TUESDAY, May 27 (HealthDay News) -- A new review of existing research suggests that cancer patients undergoing radiation or chemotherapy avoid supplements with high levels of antioxidants.

Although multivitamins may be all right in some cases, even green tea and vitamin A or E supplements can spell trouble, said review author Dr. Brian Lawenda, clinical director of radiation oncology at Naval Medical Center in San Diego.

The supplements "may decrease the effectiveness of radiation or chemotherapy or even make the toxicities of these treatments worse," Lawenda said. "I would recommend that you do not take these agents during chemo or radiation."

According to Lawenda, many cancer patients take supplements and green tea because they think antioxidants will help their treatment. Antioxidants are generally considered healthy because they help protect cells against threats in the body.

"Everybody wants to take them," said Dr. Alfred Neugut, the head of cancer prevention and control for the Herbert Irving Comprehensive Cancer Center at Columbia University in New York City. "It comes up almost every day in almost anyone's practice."

But researchers have questioned the conventional wisdom, wondering whether antioxidants might actually harm cancer patients during treatment.

In the new review, reported in the May 27 online edition of the Journal of the National Cancer Institute, Lawenda and colleagues examined studies into antioxidants and cancer therapy.

The researchers found that only three studies on radiation and antioxidants relied on randomized controlled trials, which are considered the best way to compare medical treatments.

One of the studies found that antioxidant treatment appeared to raise the likelihood of death.

The antioxidants may protect cancer cells from harm just like they protect normal cells, Lawenda said, and therefore prevent cancer treatments from killing tumors.

The researchers did find 16 randomized controlled trials that looked at antioxidants and chemotherapy, but they said the findings weren't conclusive.

"The quality of these studies, in terms of many of the details, is not sufficient enough for us to make strong recommendations to our patients on the safety of using these agents during radiation or chemotherapy," Lawenda said.

Overall, however, the effect on life spans of taking antioxidants appears to be small, he added.

Lawenda said he tells his patients they can take a daily multivitamin, but he recommends they avoid high-dose supplements.

Still, "we don't know whether even a multivitamin is OK to take," he added. "We don't know if even the USDA levels are acceptable. We just don't know the answers to so many questions, unfortunately."

Neugut said the findings would change what he tells patients about antioxidant supplements.

"Since there's been little evidence on the subject, I've tended to be laissez-faire about it, figuring it probably couldn't hurt," he said. "This article suggests that it seems a little bit more likely that it's more harmful than helpful."

More information

Learn more about vitamins from the National Institutes of Health.

SOURCES: Brian Lawenda, M.D., clinical director, radiation oncology, Naval Medical Center, San Diego, and assistant professor, radiology and radiological sciences, Uniform University of Health Sciences, Bethesda, Md., and assistant professor of radiation oncology, Indiana University School of Medicine, Indianapolis; Alfred I. Neugut, M.D., Ph.D., professor, medicine and epidemiology, Columbia University College of Physicians and Surgeons, and head of cancer prevention and control, Herbert Irving Comprehensive Cancer Center, Columbia University, and co-director, cancer prevention, New York Presbyterian Hospital, New York City; May 27, 2008, Journal of the National Cancer Institute, online

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