In the study of nearly 4,000 patients, researchers found that megadoses had no effect on the risk of cardiovascular disease or cancer, but increased the risk of heart failure.

While some doctors say vitamin E still has potential in preventing blindness and dementia, the study's authors say there is no reason for patients to take high doses.

Multivitamins typically contain only 30 international units of vitamin E, but many individual supplements include 400 units or more.

"There is this belief that you can take a miracle pill to prevent diseases like heart disease and cancer, but it's just not true," says Eva Lonn, a leader of the study and a professor of medicine and cardiology at McMaster University in Ontario, Canada. "Unfortunately, we have to pay attention to our diet and exercise and our body weight."

Encouraging results from studies in the 1990s have helped make vitamin E the most popular supplement, taken by 22% of adults over 55, according an editorial by E. Robert Greenberg of Dartmouth Medicine School published in January in Annals of Internal Medicine. Nutritional supplements had estimated sales of $19 billion in 2003.

In recent years, however, more rigorous clinical trials have failed to show that vitamin E protects the heart.

Because this study is the first to suggest vitamin E may cause heart failure — a condition in which the heart muscle doesn't pump blood as it should — Lonn says doctors should conduct additional research to make sure that this connection was not just a coincidence.

In her study, an extension of the Heart Outcomes Prevention Evaluation, or HOPE, the difference in heart failure rates among patients was small.

After seven years, 14.7% of those taking supplements suffered heart failure, compared with 12.6% of those who took placebos.

Researchers say the amount of vitamin E in regular multivitamins poses little risk. A recent analysis published in the Annals of Internal Medicine, in fact, found that low doses might be beneficial. Some scientists point to evidence suggesting that vitamin E may reduce the risk of ailments such as Alzheimer's disease, age-related macular degeneration and cataracts, says Jeffrey Blumberg, a professor in the Friedman School of Nutrition Science and Policy at Tufts University.

And while vitamin E may not reduce the overall risk of cancer, researchers continue to study whether it may prevent specific types of tumors. The National Cancer Institute has enrolled more than 35,000 men to study whether vitamin E and selenium, a mineral, prevent prostate cancer.

The trial's safety committee will hold a special meeting to review the results of the new study, says Lori Minasian, one of the scientists leading the trial.