1. Why people are told to gargle: the idea that it washes germs from the throat
Many people have been told to "wash your hands and gargle" when returning home. According to materials from the Hiroshima Prefectural Infectious Disease Control Center, the purpose of gargling is to wash away pathogens that stick to the throat lining during breathing[4]. Because the goal is to flush out these particles, timing is important. The same materials explain that gargling after going out is effective, while doing it before leaving home has little meaning[4]. If there is a patient in the home, gargling can also help prevent infection from that person[4].
The origin of the word "ugai" (gargle) has a theory linked to cormorant fishing. It is said that the action of swallowing and spitting water resembles the way a cormorant swallows and regurgitates fish[6]. This is one etymological theory, not a confirmed fact. But does gargling actually reduce colds? Surprisingly, this question had remained largely unverified for a long time.
2. Why a Kyoto University professor tested gargling, a habit with little proof
In Japan, gargling is a common habit, but it is said to be rare in other countries[3]. Professor Takashi Kawamura of Kyoto University and his team noticed that despite searching global literature, they could not find papers proving that gargling is effective for cold prevention[3]. The history of gargling is old; examples are said to appear in texts from the late Heian period and early Kamakura period, though its origins are unclear. It is also told that during the Sengoku period, Hojo Soun advised people to swallow the water used for gargling rather than spit it out[5]. Despite this long tradition, there was little evidence of its effectiveness.
So, in the winter of 2002–2003, a clinical trial was conducted with 387 healthy volunteers aged 18 to 65. Participants were randomly assigned to three groups: those who gargled with water, those who gargled with povidone-iodine mouthwash, and a control group that did nothing special[1]. Random assignment is used to prevent healthy or cautious people from clustering in one group, helping make the groups comparable apart from the variable being tested.
3. Results: people who gargled with water had the lowest rate of colds
In this trial, 130 people contracted upper respiratory tract infections during the winter. When adjusted to a rate per 100 people per month, the incidence was 26.4 for the no-gargling group, 17.0 for the water-gargling group, and 23.6 for the mouthwash group[2]. Dividing the water group’s rate (17.0) by the control group’s rate (26.4) gives approximately 0.64. This means the water-gargling group had an incidence rate about 36% lower than the control group (media reports often cited about 40% suppression). The mouthwash group’s rate (23.6) divided by 26.4 is about 0.89, showing only an 11% reduction, which is a small difference from the control group. The researchers concluded that simple water gargling was effective in preventing upper respiratory tract infections in healthy people[1].
Furthermore, it was reported that the water-gargling group tended to have milder bronchial symptoms, such as coughing and phlegm, even if they caught a cold[6]. However, this was a single study conducted over one winter with a specific group of people. From this alone, one cannot say that "gargling always works."
4. Why water gargling may have worked: a hypothesis about house dust enzymes
Professor Kawamura hypothesizes that water gargling worked because it washes away proteases in house dust, which are enzymes that cut proteins and activate viruses[2]. On the other hand, he suggests that the smaller difference in the mouthwash group may be because the medicine destroys normal bacteria that naturally live in the mouth[2]. Both of these are merely speculations. The materials do not contain results from tests that verify whether these mechanisms are actually the cause.
5. Why other countries stress handwashing and do not rely on gargling alone
Guidelines for infection control in the UK and the US include handwashing but do not mention gargling, and it is said that gargling is not a habit in Western countries[8]. The explanation given is that cold infections are thought to enter mainly through the back of the nose, while gargling targets the throat, so handwashing is considered more important abroad. This does not mean gargling is never done in the West.
In Japan, a newspaper article featuring a public health nurse explained that the word "gargle" has been removed from recent influenza prevention posters, with calls now focusing on "handwashing" and "cough etiquette"[7]. This is not a national judgment that gargling is meaningless, but rather a shift in the focus of public health messaging. There are limits to the scope of what was confirmed in this study. The research showed the difference in infection rates when healthy people gargled with water during one winter[1]. Since this was not a trial comparing gargling to handwashing, one cannot say whether gargling is more or less effective than handwashing. It is best viewed as one method to combine with handwashing and other measures.
6. How to check the study's numbers by calculating the ratios yourself
You can check the article's numbers by calculating the ratios. Using a calculator or by hand, divide 17.0 by 26.4. You should get approximately 0.64. Dividing 23.6 by 26.4 gives approximately 0.89. These two numbers represent the ratio of each group's incidence rate compared to the control group. Once you have calculated this, verify that the figure "incidence rate about 36% lower" comes from 1 minus 0.64. For the mouthwash group, 1 minus 0.89 gives about 11%.
You can also compare the sources of the numbers. If you line up the text's "26.4, 17.0, and 23.6 per 100 people" with the study's "0.26, 0.17, and 0.24", you can see that they represent the same trial using different units. Check by hand to see if the answers to these two sets of divisions result in the same ratio of 0.64.