Quercetin is a flavonoid occurring naturally in nature, and can be purchased over the counter as a nutritional supplement. Scientists have proved its anti-inflammatory and anti-viral properties in both humans and animals. In the case of Influenza A, quercetin has been shown to help suppress the viral replication cycle.
As of April 2020 there is no evidence of its effect against the coronavirus. In March, a quercetin study was initiated by McGill University to determine its efficacy in suppressing the novel coronavirus. The first phase of the study, which includes human trials, is due to be complete by August 2020.
Manufacturers typically recommend a daily dose that is considerably less than those advocated on some social media channels.