Evidence is mounting about the health risks of mouthwash so just how safe is it?

A swift swill of mouthwash is part of many people’s daily dental routines.

But while around a quarter of Britons use it, there’s growing evidence to suggest it may do more harm than good: a study has found that antiseptic mouthwash increases levels of ‘bad’ bacteria in the mouth that may in turn raise the risk of gum disease and certain cancers.

Meanwhile, other recent research has linked its use to high blood pressure and type 2 diabetes — with its bacteria-killing properties, the reason people use it, thought to be to blame.

The latest study, by the Institute of Tropical Medicine in Antwerp, Belgium, showed that using one type of Listerine mouthwash every day for three months increased the numbers of two species of bacteria — Fusobacterium nucleatum and Streptococcus anginosus — which have been linked to gum disease as well as oesophageal and colon cancers.

It’s believed that the alcohol in the mouthwash may cause the rise in these two species of bacteria — and that by altering the delicate balance of the mouth’s diverse microbiome (a community of micro-organisms) and wiping out some “good” bacteria, it allows these “bad” bacteria to proliferate.

While the study looked only at one product, Professor Chris Kenyon suggested other alcohol-based mouthwashes would have a similar effect, adding: “Most people should not be using it and if they do, they should use preparations without alcohol and limit use to a couple of days.”

But Dr Zoe Brookes, an associate professor of dental education and research at the University of Plymouth, urges caution when interpreting the study.

“We have to be careful not to over-sensationalise the findings of one study which measured only the amounts of certain bacteria in the mouth after using a Listerine mouthwash — not actual rates of cancer.

“Yes, there are some links between Fusobacterium and colon cancer, but large reviews of all the existing studies have not shown a strong link between using alcohol mouthwash and developing oral cancers.

“The picture isn’t clear cut.”

So what is the truth — and should you be using it?

It’s well known that antibiotic use can upset the gut microbiome that plays an important role in our digestive and immune systems — by wiping out the “good” as well as the “bad” bacteria.

Now attention is turning to the oral microbiome and, specifically, how using mouthwash could wipe out beneficial bacteria that help protect our bodies against conditions such as heart disease, type 2 diabetes and even cancers.

“Your mouth is filled with hundreds of species of bacteria and while some cause plaque and decay, others are actually very good for health and responsible for quite complex processes in the body,” says Dr Brookes.

“For example, there are bacteria living on the tongue that convert ‘nitrates’ from the food we eat into ‘nitrites’,” explains Dr Brookes, who led a study in 2020 on the effects of mouthwash on the oral microbiome.

Nitrites are then turned into nitric oxide in the gut — which effectively tells our blood vessels to relax, keeping our blood pressure nice and low.

Dr Brookes adds: “Studies have found that the use of mouthwash — particularly brands containing the antiseptic chlorhexidine — can lead to an increase in blood pressure, especially in people who already have raised levels.”

For instance, in 2019, researchers at the University of Puerto Rico found that people who used mouthwash twice a day or more had a greater risk of having high blood pressure compared with less frequent users.

And an earlier study by the same research group in 2017 found that overweight people using over-the-counter mouthwash at least twice daily had a 50 per cent increased risk of developing type 2 diabetes over a three-year period compared with non-users.

The researchers put this down to the fact that killing these key bacteria in the mouth reduces the body’s ability to make nitric acid, a natural compound that plays a role in regulating insulin — the hormone that keeps blood sugar levels steady.

Therefore, the destruction of this beneficial bacteria could lead to unstable blood sugar peaks and encourage the development of diabetes.

And that’s not the only potentially harmful side-effect of mouthwash.

A 2020 study published in the Journal of Intensive Care Medicine found that antiseptic mouthwash used by hospital patients may increase their risk of death from sepsis — a life-threatening condition that occurs when the body’s immune system overreacts to an infection, leading to potentially fatal organ failure.

Scientists aren’t entirely sure what is behind the heightened risk.

But they suggest that killing the oral bacteria responsible for nitric acid production prevents the body from absorbing enough of this compound which plays a key role in healthy circulation — and we know that circulation is one of the body systems that shuts down in sepsis.

But all this must be weighed against the benefits of mouthwash.

“Of course there is also good evidence that when patients use mouthwashes containing the antiseptic chlorhexidine (alongside toothbrushing) they reduce the plaque that causes tooth decay and early gum disease,” says Dr Brookes.

She explains: “But it’s a double-edged sword, because chlorhexidine is so powerful it kills so many different species of bacteria — including the good ones.

“And by unbalancing our oral microbiome in this way, we may indirectly affect not only our heart health, but perhaps increase the risk of succumbing to other problems such as sepsis and contribute to the wider problem of antibiotic resistance.”

So should we actually be ditching mouthwash?

“Not necessarily,” says Dr Brookes.

“There is concern, but on the other hand, we also have a huge amount of growing evidence that gum disease itself is associated with uncontrolled diabetes and cardiovascular disease, so keeping our teeth and gums healthy is more important than ever — especially when so many people can’t get to an NHS dentist.”

She adds: “As for the latest study linking alcohol-containing mouthwash to cancer-causing mouth bacteria, if people are worried, there are plenty of similar alcohol-free mouthwashes available.

“As a dentist, my advice is different for each patient, questioning each time whether the benefits of using mouthwash outweigh any personal risks for them,” she says.

A spokesman for Kenvue, which owns Listerine, said: “Based on our initial review, the published trial lacks several important design controls and adequate rigour to make any conclusions about potential impact to human health.”