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Mouthing off About Mouthwash for COVID

Can mouthwash “kill” the coronavirus? Sure but the implications of a laboratory study on human-to-human transmission are still unknown

Can rinsing your mouth with the right mouthwash protect you from the novel coronavirus or reduce your risk of transmitting it to someone else? A recent study involving mouthwash and the coronavirus (but no actual mouth) gives us a great opportunity to dissect the many ways in which interesting laboratory results can end up getting distorted through a game of telephone. If your cousin told your family group chat that gurgling with cetylpyridinium chloride will end the pandemic, lend me your ear.

The first thing to notice is that the study itself was written up and published on a preprint server called BioRxiv (charmingly pronounced “bio-archive”). Typically, scientific papers get sent to journal editors and are inspected by anonymous scientists in the field, whose task is to make sure these papers all look like they are in order. This is called peer review and it is an imperfect though necessary quality control step. Preprints, as the name implies, are not peer reviewed, although basic screening and checks against plagiarism are done. It’s not far removed from blogging. It doesn’t make preprints bad and peer-reviewed journal articles good, but it is important to keep in mind that rushing a study out as a preprint means it hasn’t been properly inspected by other scientists.

Another big question to ask ourselves when the subject matter is human health is: was this tested in humans? You would think a study about mouthwashes killing the coronavirus would involve actual jaws, tongues and teeth, but you’d be surprised. This was done in vitro, meaning in laboratory glassware. The scientists simulated the secretions of our mouth in a plastic receptacle, added the coronavirus, then dropped in the mouthwash they were testing before inactivating it after thirty seconds to simulate spitting out the solution into the sink. This was not done in humans. The scientists did not test to see if using mouthwash when you’ve just been infected prevents the illness, and they certainly did not verify to see if people who test positive for COVID are no longer infectious after rinsing their mouth.

Knowing all of these limitations, we can look at the results. The virus is not technically alive, so when we say scientists tried to “kill” the virus, it actually means deactivating it in such a way that it can’t copy itself anymore. The mouthwashes they tested which were the best at “killing” the virus in these laboratory simulations contained either cetylpyridinium chloride or 23% ethanol with a molecule called ethyl lauroyl arginate (abbreviated LAE). The real question is: how useful is it for us to know this?

These findings are part of a long scientific process of understanding how the virus behaves and what we can do to stop it in its tracks. Dentist offices typically now ask patients to gargle with mouthwash before the examination begins, not as a stand-alone solution to prevent spread but as part of a strategy that includes good air circulation, masks, gloves and visors. Mouthwash alone is unlikely to end the pandemic. Keep in mind that we can’t sterilize our mouth. Even if every coronavirus in our mouth was deactivated by a potent rinse, the virus exists inside the cells of our respiratory system, and copies of the virus are regularly churned out. The effect of mouthwash is temporary. It’s like killing the termites you see on your floor without addressing the ones underneath the floorboards.

As always, when we hear about promising research in our race to defeat the coronavirus, we should ask ourselves: was this tested in humans? To verify that, going back to the original study--getting it from the horse’s mouth, as it were--is usually better than hearing about it through word of mouth.

Take-home message:
- The results of a study on the effect of different mouthwashes on “killing” the coronavirus were released on an online repository but have not yet been officially reviewed by other scientists
- The scientists identified the active ingredients cetylpyridinium chloride and a combination of ethanol and LAE as the best at “killing” the coronavirus in laboratory glassware containing a solution meant to mimic the inside of the human mouth
- The study did not test to see if mouthwash can reduce COVID-19 transmission from person to person


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