Vitamin C (ascorbic acid) is commonly used to support sinus health, primarily due to its role in immune function and its antioxidant properties. Scientific research indicates that vitamin C contributes to the maintenance of the immune system, which may help the body respond to respiratory infections that can affect the sinuses. Some studies have found that vitamin C supplementation can slightly reduce the duration and severity of common cold symptoms, which are often associated with sinus congestion and discomfort. For example, a 2013 Cochrane review concluded that regular vitamin C supplementation slightly reduced the duration and severity of colds in the general population, though it did not significantly reduce the incidence of colds. Given that viral infections are a major cause of sinusitis, this modest benefit may extend to sinus health indirectly.

However, direct evidence specifically linking vitamin C supplementation to improved outcomes in sinusitis or chronic sinus conditions is limited and not robust. Most clinical guidelines do not recommend vitamin C as a primary or adjunctive treatment for sinusitis. The use of vitamin C for sinus support is therefore based on its general effects on the immune system rather than on strong evidence for direct sinus-specific benefits. In summary, while there is some scientific rationale and modest evidence for vitamin C supporting immune function and potentially influencing sinus symptoms indirectly, the evidence is not strong or specific to the sinuses.