The supplement beta-carotene may increase the risk of bowel cancer, researchers have warned.
An estimated 10m Britons take vitamin supplements
The substance has been linked to an increased risk of lung cancer in smokers and people exposed to asbestos.
Now a study suggests that in some people it can double the risk of benign tumours known as adenomas that can lead to colorectal cancer.
The finding relates to those who smoke cigarettes and have more than one alcoholic drink a day.
"Supplementation [with beta-carotene] was beneficial among subjects who did not drink or smoke but, if anything, increased risk among those who drank and/or smoked," said study author Dr John Baron, of the Dartmouth Medical School in the United States.
The conclusion is based on a study of 864 patients who had had previous adenomas removed.
Paradoxically those who did not smoke or drink and took beta-carotene were at decreased risk of adenomas.
More harm than good
However, Dr Tim Key of Cancer Research UK at Oxford University, said there was no evidence of benefits for beta-carotene and more evidence of harm.
"The advice is you should not take supplements of beta-carotene to prevent cancer," Dr Key told BBC News Online.
"The main thing for smokers and drinkers is to give up smoking and don't drink too much."
The warning comes after experts from the UK's Food Standards Agency said that high levels of supplements like beta-carotene and zinc over a long period may have irreversible harmful effects.
Beta-carotene is a substance from plants that the body converts into vitamin A. Natural beta-carotene is found in food such as fruit and vegetables.
The research is published in the Journal of the National Cancer Institute.