Extra vitamin C does not prevent colds, cardiovascular disease or diabetes in ordinary people, and this has been demonstrated in large-scale research. For intensive athletes and people following a carnivore diet, there are genuine reasons for supplementation. People who take vitamin C regularly and still fall ill tend to have a somewhat milder cold on average; higher doses (6-8 g/day) appear to be more effective for that purpose, but this is an individual choice, not a broad recommendation.
For the average healthy person, taking extra vitamin C has no proven effect on preventing the common cold. Controlled studies demonstrate this clearly for doses above 1 gram per day. The same applies to cardiovascular disease and type 2 diabetes: a large meta-analysis of nearly 884,000 participants from 884 randomised studies shows no benefit whatsoever from vitamin C supplementation on these risks.
One group is an exception to this: people who engage in heavy physical exertion, such as marathon runners or military recruits. Five trials show that vitamin C roughly halves the number of colds in this group. This benefit does not apply to people with an ordinary, sedentary or lightly active lifestyle.
People who take vitamin C regularly and still catch a cold do get some benefit from it. Fifteen trials involving more than 6,000 participants in total show a reduction in the severity of the cold of around 15% on average. Higher daily doses of 6 to 8 grams appear to shorten the duration more effectively than lower doses of 3 to 4 grams. The researchers describe this as 'not unreasonable to try on an individual basis', not as a firm recommendation. Starting vitamin C once you are already ill does not help in any consistent way: the studies give conflicting results.
For the brain and memory, the evidence is thin. A combination of low doses of vitamin C with vitamin E produced no improvement in cognitive functioning. Studies using higher doses of vitamin C on its own showed positive signals in some cases, but this is insufficient to recommend supplementation on that basis.
An important nuance is that vitamin C does not simply work on a 'more is better' principle. In addition to its well-known role as an antioxidant, vitamin C can, under certain circumstances, also cause oxidative damage. Whether that is harmful or actually beneficial depends strongly on context, and is a reason to be cautious about the assumption that higher doses are always harmless.
There is one situation in which supplementation is almost certainly beneficial: people who eat exclusively animal products (a carnivore diet) are likely to consume too little vitamin C and run the risk of a deficiency. Outside that situation, most people on a normally varied diet get enough vitamin C, and taking extra offers little for the general population, except for those who exercise intensively on a regular basis and want to fall ill less often.
All claims are based on randomised trials and meta-analyses (PMID 39803741, 36480969, 35268010, 32455696, 31963293, 39796574). For PMID 31963293, two authors are employed by Bayer Consumer Care, a vitamin manufacturer; the positive conclusions about immune support should be read with that interest in mind. The cognition claims (PMID 35268010) are partly associational and limited in scope.