to help control their glucose levels, might cause patients with both diabetes and heart failure more harm than good by causing lactic acidosis, a potentially fatal condition where lactic acid builds up in the cells. This leaves only sulfonylurea, acarbose, and insulin as "safe" options. However, other studies suggest that there is little evidence to prove that metformin causes lactic acidosis, and taking metformin might actually lower the risk of death in these patients.

Why did the researchers do this particular study?

The authors studied people on metformin and/or sulfonylureas to examine the effects of these medications in people with diabetes and heart failure.

Who was studied?

The researchers of this study used the Saskatchewan Health database to find new users of metformin and sulfonylureas based on prescription claims between 1 January 1991 and 31 December 1996. These subjects were at least 30 years old, had health coverage with Saskatchewan Health, and were able to use their drug benefits at least 1 year before they received their first prescription. Federal employees, Native Indians, and prison inmates were not included in this study. Of 12,272 people in the Saskatchewan Health database, 1,833 were chosen for the study.

How was the study done?

A total of 773 patients were treated with sulfonylureas only, 208 were treated with metformin only, and 852 were treated with both. The researchers then found out who had been to the hospital for heart failure and who had been treated for heart failure by a doctor. The medical records of the patients were checked until the patients died, until they stopped using Saskathewan Health coverage, or until 31 December 1999.

What did the researchers find?

After 1 year, there were 200 deaths (from all causes) in the sulfonylurea-only group (26%), 29 in the metformin-only group (14%), and 97 in the group taking both (11%). This shows that metformin either alone or with sulfonylureas was linked to fewer deaths than the sulfonylurea-only group.

After about 2 years, there were 404 deaths in the sulfonylurea-only group (52%), 69 in the metformin-only group (33%), and 263 in the group taking both (31%). The total number of deaths related to heart disease was 224 in the sulfonylurea-only group (29%), 36 in the metformin-only group (17%), and 145 in the group taking both (17%). There was no difference in the deaths related to diabetes among the groups.

After 1 year, the number of people who had to go to the hospital was 406 in the sulfonylurea-only group (53%), 102 in the metformin-only group (49%), and 435 in the group taking both (51%). At the end of the study period, there were 538 people who had to go to the hospital in the sulfonylurea-only group (70%), 143 in the metformin-only group (69%), and 632 people in the group taking both (74%).

What were the limitations of the study?

There were three main limitations to this study. First, the researchers did not take into account the patients' glucose levels, and people who control their glucose levels are less likely to have heart failure. Although metformin and sulfonylureas both help keep glucose levels low, people who take metformin usually have "less severe" diabetes than those who take sulfonylureas. However, that would also mean that the people taking both metformin and sulfonylureas are the most severe cases of diabetes, and should have the most deaths and stays at the hospital.

Second, there is a chance that doctors did not prescribe metformin to patients who were at risk for heart failure.

Third, the researchers did not take into account kidney function and kidney failure, which can predict the outcomes of heart failure.

What are the implications of the study?

According to the researchers of this study, there is little or no evidence that suggests that metformin is dangerous to people with both diabetes and heart failure. In fact, there is evidence that the opposite is true. Doctors should carefully weigh the pros and cons of prescribing metformin to patients with heart failure.

FOR MORE INFORMATION

Metformin May Help Diabetics With Heart Failure

Other Diabetes Medications

Diabetes, Heart Disease and Stroke