I present an article from my own website as evidence here is that ok?
In short the answer is this:
In 2007, the Cochrane Collaboration published a report which examined, collectively, all
of the good studies that had investigated the role of Vitamin C in prevention and cure of
the common cold. Their conclusions were unremarkable.
First of all, the good news. If you take high doses of Vitamin C (around 1g per day –
approximately 16x RDA or 22 oranges) every day then there is a suggestion that, when you
get a cold, you’ll experience a very modest 7% reduction in the length of the illness
(for children the effect is larger – around a 13% reduction). This amounts to an average
half a day’s reduction in service to Kleenex.
However, the investigation (analysing the results of up to 29 different studies,
involving more than 11,000 people) found no reliable evidence that regularly dosing
yourself with Vitamin C greatly decreases the risk of actually catching a cold in the
first place. Nor has there been convincing evidence that you can shorten the duration
of a cold or reduce the severity of its symptoms by upping your intake of Vitamin C
after you’ve started feeling ill. One study, alone, suggested that an enormous 8g dose
of Vitamin C taken at the onset of symptoms could modestly reduce the length of a cold,
but the study was beset with methodological problems.
It seems, then, that the practise of reaching for the Jaffas at the first hint of a
sniffle is misguided. It’s a popular belief drawn from the flawed hypothesis of an old
scientist who was speculating about topics outside of his field. There admittedly is
evidence that suggests long-term intake of high doses of Vitamin C (around 22 oranges
worth, no less) may help you recover less than half a day quicker than usual, but this is
confounded by the notorious difficulty in measuring the beginning and end of a cold. Can
you say when the last cold you had ended with an accuracy of less than half a day?
So the evidence that eating jaffas help is weak and there has been no suggestion in the trials that doing so is harmful in any way.
It is possible to overdose on vitamin C but I suspect in the duration of an average cold you would have to consume a heroic amount of very potent oranges to suffer any ill effects.