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Millions of Canadians take daily vitamins to ward off disease, but a major new study has found people who take popular supplements are just as likely to develop lung cancer as those who don't, a finding that could put the "assumed benefits" of vitamins into question.

They found that smokers who take vitamin E may have an increased lung cancer risk.

The study, published in the March issue of the American Journal of Respiratory and Critical Care Medicine, sends a strong warning about the potential downside of some vitamins and could have implications for the way doctors counsel some patients.

"There's so many people taking vitamins and yet there's not a lot of evidence out there that they're all that helpful," Chris Slatore said.

Dr. Slatore is a senior fellow in the pulmonary division at the University of Washington, and one of the study's authors.

This isn't the first study to sound a note of caution about the potential negative effects of vitamins. Past studies have shown that smokers who take vitamin supplements containing beta carotene are at an increased risk for developing lung cancer. Last year, researchers at the National Cancer Institute in the United States published a study that found men who took more than seven multivitamins a week greatly increased their chances of developing prostate cancer.

"Vitamin pills are widely used with the idea that supplementing our diet with extra vitamins must be a good thing," Tim Byers, deputy director of the University of Colorado Comprehensive Cancer Center, wrote in an editorial accompanying the study. "Foods that are rich in vitamins seem to be associated with reduced risk of cancer, but vitamins packaged as pills clearly do not have the same effect."

The study followed a group of nearly 78,000 participants in the Seattle area who were aged between 50 and 76 to determine the role those vitamins played in their health. After examining the group's reported history of vitamin intake and monitoring their health for several years, researchers determined that multivitamins, folate and vitamins C and E did not lower a person's risk of developing lung cancer.Smokers who took vitamin E daily actually had a slightly elevated risk. Although more research is needed, Dr. Slatore said the results should send a strong message that people shouldn't assume vitamins are a "magic pill" that can ward off disease.

"For lung cancer especially, it would suggest that if you're going to keep smoking and hope that vitamins somehow protect you against cancer, that's not going to help you."