Dr. Jeffrey A. Johnson of the University of Alberta in Edmonton said that "Stable heart failure and diabetes can be safely treated with metformin."

Warnings against using metformin are based on past experience with phenformin -- a similar drug that was taken off the market in the 1970s after being linked to hundreds of cases of lactic acidosis, a potentially life-threatening build-up of lactic acid in the blood that can damage vital organs.

However, there is a scarcity of information linking the newer drug metformin to this side effect, and recent studies suggest metformin may actually be more effective than so-called sulfonylurea drugs in reducing death from cardiac causes. According to Johnson, as many as 10 to 15 percent of diabetic patients with heart failure are prescribed metformin despite the labeling, largely due to the scarcity of other treatment options.

To investigate whether contraindications are warranted, the researchers identified 12,272 new users of oral antidiabetic drugs, 1833 of whom developed heart failure, and classified them based on the type of drug they were taking.

Compared to treatment with a sulfonylurea, treatment with metformin was associated with reduced hospitalization and death among patients with heart failure.

One third of patients on metformin died, compared with 52 percent of patients on sulfonylureas only. Seventy-seven percent of patients on metformin died or were hospitalized, compared with 85 percent of patients on sulfonylurea alone.

Given that this is a comparative study, Johnson notes, it's not possible to say whether metformin improved outcomes or sulfonylureas worsened them. He points out that some older sulfonylureas have been shown to have some cardiotoxicity, while newer members of the drug family likely are safer.