Millions of people who take dietary supplements to ward off cancer may be toying with a "two-edged sword" that might do them harm, experts have warned.
A team of US scientists said there was no good evidence that supplement pills and capsules reduced the risk of cancer in healthy individuals.
They pointed out that antioxidants such as beta carotene, and vitamins C and E might even have biological effects that promote cancer.
Antioxidants are believed to counter the destructive effects of rogue oxygen molecules called free radicals.
Oxidative stress by free radicals, which attack cell membranes, proteins and DNA, has been linked to cancer and heart disease.
But the US authors, writing in the Journal of the National Cancer Institute, argue that the supposed benefits of antioxidant supplements are largely a myth.
People were being misled by "messages from supplement manufactures" stressing the health benefits of their products, including cancer prevention.
The panel of five experts, led by Dr Maria Elena Martinez, from the University of California at San Diego, wrote: "Undoubtedly, use is driven by a common belief that supplements can improve health and protect against disease, and that at worst, they are harmless.
"However, the assumption that any dietary supplement is safe under all circumstances and in all quantities is no longer empirically reasonable."
Health supplements are booming in the US, with annual sales estimated at 30 billion dollars (18.6 billion), said the scientists, who assessed the evidence relating to several supplements including antioxidants, folic acid, vitamin D and calcium.
A number of animal, laboratory and observational studies had appeared to show that dietary supplements could lower cancer risk, they said. However, these findings were not confirmed by the "gold-standard" in evidence-based medicine, randomised controlled trials (RCTs).
Only a small number of RCTs had been carried out to test the effectiveness of dietary supplements, said the experts - and several of these had reported increased risks.
"Supplementation by exogenous anti-oxidants may well be a two-edged sword," the scientists wrote. "These compounds could, in vivo (outside the laboratory), serve as pro-oxidants or interfere with any number of protective processes such as apoptosis induction."
Apoptosis, or programmed cell death, causes malfunctioning cells effectively to "commit suicide".
Experimental studies had shown that different tissues with different cancer-triggering pathways may not respond the same way to a particular nutrient.
"In fact, a nutrient may be associated with protection in one tissue and harm in another," said the experts.
They added that supplement users were "sometimes quick to discredit caution" and distrustful of mainstream science which they suspected of being corrupted by links to the drug industry.
Users may also assume the supplements they bought to be as well regulated as over-the-counter medications.
"These beliefs underscore the need for efforts by scientists and government officials to encourage the public to make prudent decisions based on sound evidence with respect to use of dietary supplements for cancer prevention," the scientists concluded.