The drug fenfluramine, half of the fen-phen diet drug combination, was pulled from the market in 1997 when reports showed it caused heart valve damage in some people.

In this story: Manufacturer: no related signs of heart disease Abnormalities don't limit normal lives RELATED STORIES, SITES

NEW ORLEANS (CNN) -- In the largest study of its kind, researchers have found that people who took the once-popular drug combination "fen-phen" for longer than six months experienced heart valve disease at an unusually high rate.

The vast majority of the patients had only minor problems, but it is unknown whether these problems get worse over time, said Dr. Thomas Ryan, a cardiologist at Duke University Medical Center and an author of the study. But people who took the drugs for less than six months did not suffer from heart valve damage, Ryan's study found.

Ryan presented his findings Tuesday at the annual meeting of the American College of Cardiology.

The drug fenfluramine, which is one half of the fen-phen combination, was voluntarily pulled from the market in September 1997, when reports showed it caused heart damage. The reports were on a small number of cases, but ever since, doctors have been trying to figure out exactly who is at risk.

Even though the Food and Drug Administration never approved the drug combination itself, it became popular. Doctors wrote an estimated 25 million prescriptions for the drug combination.

According to the study, people who never took fen-phen had a 3.6 percent incident rate of heart valve disease. Of 1,163 former fen-phen users in the study, those who took the drugs for less than six months had a 4.5 percent incident rate, which was not a statistically significant difference.

People who took the drugs for six months to a year had a 7.0 percent incident rate of the heart disease. For one to two years of usage, the rate was 13.6 percent; for more than two years, the rate was 17.4 percent.

Manufacturer: no related signs of heart disease

The study was funded by Wyeth-Ayerst, which made fenfluramine and another diet drug called dexfenfluramine, which was taken off the market for the same reasons.

The patients with valve problems suffered from aortic regurgitation, which is when blood leaks from the aorta -- the large artery that distributes blood to the body -- back through the aortic valve into the left ventricle. Normally, blood should flow forwards, not backwards through the valve.

Nearly all patients -- about 98 percent -- with valve problems had only trace or mild regurgitation, which does not pose an immediate health threat.

In a statement released with the study, Wyeth-Ayerst emphasized that people who had taken the drugs did not have symptoms of heart disease.

"There were no statistically significant differences in cardiovascular physical findings or serious cardiovascular findings across the study groups," the company said.

Abnormalities don't limit normal lives

Ryan agreed that at the present time, the people with valve abnormalities did not have symptoms.

"Almost everyone with trace or mild regurgitation is asymptomatic and we don't treat it," he said. "As long as they stay at that level, they live normal lives."

"What we don't know is what this means down the road. We don't know about the long-term progression of these patients. It's hard to predict, so I remain concerned," he added.

No one knows exactly why there seems to be an association between fenfluramine and dexfenfluramine and valve problems. Ryan said that the cause is important, because valve disease progresses differently depending upon its cause.

Ryan said he agrees with the recommendations of the American College of Cardiology and the American Heart Association that everyone who had taken fen-phen or dexfenfluramine should see his or her doctor.

The first lawsuit over fen-phen is scheduled to go to court in Texas in three weeks, according to Kip Petroff, a lawyer involved in the case, which was filed in Dallas.