Studies released Sunday at the World Cardiology Congress in Barcelona raised new concerns about the risks that may accompany the drug-coated stents, which were introduced in 2000 as an improvement on bare-metal stents.

Nearly 6 million people worldwide now have the drug-lined versions. The devices are intended to keep arteries open after having been cleared of fatty deposits and are often credited with saving patients from future heart attacks or bypass surgery.

A Swiss-Dutch study tracked 8,146 patients and found that recipients of drug-coated stents were at increased risk of thrombosis, or blood clots, that can occasionally result in death.

Two other Swiss studies, analyses of presented and published information discussed at the cardiology conference, also found that first-generation drug-coated stents had higher links to thrombosis compared to bare metal stents.

In bare metal stents, heart cells naturally grow to cover the stent, providing a natural biological lining. But in the drug-coated versions, the drugs prevent tissue growth - which is both their intent and their possible downfall.

Drug-coated stents were previously viewed as a great advance since the drugs they emitted prevented cells that could block the arteries from growing. A thick growth of cells is undesirable, but a thin layer of cells lining the artery is essential. In some instances, drug-coated stents have prevented this minimal protective cell layer from growing, leaving exposed metal, which essentially can act as a clot magnet.

"This is potentially explosive information," said Dr. Steven Nissen, president of the American College of Cardiology and director of cardiology at the Cleveland Clinic.

"It certainly makes me pause with substantial concern," said Nissen. He said there is already a shift in the U.S. away from using drug-coated stents in favor of their uncoated predecessors.

Doubts about drug-coated stents were initially raised in March by a small-scale Swiss study, though there have long been naysayers about their potential adverse effects. Results from the Swiss-Dutch study found that the incidence of thrombosis does not diminish as time passes.

Researchers found that the possibility of thrombosis continued to pose a risk, though minimal, during the first three years following stent implantation. Other issues, however, such as patient compliance with taking prescribed medications, may play a role in blood clot formation.

Some medical experts worry that the financial implications of reducing usage of the profitable drug-coated stents may be taking priority over improving patients' health.

"It's time to stop and re-evaluate," said Dr. Salim Yusef, professor of medicine and director of cardiology at McMaster University, Hamilton, Canada.

"Having done six million of these procedures, isn't it a terrible indictment on us that we don't have long-term safety data on these?" he said.

The drug-coated stent market last year was estimated as being worth more than $5 billion, and is dominated by Boston Scientific and Johnson & Johnson. Drug-coated stents are also far more profitable, selling for about $2300 each compared to the cheaper $700 bare metal versions.

While significant, these new findings are not expected to radically alter the use of drug-coated stents in the immediate future.

"This won't change clinical practice right now, but it shows us that this a problem that needs to be investigated with larger trials," said Dr. Raymond Gibbons, president of the American Heart Foundation.

Nissen, however, was more skeptical.

"There is a tendency for physicians to switch practices even before data is solid," he explained. "If there's a suspicion, why take the risk?"