GlaxoSmithKline Plc's diabetes drug Avandia should stay on the U.S. market, even though data suggest it may increase the risk of a heart attack for some patients, an advisory panel said on Monday in a 22-1 vote.

Several panel members said the Food and Drug Administration should consider asking for strong new warnings on Avandia, but that evidence was not strong enough to justify halting sales.

Data does suggest there is an increased risk, said Dr. Clifford Rosen, the panel's chairman.

Glaxo shares rose 5.2 percent in extended U.S. trading to $52 from their close of $49.43 on the New York Stock Exchange.

The FDA usually adopts the recommendations of advisory panels. Before the panel vote, FDA officials said there has been internal disagreement about how to handle Avandia.

Millions of diabetics around the world take Avandia. The drug's safety came into question in May, when a Cleveland Clinic analysis linked the drug to a 43 percent higher chance of having a heart attack.

Senior FDA officials and Glaxo said other research conflicts with that finding.

But two scientists in one FDA division, which monitors side effects from drugs after they reach the market, said they had concluded that Avandia did increase the chances of having a heart attack and offered no advantage over alternatives. They said Avandia should be withdrawn.

About 21 million Americans have type 2 diabetes, a disease in which the body does not adequately control blood sugar. Patients are at risk for heart disease, kidney failure, limb amputations and other serious complications.

Glaxo said Avandia, its second-best-selling drug, should remain an option because many patients need multiple drugs to keep blood sugar under control, and a portion of them cannot take some of the other therapies.

There is no overall evidence that (Avandia) is different from other oral anti-diabetic agents with regard to heart attack risk, Murray Stewart, Glaxo vice president for clinical development, told the panel.

(Reporting by Lisa Richwine, editing by Mark Porter/Jeffrey Benkoe)