SAN ANTONIO — Six million Americans who are taking a class of drugs to lower blood pressure may instead be increasing their risk of heart attacks by 60 percent, researchers reported Friday.

The drugs, called calcium channel blockers, are nifedipine, diltiazem and verapamil. They are sold under various brand names, including Adalat, Calan, Cardizem, Dilacor, Isoptin, Procardia and Verelan.

National guidelines recommend the use of beta blockers and diuretics to lower high blood pressure.

Calcium channel blockers have been widely used to treat high blood pressure, but in recent years studies have shown they don’t work very well and are not considered the best treatment, said Dr. Paul Ridker, a cardiologist and epidemiologist at the Brigham and Women’s Hospital in Boston.

However, many American cardiologists are still prescribing them for that purpose, said study author Dr. Bruce Psaty, an epidemiologist at the University of Washington in Seattle.

“We are very concerned about these results,” he said. “We believe these findings are real. From a public health point of view, I think it’s important.”

Previous studies have suggested that calcium channel blockers can be dangerous when given to patients immediately after a heart attack, or when they are given some time later to prevent a second heart attack.

This is the first suggestion that they are dangerous in the large group of outwardly healthy people who are trying to prevent heart attacks by lowering their blood pressure, Psaty said. Researchers said they didn’t know why the drugs may increase the risk of heart attack.

About 50 million Americans suffer from high blood pressure, greatly increasing their chances of a heart attack.

Half of those are being treated, and about 25 percent of the patients being treated are getting calcium channel blockers, Psaty said. That amounts to about 6 million Americans.

Ridker, who wasn’t involved in the study, said doctors continue to use calcium channel blockers in part because drug companies have “marketed them beyond what the data shows.”

Another reason may be that many cardiologists are not familiar with the most recent research data on the drugs’ effectiveness.

“There have been several studies saying these are hazardous for your health, but they’re still being used,” Ridker said.

Psaty studied 623 people who had had heart attacks and a control group of 2,032 who hadn’t. When he determined the drugs they had been taking, he found that those on calcium channel blockers had a heart disease risk 60 percent higher than those taking beta blockers or diuretics. The study was funded by the National Institutes of Health.

Psaty said he prescribes calcium channel blockers for some patients who don’t respond to beta blockers or diuretics. But calcium channel blockers shouldn’t be used routinely, he said.

Dr. John T. Henderson, a senior vice president at the Pfizer pharmaceutical company in New York, said he didn’t think all calcium channel blockers were likely to have the same risks. “These drugs aren’t absolutely identical,” he said. Pfizer makes Procardia.

Psaty said he looked at several different kinds of calcium channel blockers in his study and could find no difference in risk among them. Henderson said Psaty’s study might not have been large enough to find differences.

Following release of the study Friday, Pfizer stock fell $1 a share to $81.87 1/2 on the New York Stock Exchange. Marion Merrell, which makes Cardizem, saw its shares fall 87 1/2 cents to close at $24.

Dr. Jeffrey A. Cutler, an epidemiologist at the National Institutes of Health in Bethesda, Md., is in charge of a new clinical trial that will compare the use of various blood pressure drugs in 40,000 Americans.

Cutler, who wasn’t involved in this study, said he found Psaty’s evidence convincing. “It was obviously a carefully conducted study,” he said.