Investigation of potential effects of quercetin on COVID-19 treatment: a systematic review of randomized controlled trials

Objectives: The COVID-19 pandemic has rapidly become a global health crisis. Currently, there are no proven, reliable, specific treatments for COVID-19. Alongside drug interventions, supportive treatments are implemented during the disease. Quercetin, recognized for its antiviral, anti-inflammatory, anti-aging, and antioxidant properties, is under evaluation in this study for its potential impact on preventing, influencing the course, and mitigating the severity of COVID-19. Methods: A thorough search was conducted across scientific databases, including PubMed, Embase, Web of Science, SAGEpub, Copernicus, Cochrane Library, ScienceDirect, Elsevier, Scopus, Google Scholar, EBSCOhost, Crossref, Ovid-LWW, and DergiPark databases, between 1 November 2021 and 1 April 2022 to ensure a comprehensive inclusion of relevant studies. Results: Thirteen randomized controlled clinical trials (five published, eight unpublished) were identified. Existing literature supports quercetin’s role as a potent free radical scavenger with robust antioxidant properties. It exhibits anti-inflammatory characteristics by inhibiting lipid peroxidation and restraining pro-inflammatory enzymes such as lipoxygenase and phospholipase A2. Scholarly discourse suggests that quercetin supplementation within the 500-1500 mg range leads to favorable outcomes, including quicker patient discharge, reduced inflammation, increased respiratory rate, accelerated viral clearance, and an improved disease prognosis. However, it is noted that intervention durations vary across studies. Conclusions: The analysis of the studies suggested that quercetin is a promising therapeutic agent that can cause a decrease in disease symptoms, frequency of hospitalization, hospital stay, need for non-invasive oxygen treatment, need for intensive care, and mortality. Nonetheless, more clinical studies are needed to better understand quercetin’s curative effects on COVID-19 infection.