Vitamin supplements do nothing to prevent gut cancers and may shorten life expectancy, research suggests.
Popping pills may not be the answer
A review of 14 trials involving more than 170,000 people found antioxidant vitamins, like vitamin E, offered no protection against these cancers.
People taking some supplements died prematurely, the European researchers said in the Lancet.
Cancer Research UK cautioned the findings were preliminary and did not offer convincing proof of hazard.
The study authors themselves emphasised that they had only studied the effect of certain antioxidant supplements.
"The results should not be translated to the potential effects of vegetables and fruit, which are rich in antioxidants and other substances," they said.
Antioxidants are thought to stop cancer by preventing or slowing damage caused by certain oxygen compounds.
Dr Goran Bjelakovic and his colleagues, working at the Copenhagen Trial Unit in Denmark, looked at the supplements beta-carotene, vitamins A, C and E and selenium as different combinations or separately.
They compared the rate of gastrointestinal cancers, such as stomach, liver or bowel cancer, among people taking the antioxidant supplements and people taking fake tablets with no active ingredient.
Other than selenium, regular use of antioxidant supplements did not prevent gastrointestinal (GI) cancers.
In half of the 14 trials reviewed, vitamin tablets appeared to shorten life expectancy.
Food better than pills
The combination of beta-carotene and vitamin A or vitamin E increased risk of premature death by 30% and 10%, respectively.
Dr Bjelakovic said: "We could not find evidence that antioxidant supplements can prevent gastrointestinal cancers.
"On the contrary, they seem to increase overall mortality."
He said the potential merits of taking selenium to prevent GI cancers should be further investigated.
Dr Richard Sullivan of Cancer Research UK said the research could have been biased because many of the people in the study were smokers, who have a higher death risk anyway.
"There are no shortcuts to prevent bowel cancer. If you're taking vitamins to protect yourself against the disease, you're wasting your money.
"The best way to lower the risk is to eat a healthy diet and not smoke."
He said the study added to the evidence that trials into the benefits of selenium are warranted.
"There is currently a study of the protective effects of selenium in prostate cancer in the US, but further research is needed into the wider role of selenium."
Selenium is found in nuts, white fish, liver and kidney, shellfish, cereals, bread and dairy products.
Douglas Altman from Cancer Research UK told the Lancet in an editorial: "If their findings are correct, 9,000 out of every million users would die prematurely as a result."
He said this was a "scary speculation" given the vast quantities of vitamin supplements used.
He said more research was needed and described the study as "work in progress" that offered no convincing proof of hazard.
Martin Ledwick from CancerBACUP said: "Further investigation is needed into the effects of vitamin supplements.
"In the meantime, it's probably sensible to avoid taking very large doses of vitamin supplements and to try to get the necessary vitamins from eating five or more portions of fruit and vegetables every day."