Abstract: Turkish Journal of Biology Turk J Biol (2021) 45: 518-529 © TÜBİTAK doi:10.3906/biy-2104-16 http://journals.tubitak.gov.tr/biology/ Research Article Treatment of COVID-19 patients with quercetin: a prospective, single center, randomized, controlled trial 1, 1 1 1 2 Hasan ÖNAL *, Bengü ARSLAN , Nurcan ÜÇÜNCÜ ERGUN , Şeyma TOPUZ , Seda YILMAZ SEMERCİ , 1 1 3 4 3 Mehmet Eren KURNAZ , Yulet Miray MOLU , Mehmet Abdussamet BOZKURT , Nurettin SÜNER , Ali KOCATAŞ  1 Department of Pediatric Nutrition and Metabolism Clinics, İstanbul Kanuni Sultan Süleyman Training and Research Hospital, İstanbul, Turkey 2 Department of Neonatology, İstanbul Kanuni Sultan Süleyman Training and Research Hospital, İstanbul, Turkey 3 Department of General Surgery, İstanbul Kanuni Sultan Süleyman Training and Research Hospital, İstanbul, Turkey 4 Department of General Medicine, İstanbul Kanuni Sultan Suleyman Training and Research Hospital, İstanbul, Turkey Received: 06.04.2021 Accepted/Published Online: 02.07.2021 Final Version: 30.08.2021 Abstract: Scientific research continues on new preventive and therapeutic strategies against severe acute respiratory syndrome Coronavirus-2 (SARS-CoV-2). So far, there is no proven curative treatment, and a valid alternative therapeutic approach needs to be developed. This study is designed to evaluate the effect of quercetin in COVID-19 treatment. This was a single-centre, prospective randomized controlled cohort study. Routine care versus QCB (quercetin, vitamin C, bromelain) supplementation was compared between 429 patients with at least one chronic disease and moderate-to-severe respiratory symptoms. Demographic features, signs, laboratory results and drug administration data of patients were recorded. The endpoint was that QCB supplementation was continued throughout the follow-up period from study baseline to discharge, intubation, or death. The most common complaints at the time of hospital admission were fatigue (62.4%), cough (61.1%), anorexia (57%), thirst (53.7%), respiratory distress (51%) and chills (48.3%). The decrease in CRP and ferritin levels was higher in the QCB group (all Ps were < 0.05). In the QCB group, the increase in platelet and lymphocyte counts was higher (all Ps were < 0.05). QCB did not reduce the risk of events during follow-up. Adjustments for statistically significant parameters, including the lung stage, use of favipiravir and presence of comorbidity did not change the results. While there was no difference between the groups in terms of event frequency, the QCB group had more advanced pulmonary findings. QCB supplement is shown to have a positive effect on laboratory recovery. While there was no difference between the groups in terms of event frequency, QCB supplement group had more advanced pulmonar findings, and QCB supplement is shown to have a positive effect on laboratory recovery/results. Therefore, we conclude that further studies involving different doses and plasma level measurements are required to reveal the dose/response relationship and bioavailability of QCB for a better understanding of the role of QCB in the treatment of SARS CoV-2. Key words: Bromelain, COVID-19, quercetin, vitamin C