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

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.

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.

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.

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.