Vitamin C tablets really can combat colds and flu but only when taken in very high doses, according to new research.
The vitamin is found in citrus fruit, green leafy vegetables and supplements taken by millions of Britons.
Now a study has shown taking 6g or 8g a day shortens the length of time symptoms last by 17 and 19 per cent respectively.
However these amounts are up to eight times more than the NHS and Department of Health advises to avoid stomach upsets.
Study leader Dr Harri Hemila, of the University of Helsinki, believes even higher doses could lead to even higher reductions.
He said: “Given the consistent effect of vitamin C on the duration of colds, and its safety and low cost, it would be worthwhile for individual common cold patients to test whether therapeutic 8g/day vitamin C is beneficial for them.
“Self dosing of vitamin C must be started as soon as possible after the onset of common cold symptoms to be most effective.”
Previous research has found regular Vitamin C supplements have no effect on common cold incidence although it may shorten the time the disease lasts.
But large doses could cause nausea, diarrhoea and stomach cramps.
Dr Hemila and colleagues analysed the findings of two trials, the first which administered either 3 g/day of the nutrient, 6 g/day or a dummy pill to volunteers who were assigned to each group at random.
Dr Hemila said each study revealed a significant relationship between dosage and duration.
He pointed out it has been proposed vitamin C doses should be more than 15 g/day for the best treatment.
But the highest amounts that have so far been investigated have been much lower.
The majority of controlled trials have used a modest dose of only 1 g per day.
But the optimal doses and the maximal effects of vitamin C on the common cold are unknown.