Taking high doses of vitamin E supplements can increase the risk of lung cancer, research suggests.
High vitamin E doses were found to increase risk
The US study of 77,000 people found taking 400 milligrams per day long-term increased cancer risk by 28% - with smokers at particular risk.
It follows warnings about similar risks of excessive beta-carotene use.
Writing in the American Journal of Respiratory and Critical Care Medicine, an expert said people should get their vitamins from fruit and veg.
Dr Tim Byers, from the University of Colorado, said a healthy, balanced diet meant people took in a whole range of beneficial nutrients and minerals, which might help to reduce cancer risk.
The researchers followed people aged between 50 and 76 for four years and looked at their average daily use of vitamin C and folic acid, and vitamin E supplements.
Over the course of the study, 521 people developed lung cancer.
Smoking, family history and age all had unsurprisingly strong links to cancer risk.
And while neither vitamin C or folic acid use had any effect on lung cancer risk, vitamin E use did.
The researchers extrapolated their findings, and concluded that over a decade, there was an additional 7% increase in risk for every 100 milligrams taken per day.
The vitamin E trend was most prominent among smokers, but was not confined to them.
Vitamin E is known to be an antioxidant - protecting cells from molecules called free radicals.
But the US researchers speculate that, in high doses, it may also act as a pro-oxidant - causing oxidation and therefore damage to cells.
Dr Christopher Slatore of the University of Washington in Seattle, who led the study, said: "In contrast to the often assumed benefits or at least lack of harm, supplemental vitamin E was associated with a small increased risk of lung cancer.
"Future studies may focus on other components of fruits and vegetables that may explain the decreased risk of cancer that has been associated with fruit and vegetables.
"Meanwhile, our results should prompt clinicians to counsel patients that these supplements are unlikely to reduce the risk of lung cancer and may be detrimental."
But Henry Scowcroft, senior science information officer at Cancer Research UK, said: "The jury's still very much out on whether vitamin and mineral supplements can affect cancer risk.
"Some studies suggest a benefit, but many others show no effect and some, like this one, suggest they may even increase risk."
He added: "Research repeatedly shows that a healthy, balanced diet can reduce your risk of some cancers while giving you all the vitamins you need.
"Quitting smoking remains the most effective way to avoid many cancers. There's no diet, or vitamin supplement, that could ever counter the toxic effects of cigarette smoke."
In 2002 a Finnish study of 29,000 male smokers found taking beta-carotene - which is converted into vitamin A in the body - was linked to an 18% increased risk of developing lung cancer