BOSTON -- More than 20 millions Americans supplement their diet with vitamin E, hoping to protect themselves from cancer and heart attacks.

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NewsCenter 5's Liz Brunner reported Tuesday that a large study suggests it doesn't work and there could be risks.

Carole Ferguson, a nurse practitioner, started taking vitamin E supplements 10 years ago after reading a study that touted their potential benefits.

"The results looked like they were promising enough to throw in a vitamin," Ferguson said.

But the results of a new study have her reconsidering her daily dose of vitamin E. Canadian researchers followed 4,000 patients age 55 or older with vascular disease or diabetes.

Half the group took 400 IUs of vitamin E daily, the other half took a placebo. After seven years, the study found taking vitamin E not only did not protect against cancer or heart attacks but actually increased the risk of heart failure by 13 percent.

"This was a clinical trial conducted just like a prescription drug, and that's a new thing for these vitamins and antioxidants," Lahey Clinic Dr. Richard Nesto said.

Nesto is advising all his patients to discontinue their use of vitamin E.

"I am telling my patients that I can't guarantee that they're going to be better. There's a downside that it could increase their risk, and that I would review all the drugs they take just to be sure they're getting the benefit they think they're getting," Nesto said.

But not all researchers agree. Jeffrey Blumberg is a nationally known researcher and also serves on advisory boards for several vitamin manufacturers.

"In terms of generally healthy people, I don't think there's any indication that vitamin E is in any way harmful and indeed the NIH and Institutes of Medicine have declared that doses of 1,000 IU a day were perfectly safe," Blumberg said.

Ferguson says she will read the study for herself before making any change.

"You're always having to make your best judgment on the information you have at the time," she said.

This is the third study in a couple of months that has called into question the benefits and risks of Vitamin E. If you are taking vitamin E now, check with your doctor to see if you should continue.