Gargling: Clinical trials on reducing respiratory infections

For many, gargling or swishing with mouthwash is a habit associated with fresh breath and clean teeth. But in some Asian countries, gargling is a traditional way to cut down on colds and the flu – and it actually seems to work.

During this COVID-19 pandemic, common-sense prevention and early treatment methods aren’t recommended or discussed. I assumed that it was because there were no clinical trials on the old-fashion ways of preventing respiratory infections, but that turns out not to be true.

Research studies and clinical trials do show that gargling decreases colds and flu — reducing the risk of catching the disease, the severity of the disease, and (importantly) spreading the virus to others. In this brief article, I’ll explain some of the research and then let you decide whether gargling is worth a shot.

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Gargling as a prevention for colds and flu:

A common practice in Japan, gargling has been studied for a couple of decades to see if it is linked to a statistical reduction in upper respiratory tract infections. In the early 2000s, one of Japan’s guideline preventative measures for an influenza pandemic was to recommend gargling to their citizens.[ref]

Upper respiratory tract infections include colds (coronaviruses, rhinoviruses, adenoviruses, etc.) and the flu. For many respiratory infections, the virus colonizes the upper respiratory tract, including the nose and throat — and this is also a site where the virus often spreads to others via aerosol transmission. Thus, theoretically, gargling can reduce the viral load, prevent illness, and cut down on transmission to others.[ref]

But… does gargling really work for reducing upper respiratory infections?

Let’s take a look at some of the clinical trials and studies over the past couple of decades:

A small randomized controlled trial found that gargling either with water or a povidone-iodine solution reduced the risk of respiratory infections and reduced symptoms in the people who did get sick.[ref]
An economic analysis in Japan showed that reducing respiratory tract infections by 36% (one study showed gargling does) would save about $2 billion annually in healthcare costs.[ref] That seems pretty significant for something with little to no cost or downside.
A study of Iranian pilgrims during Hajj found that gargling daily with salt water reduced the risk of respiratory infections by half. The study concluded, “This study showed that measures such as seasonal influenza vaccination, use of face masks and personal prayer carpet have no effect on the incidence of respiratory tract infections. However, washing throat and mouth with salt water can be considered the most effective preventive measures.”[ref] (Note that this study was done during one flu season in 2010, so it may have been one of those years that the flu vaccine was a mismatch for the strain going around.)
A study conducted with almost 20,000 Japanese children found that gargling (green tea or water) reduced the risk of fever with illness by about half. In kids aged 4+, gargling reduced the risk of sickness by about 30-40%. (It wasn’t statistically effective in 2-year-olds, but I can’t imagine trying to get a 2-year-old to gargle…)[ref]

Historically, gargling with salt water was the standard recommendation for a sore throat. In Germany, it was recommended during the Spanish flu epidemic and during the Cold War era, gargling with an antimicrobial mouth rinse was done before entering summer camp.[ref]

Recap: This sampling of studies showed that gargling with water, green tea, or saltwater cuts the risk of flu and colds. It’s not a miracle prevention, but it’s safe, cheap, and half-way effective.

Beyond water or green tea: gargling with iodine or mouthwash

Iodine is a general disinfectant that has been used for more than a century. I know that it was the first thing my parents reached for when I had a scraped knee as a kid.

Researchers found that iodine throat spray or iodine gargle (both at 0.23% iodine) would kill the avian flu virus with 10 seconds of application.

After the original SARS-CoV outbreak occurred, researchers found in 2004 that two-minute exposure to a povidone-iodine solution would reduce the virus infectivity to zero.[ref]

Other options for killing viruses and bacteria include gargling with antimicrobial mouthwashes such as Listerine or chlorhexidine.

For SARS-CoV-2, studies show that both povidone-iodine solution and Listerine (essential oils) effectively inactivate the virus.

Another mouthwash (available in Europe) called Dequonal was also very effective.[ref]

What about COVID-19?

Let’s look at the studies on prevention or treatment of COVID-19 with gargling:

Overall, at least ten clinical trials (as of Dec 2021) on povidone-iodine gargling and/or nasal rinsing. When combining the data, the trials show an average reduction in covid severity of 71% when used early and a 45% decreased risk of getting covid when used as a prevention.[ref]

Preventing infection: A randomized controlled trial in a dormitory in Singapore investigated the efficacy of several ways to prevent SARS-CoV-2 infection. The control arm was just given vitamin C (not a placebo), which meant that the interventions were all compared to 500mg/day of vitamin C. The risk reduction for using povidone-iodine throat spray was a 24% decrease in infections. It was better than the risk reduction from hydroxychloroquine, ivermectin, or zinc.[ref] What I like about this study is that it was large (3,000+ participants) and conducted in the worst case for spreading a respiratory illness (high population density dormitory). Keep in mind that the statistically significant reduction in SARS-CoV-2 infection was above and beyond the control arm, which was taking vitamin C.

Iodine vs. Listerine: Another (small) clinical trial examined gargling with iodine (1% solution) vs. Listerine or tap water. The results showed viral clearance in 6 days for 100% of people with iodine gargling and 80% of people with Listerine gargling. The group that gargled with water had 0% clearance of the virus at day 6.[ref]

Possibility of thyroid side effects with iodine: Iodine is essential to thyroid function, and we normally take in trace amounts each day. Iodine can be absorbed through the skin (or throat or nose).

A study published in JAMA involved patients with COVID-19 who had a high viral load. The patients gargled with a 1% povidone-iodine solution (4 times) and also used an iodine nasal spray as well as an iodine nasal swab.

The study concluded, “Nasopharyngeal decolonization may reduce the carriage of infectious SARS-CoV-2 in adults with mild to moderate COVID-19. Thyroid dysfunction occurred in 42% of the patients exposed to PI, with spontaneous resolution upon treatment discontinuation, as previously reported.” The thyroid dysfunction, in this case, was an elevated TSH level for a couple of days.[ref]

Lifehacks:

The studies on gargling with water show that it is somewhat effective for colds and flu. It is free and easy to do, with no drawbacks that I can see.

For more severe viral infections, gargling with an iodine-water solution (0.23% – 1% solution) or with an antimicrobial mouthwash seems more effective than water for prevention and reducing the spread of the virus.

Most studies on active infections included gargling 2-4 times a day for 30 seconds.

The drawback to iodine is that there could be a transient effect on TSH levels. For someone with thyroid problems, I recommend talking to your doctor before using an iodine solution daily. An antimicrobial mouthwash may be a safer option for daily use.

My two cents: Personally, I’m going to gargle at the first sign of a cold. I will likely use an iodine solution if I’m starting to feel sick (no thyroid problems). I also now have some antimicrobial mouthwash on hand and will use it prophylactically before being in situations where I could spread a virus unknowingly or in crowds where I could pick up a virus.

While I focused on gargling, nasal rinsing and iodine-based nasal sprays may be another option to consider.

Gargling as a prevention for colds and flu:

Beyond water or green tea: gargling with iodine or mouthwash

What about COVID-19?

Lifehacks:

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