Taking a daily supplement of vitamin C will not protect most people from common colds, scientists say.
Vitamin C, found naturally in citrus fruit, might not help prevent colds
A review of 30 studies, involving 11,350 people, also found doses of at least 200mg per day did little to reduce the length or severity of colds.
But people exposed to periods of high stress, like marathon runners, could reduce their risk of catching colds by half if they took the vitamin daily.
The Australian and Finnish team's study is published in the Cochrane Library.
Researchers at the Australian National University and the University of Helsinki concluded that, for most people, the benefits of taking vitamin C daily were so slight that they were not worth the effort or expense.
Although they found it could reduce the duration of colds by up to 8% in adults and 13.6% in children, as most people only have two or three colds every year, this benefit is only very small.
Author Professor Harri Hemilš, said: "It doesn't make sense to take vitamin C 365 days a year to lessen the chance of catching a cold."
However, they say it may be justified in those exposed to severe cold or physical stresses, where vitamin C doses reduced the risk of getting a cold by half.
Furthermore, they say there is "tantalising" evidence from one study that very large doses of vitamin C taken at the beginning of a cold might have a useful effect.
But more research would be needed to evaluate this possibility.
In the 1970s, Nobel Prize-winning chemist Linus Pauling encouraged people to take 1,000mg of vitamin C daily to ward off colds.
But since then the effects of the vitamin on colds has remained controversial.
The current recommended daily allowance of vitamin C is just 60mg, and Catherine Collins, a registered dietician with the British Dietetic Association, says an optimum amount would be 200mg every day.
But she said most people could easily obtain this amount through eating five portions of fruit and vegetables every day.
Furthermore, eating too much vitamin C means it cannot be absorbed and so is removed from the body.
Although infection-fighting white blood cells do use vitamin C, Ms Collins said there was very little evidence to suggest that it could help fight off colds.
She said: "It sounds biologically plausible because vitamin C does help improve our immune systems, but it only seems to work if people are deficient in it in the first place, which is very rare."
However, she added that the study was useful in adding to the existing body of evidence concerning Vitamin C.