Starting vitamin C once you feel a cold coming did nothing
Seven comparisons covering 3,249 cold episodes found no consistent effect on duration or severity, including a trial that began dosing within four hours of symptoms.
What the study found
Started at the first symptom, vitamin C showed no consistent effect
Reaching for vitamin C once symptoms start is a different experiment from taking it daily, and it is the one that keeps failing. Seven comparisons covering 3,249 cold episodes found no consistent effect on duration or severity, including a trial that began dosing within four hours.
The caveat · Top trial lost 251 of 400 people
The strongest single trial here lost 251 of its 400 volunteers before any diary came back, and how ill someone felt could plausibly decide whether they filled one in. Its comparison group still took 0.03 grams of vitamin C rather than nothing. None of these trials went past 3 grams a day.
What we don't know
Vitamin C started at the first symptom failed in seven trials, all stopping at 3 to 4 grams a day. Two older trials hint an effect only emerges at 6 to 8 grams, and that dose has never been tested directly.