Vitamin C (ascorbate) has been investigated for its potential role in supporting or treating respiratory allergies, such as allergic rhinitis and asthma. There is some scientific basis for its use, primarily due to its antioxidant properties and its role in modulating immune responses. Vitamin C can help reduce oxidative stress in the airways, which is often elevated in allergic reactions. It may also influence histamine levels; some studies suggest that vitamin C can reduce histamine secretion and promote its breakdown, which could theoretically alleviate some allergy symptoms.

However, the clinical evidence supporting the use of vitamin C for respiratory allergies is limited and mixed. Some small clinical trials and observational studies have reported modest improvements in symptoms such as nasal congestion and wheezing, particularly in individuals with low baseline vitamin C levels. For instance, a few studies have shown that intravenous vitamin C may reduce allergy-related symptoms, but these studies often have small sample sizes and are not always rigorously controlled.

Overall, while there is a plausible mechanism and some early clinical evidence, robust, high-quality randomized controlled trials are lacking. Most clinical guidelines do not currently recommend vitamin C as a primary or adjunct therapy for respiratory allergies. Therefore, while its use is partially supported by scientific investigation, the strength of evidence is low.