LONDON (Reuters)—European regulators are launching a new probe into the safety of GlaxoSmithKline P.L.C.'s diabetes pill Avandia, adding to pressure on a medicine that already faces a grilling from U.S. experts next week.

The European Medicines Agency on Friday said the decision to review the risks and benefits of the drug followed publication of two recent studies by U.S. researchers Steven Nissen and David Graham questioning its cardiovascular safety.

Experts from the agency's Committee for Medicinal Products for Human Use, who last reviewed the product in early 2010, will discuss the issue at their next plenary meeting in London on July 19-22.

Avandia, known generically as rosiglitazone, is currently authorized for sale in Europe on its own, in combination with metformin as Avandamet, and with glimepiride as Avaglim.

"Once all relevant data on the benefits and the risks of rosiglitazone have been looked at, the CHMP will issue an opinion on whether or not the marketing authorizations for these medicines should be revoked, suspended or changed," the EMA said in a statement.

Shares in British-based Glaxo were 2.2% lower by 1140 GMT following the news, underperforming a little-changed European drugs sector.

Ben Yeoh, an industry analyst at Atlantic Equities, said the new EMA review was not a huge surprise in the light of the fresh studies. But it increases uncertainty as to whether Avandia will be allowed to stay on the market.

More lawsuits?

Avandia was once Glaxo's second-biggest drug, though its sales have plunged since safety fears first erupted three years ago. Revenues totaled £771 million ($1.17 billion) in 2009, equivalent to 2.7% of Glaxo's group sales.

The big worry for investors, though, is litigation.

Concerns about the medicine's adverse impact on the heart have triggered a slew of lawsuits in the United States and fresh criticism of the drug at a U.S. Food and Drug Administration advisory panel on July 13-14 could make matters worse.

"There is an increased chance of more lawsuits," said Mr. Yeoh. "A lot of them will settle because proving causality in this area is relatively difficult since diabetic patients are already ill, so dying of a heart attack is within a normal range of things."

Glaxo has vigorously defended its product and Tony Hoos, Glaxo's European medical director, said on Friday that Avandia was "an important treatment option for appropriate Type 2 diabetes patients."

Persuading doctors and patients of the benefits of a drug that was already slammed in a highly critical report from U.S. senators in February is becoming an uphill battle, with the two latest anti-Avandia studies adding to Glaxo's difficulties.

In one study, Dr. Steven Nissen, a cardiologist at Ohio's Cleveland Clinic and an outspoken Avandia critic, offered even more ammunition to his 2007 analysis that first suggested Avandia raises heart risks.

In the second, Dr. David Graham of the Food and Drug Administration and others found Avandia was more dangerous to the heart than Actos, a pill in the same class of drugs made by Japan's Takeda Pharmaceutical Co.