Gargling water three times a day ‘cuts infection risk by 36%’, but there is a catch

A centuries-old Japanese habit has been linked to fewer colds. The headline figure is a relative reduction from one trial, and a later study did not reproduce it.

Rinsing the throat with plain tap water is as routine in Japan as washing hands. The practice, known as ugai, is done at school, at home and in the office, and is often fitted into the same bathroom visit. A 2005 trial found that people who gargled water had a 36% lower rate of upper respiratory tract infections. Later research has not confirmed that result, and the 36% figure overstates the real-world difference.

The trial behind the headline

The research was published in the American Journal of Preventive Medicine in November 2005. It was carried out during the winter of 2002-03 and recruited 387 volunteers, 384 of whom were included in the main analysis. It tested gargling plain water against not gargling, and against gargling with a povidone-iodine rinse, an antiseptic sold as Betadine for sore throats in US pharmacies. Plain water beat both.

The idea behind the habit is that gargling dislodges and washes away harmful microbes before they can cause infection.

What 36% really means

The 36% is a relative reduction in the rate of infections. It is not a 36-percentage-point drop in anyone’s chance of falling ill. A follow-up analysis of the Japanese trial reported that about 30.1% of the water-gargling group developed an infection over 60 days, against 40.8% of controls. That is an absolute difference of 10.7 percentage points. On those figures, roughly 10 people would need to gargle for 60 days to prevent one additional symptomatic infection. Because the trial took place more than 20 years ago in Japan, that estimate should not be assumed to apply to the general population today.

The routine was also more demanding than a quick rinse. Participants gargled about 20ml of water for 15 seconds, repeated it three times in a row, and did so at least three times a day. That adds up to about 135 seconds of gargling daily.

The outcome relied on participants’ own symptom diaries and not on laboratory confirmation of a virus. Because people knew whether they were gargling, their expectations could have influenced what they reported. The trial measured symptomatic upper respiratory infections generally, and not laboratory-confirmed flu, RSV or COVID-19.

A later trial did not match it

A randomised trial published in 2014 by researchers at McMaster University in Canada tested gargling in university students, alongside a separate test of weekly vitamin D3 supplements. It used laboratory testing to identify respiratory infections, which addressed a limitation of the Japanese study. Comparing water gargling with no gargling, the researchers found a relative risk of 0.82. The 95% confidence interval ran from 0.53 to 1.26, which includes no benefit at all, and the p-value was 0.36. The result was not statistically significant.

A Cochrane review published in 2023 combined the two gargling trials, covering 830 participants. It found a pooled relative risk of 0.91, with a confidence interval of 0.63 to 1.31. That result is compatible with a reduced risk, no effect or even an increased risk, so the review found no convincing protective effect.

Possible benefits for colds already under way

A different line of research points to a more modest benefit once people are already ill. University of Edinburgh researchers published a pilot trial in Scientific Reports on 31 January 2019. It involved 68 adults who had recently developed a cold. Those assigned to hypertonic saltwater nasal rinsing combined with gargling recovered after an average of 6.8 days, compared with 8.7 days for those on standard care. That is 1.9 days fewer. The pilot also recorded a 35% reduction in household transmission, and use of over-the-counter cold medicines was 36% lower. That figure is separate from the 36% infection reduction in the Japanese prevention trial, and the two should not be confused.

The study was small and not blinded, and the authors said a larger trial was needed to confirm the results. It was a study of treatment, not of preventing infection in healthy people. Nasal rinsing and gargling were used together, so the benefit cannot be pinned on gargling alone.

A 2025 trial in people with confirmed COVID-19 compared two concentrations of saltwater. Fifty-eight patients were assigned, and three were lost to follow-up. There was no statistically significant difference between the two regimens. Both groups had lower hospitalisation rates than a separate observational reference population, but that population was not randomly assigned, so the figures cannot show that saltwater caused the difference. A separate 2025 research review concluded that saline nasal irrigation and gargling might ease symptoms if started early, but said larger prospective trials were still needed.

What the NHS says

The NHS recommends gargling with warm salty water to soothe a sore throat during a cold. It does not list gargling plain water three times a day among its main ways to prevent infection. Instead it highlights handwashing, not sharing items that may be contaminated, and avoiding touching the eyes and nose after contact with potentially contaminated surfaces. Its guidance says saltwater gargling is not recommended for children.

The evidence at a glance