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Taking Large Amounts of Vitamin C Prevents Colds
For me, the bottom line is that, like chicken soup, it doesn't hurt and may actually help. From personal experience, my wife and I have found that taking 1,500 mg of vitamin C every few hours, when we first feel symptoms coming on, often does stop a cold in its tracks. But not always!
The recommended daily intake of vitamin C is 250 mg, about the equivalent of seven servings of fruits and vegetables. Since smoking also reduces a person's level of vitamin C, the advice is either to stop smoking, eat more fruits and vegetables, or take a daily supplement.
Source: Old Wives' Tales