The Endeavor stent, which Medtronic hopes can hold on to a respectable chunk of the $4 billion worldwide stent market, leads to more complications than rival devices and doesn’t reduce the need for future treatment, according to research presented today at the Transcatheter Cardiovascular Therapeutics conference, Bloomberg News writes.

Endeavor patients had more heart attacks and deadly stent thrombosis blood clots than those getting Johnson & Johnson’s older drug-coated Cypher, according to a registry of patients treated in Denmark. The study also found patients were significantly more likely to have arteries that reclogged and required additional treatment.

Doctors should use caution interpreting results, lead investigator Jens Flensted Lassen told the media. Study patients have only been followed for about nine months and most are taking drugs including aspirin and the Plavix blood thinner. Once they stop the med, more Cypher patients may develop problems, he said, and the sickest patients may have been given Endeavor, skewing the findings.

“We have to be careful, but I think there are some signals in what we have seen today,” Lassen said during a press conference. It’s only half the story…but the signal in the registry is still there. I think it’s kind of a warning of a safety problem.”

“You don’t ultimately know the performance of a device until you go to these kinds of studies where you are evaluating complex patients,” says Alexandra Lansky, director of the Women’s Health Initiative at the Cardiovascular Research Foundation, who moderated the press conference. “You begin to understand truly the importance of the device.”

Lansky, a cardiologist at New York-Presbyterian Hospital- Columbia University Medical Center, adds she would no longer choose Endeavor as a first option for her patients based on the “sobering results.”

Endeavor’s portion of US stent sales fell to 13 percent in the third quarter, down from 18 percent the prior three months, Michael Weinstein, a JP Morgan Chase analyst wrote in a recent investor note, according to Bloomberg, adding that Abbott’s Xience and a copy sold by Boston Scientific as Promus are now the top-selling stents in the country.

“Extraordinary claims require extraordinary evidence,” a Medtronic spokesman wrote Bloomberg. “These data are interesting, but they are far from extraordinary proof,” pointing out that the findings were unexpectedly and unusually positive in the Cypher-treated patients. “We also do not know how the use of Endeavor might have been biased because of its reputation as a safer stent.”