Does vitamin C prevent colds or help after symptoms begin?
Regular vitamin C does not usually prevent colds in the general population, although it may modestly shorten them. Starting high doses after symptoms appear has not shown consistent benefit. Meeting nutritional needs matters, but more does not necessarily mean better immunity.
Details
Deficiency and extra supplementation differ
Treating deficiency is different from giving large amounts to someone with adequate intake. Fruit and vegetables provide vitamin C; winter alone is not a reason for everyone to take a daily supplement.
High doses can cause harm
Diarrhoea, nausea and cramps are possible. People with kidney disease, a history of certain stones or iron overload should discuss high-dose supplements with a clinician.
Manage the illness, not just nutrient intake
Use fluids, rest and suitable symptom care. Seek assessment for breathing difficulty, chest pain or severe deterioration. Do not assume influenza, COVID-19 and the common cold are interchangeable.
Related reading: Can zinc prevent a cold or shorten its duration?.