Listerine, Cancer: Is there any connection?

Listerine has a fascinating history. Originally invented as a surgical antiseptic, the mixture of menthol and essential oils was then marketed as a general germicide and a cleaning solution (for example, for floors) before finally taking off as a solution for chronic halitosis, aka bad breath. The brand is now a worldwide name, being sold globally as a way to improve oral hygiene and reduce the risk of problems with your teeth.

But that is, allegedly, not all it can do. Apparently, new research has shown that Listerine might cause cancer if you use it frequently enough. This terrifying finding has made its way into headlines around the globe. What’s more, “other brands pose a similar threat,” claimed the Daily Mail. It’s quite upsetting for those of us who gargle mouthwash on a daily basis.

Fortunately, the data is not nearly as worrying as those stories might suggest. While it’s possible that Listerine could be related to some increased risk of cancer, it’s not very likely. The new research doesn’t show much of a connection at all.

The recent hubbub has sprung up due to a new study looking at mouthwash and the oral microbiome, the tiny organisms—such as bacteria—that live in your mouth. This study was a reanalysis of a previously run randomized trial that included gay men who were taking pre-exposure prophylaxis—or PrEP, a medication that prevents HIV—and who used either Listerine Cool Mint or a placebo mouthwash for three months. The original trial looked at whether this reduced their risk of sexually transmitted infections such as chlamydia or gonorrhoea. (It concluded that Listerine didn’t really help.)

The new study examined data from a subset of 59 men from the original trial of hundreds and compared their oral microbiome before and after using the Listerine or placebo mouthwash, which was flavored but, unlike Listerine, didn’t contain any alcohol. They found that there were some changes after using Listerine for three months, particularly an increase in two types of bacteria called Streptococcus anginosus and Fusobacterium nucleatum.

What does this have to do with cancer? Well, there is some previous observational evidence suggesting that there is an association between these bacteria and some forms of oral cancer. The authors of the new paper mentioned this in their discussion, and this was picked up in the media as “mouthwash causes cancer” (with some help from one of the authors, who provided a sound bite with a pretty firm bottom line about Listerine: “Most people should not be using it”).

But these connections are, in a word, tenuous. It’s possible that mouthwash causes some bacteria to be more abundant, and that this could lead to chronic issues that then lead to cancer. But all of that is pretty much just a hypothesis at this point. If nothing else, it’s not clear that you can take a study in 59 men who are on pre-exposure prophylaxis for HIV and apply the findings to everyone else in the world.

In addition, it’s important to note that this study didn’t actually show that alcohol-based mouthwash was responsible for the changes to the bacteria in mouths of participants. There were differences in bacterial growth before and after Listerine. But there were no differences—or at least, no statistically significant ones—between the Listerine group and the placebo control group. That means that it’s very unlikely that three months of Listerine actually caused these bacteria to grow. Rather, it is likely just statistical noise. (This is why we have placebos!)

This is not the first study that’s tried to say something about alcohol-based mouthwash and cancer. A systemic review from a few years ago found that there was a weak association between the two, perhaps due to the alcohol’s effect on mouth tissue—though the authors write that they did not find the evidence sufficient “to accept the proposition that the use of mouthwashes containing alcohol can influence the development of oral cancer.” We know that alcohol isn’t good for you. Alcohol causes all sorts of problems with the body, including a wide range of cancers in pretty much every organ system. If you wanted to almost entirely eliminate even the potential risk that mouthwash could have, the best thing to do would be to start using an alcohol-free mouthwash rather than one that has some booze in it.

Either way, this new research is incredibly unconvincing and doesn’t add anything to our understanding of mouthwash and cancer. Since there were no differences between Listerine and the placebo control, it’s quite likely that Listerine had no problematic impacts on the oral microbiome. If you like mouthwash, there’s no reason to stop using it, at least based on this data. If you’re really worried about oral cancer, perhaps switch to an alcohol-free version.