The diabetes drugs Avandia and Actos will be labeled with severe warnings about a risk of heart failure to some patients, health officials said this week.

The makers of the drugs, GlaxoSmithKline Plc and Takeda Pharmaceutical Company Ltd., have agreed to add the "black-box" warnings, the Food and Drug Administration said. The warnings, the most severe that prescription drugs can bear, stress the medicines may cause or worsen heart failure and that patients should be closely monitored.

The warnings also apply to combination drugs that include the active ingredients in Avandia, made by Glaxo, or Takeda's Actos. The drugs help patients with Type 2 diabetes control their blood sugar levels.

The warnings, which the FDA said in June it would seek, are separate from concerns that Avandia also raises the risk of heart attack. FDA advisers said last month the risk appeared real but that the evidence wasn't conclusive enough to merit pulling Avandia from the market. They did recommend Avandia's label be updated to include information on that risk. The FDA said it was continuing its review of the issue.

Separately, an FDA review of reports of side effects in patients taking either Avandia or Actos found cases of significant weight gain and build up of fluids, both of which are warning signs of heart failure, the agency said.