MONDAY, Oct. 27 (HealthDay News) -- Calcium channel blocker drugs, which are widely prescribed for coronary conditions, might reduce the effect of the clot-preventing drug Plavix, a new study says.

Calcium channel blockers appear to inactivate an enzyme essential for the activity of Plavix, said the report by Austrian physicians published online Oct. 28 in the Journal of the American College of Cardiology.

Studying 200 people who were taking Plavix after artery-opening procedures and stent placement, the University of Vienna researchers found a higher level of activity of platelets -- the blood cells that can clump together to form blood clots -- in those also taking calcium channel blockers.

"Moreover, intake of calcium channel blockers was associated with adverse clinical outcome," the researchers wrote.

Is the report something that doctors and people taking Plavix should worry about?

"We don't know," said Dr. Neal S. Kleiman, director of the cardiac catheterization laboratory at the Methodist DeBakey Heart and Vascular Center in Houston. He wrote an accompanying editorial in the journal.

"Is it possible that it is clinically significant? Yes, it is," Kleiman said. "Is it a test-tube finding that doesn't mean much? That is possible, too."

Those questions can be answered by analyzing studies containing information about people taking both drugs, Kleiman said. "There are many studies that have large databases that can be queried," he said. "There are ways to query them and see if there is an increased risk of stent thrombosis in patients taking calcium channel blockers."

Plavix (clopidogrel) is recommended for anyone having a stent implanted, to reduce the incidence of thrombosis, which is blockage of the reopened artery. One problem for the working cardiologist is that people who have stents -- wire mesh tubes -- implanted usually are taking a number of other drugs, often including calcium channel blockers, Kleiman said. Calcium channel blockers are commonly prescribed for high blood pressure and the chest pain called angina, he noted.

"What you do with the information in this report is that you remember it," he said. "When a patient comes to you with a long list of medications, if possible, you try to avoid those that might have interactions."

There is a role in the decision-making for the people taking the drugs, Kleiman said. "The patient should ask, 'Why am I taking these medications, what do they do, do I need to be on all of them?'" he said. "The answer is usually 'yes.' "

Dr. John H. Alexander, an associate professor of medicine at Duke University School of Medicine's Division of Cardiovascular Medicine, said the finding could affect medical practice. "The clinical impact is to lead physicians to look for alternatives to calcium channel blockers," he said. "There are lots of alternatives, including beta blockers and ACE inhibitors."

The report shows the need for continual appraisal of commonly used medical treatments, Alexander added. "Clopidogrel has been used in millions of patients for a long time, and we are still starting to learn about potential interactions," he said. "This is a drug interaction we should have thought of earlier."

More information

Learn more about calcium channel blockers from the Texas Heart Institute.