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."
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 pre
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