Doctors have been widely prescribing drug-coated heart stents for uses not approved by the FDA, according to two studies recently published in the Journal of the American Medical Association.

Stents are wire-mesh tubes that are widely used to prop open heart valves to prevent them from collapsing due to cardiovascular disease. They are often used in conjunction with medical procedures such as angioplasty.

In 2004, three years ago, the FDA approved stents coated with medication that makes blood vessels less likely to close again. They do this by suppressing the immune response that may stimulate growth of smooth muscle tissue in the stented area. Because of the effectiveness of this treatment in preventing restenosis (reclosing of the blood vessel), the new stents immediately became wildly popular with doctors. By 2006, 90 percent of all stents being prescribed were of the drug-coated variety, a market of $6 billion.

According to the two new studies, in the rush to adopt the new devices, doctors ended up prescribing them for conditions for which they were not approved and their safety or effectiveness had not been tested. The studies found that approximately 50 percent of the coated stents installed in 2004 and 2005 were prescribed for unapproved health conditions.

The use of drug-coated stents dropped off slightly last fall, after they were found to increase the chance of deadly blood clots. However, 70 percent of the stents prescribed are still drug-coated.

One of the new studies, which looked at 5,541 patients, found that patients treated with drug-coated stents had their risk of blood clots, heart attack and death more than doubled. Because the overall complication rate was only 2.5 percent even in sicker patients, however, the researchers pronounced the devices safe.

The authors of the second study, which looked at 3,323 patients, disagreed.

"Clinicians should be cautious about extrapolating the benefits of drug-eluting stents over bare metal stents," they said.