Effects of Green Tea Gargling on COVID-19: a Multicenter RCT

INTRODUCTION: Catechins, a type of flavonoid and a major component of green tea, are expected to serve as a complementary preventive measure against COVID‐19. However, to date, clinical evidence has not been established. This study aimed to evaluate the preventive effects of green tea gargling against COVID‐19 in a multicenter RCT. METHODS: A total of 1,012 adults aged 18‐70 years who provided written informed consent and met the eligibility criteria were randomly assigned to either a green tea or a water gargling group. Participants gargled 3 times a day for 12 weeks from December 2023 to February 2025 using commercially available green tea powder according to the package instructions. Background characteristics, incidence of COVID‐19, and other preventive measures were assessed using self‐administered questionnaires. Group differences were analyzed using multivariate logistic regression and Cox proportional hazards models. RESULTS: The incidence of COVID‐19 in the full analysis population was 5.4% (27 of 503) in the green tea gargling group and 5.4% (27 of 500) in the water gargling group. In the per‐protocol set population with a gargling adherence ≥80%, the incidence was 4.4% (17 of 386) in the green tea group and 5.7% (23 of 403) in the water group. Multivariate logistic regression adjusted for preventive measures showed an OR of 0.818 (95% CI=0.422, 1.556), and Cox proportional hazards analysis yielded a hazard ratio of 0.824 (95% CI=0.439, 1.546). Pneumonia and hospitalization occurred in 1 participant in the water group. No gargling‐related adverse events were observed during the 12‐week intervention. CONCLUSIONS: In the per‐protocol set population, green tea gargling showed a 22.8% relative reduction in COVID‐19 incidence compared with water gargling, although the difference was not statistically significant. Further studies are needed to confirm the preventive effects of optimizing gargling frequency and catechin concentration.