Vitamin C (ascorbic acid) has been studied in the context of dementia primarily due to its role as an antioxidant and its importance in neural function. The brain is particularly susceptible to oxidative stress, which is implicated in the pathogenesis of neurodegenerative diseases such as Alzheimer's disease. Some observational studies have found that patients with dementia tend to have lower plasma levels of vitamin C compared to healthy controls. Additionally, preclinical (animal and in vitro) research suggests that vitamin C may help protect neurons from oxidative damage.

However, clinical evidence supporting vitamin C supplementation as an effective intervention for preventing or treating dementia in humans is weak. Most randomized controlled trials in this area have been small, short-term, or methodologically limited, and systematic reviews and meta-analyses have not found convincing evidence of benefit. The 2020 Cochrane review, for example, concluded there is insufficient evidence to support the efficacy of vitamin C (alone or in combination with vitamin E) in treating or preventing dementia or cognitive decline.

Thus, while there is a plausible biological rationale and preliminary observational support, high-quality clinical trials do not substantiate the routine use of vitamin C for dementia. Its use is, at present, not considered evidence-based for this indication.