The potential of herbal medicines to affect outcomes of the novel coronavirus disease, COVID-19, has been much discussed by researchers throughout the pandemic (Brendler et al. 2020).

Quercetin (2‐(3,4‐dihydroxyphenyl)‐3,5,7‐trihydroxychromen‐4‐one or 3,3′,4′,5,7‐pentahydroxyflavone) represents one of the best studied flavonoid with numerous health-promoting effects such as antiviral, anti-inflammatory, anticancer, and cardiovascular protective activities (Wang et al. 2016). In particular, the anti-inflammatory capacity of flavonoids in viral diseases—such as that of quercetin—can be of great value for the adjuvant therapy of COVID-19 (Solnier and Fladerer 2020).

In some studies, quercetin was found to reduce the release of inflammatory cytokines (e.g., IL-8) which may alleviate infection-related symptoms and suppress overwhelming inflammatory responses that are often associated with severe COVID-19 conditions (McKee et al. 2020; Kinker 2014). Furthermore, the antioxidative effects of quercetin preventing tissue damage by scavenging free radicals (Kinker 2014) might be another benefit for the recovery process of viral diseases like COVID-19.