The reality about mouthwash – after research discovered hyperlink to most cancers

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

But while around a quarter of the UK population uses mouthwash, there’s growing evidence to suggest it may do more harm than good: a new study has led to claims that antiseptic mouthwash may raise users’ risk of oesophageal and bowel cancer, while other recent research has linked its use to high blood pressure and type 2 diabetes, with its bacteria-killing properties – which is why people use it – thought to be to blame.

The latest study, by the Institute of Tropical Medicine in Antwerp, Belgium, showed that using Listerine Cool Mint 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 cancer.

It’s believed that the alcohol in the mouthwash may be to blame, by altering the delicate balance of the mouth’s microbiome and wiping out some ‘good’ bacteria and allowing these ‘bad’ bacteria to proliferate.

While the study looked only at one product, Professor Chris Kenyon, one of the scientists involved, suggested that other alcohol-based mouthwashes would have a similar effect.

He said: ‘Most people should not be using it [mouthwash] and if they do, they should use preparations without alcohol and limit use to a couple of days.’

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

‘We have to be careful not to over-sensationalise the findings of one study which only measured 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 about mouthwash – and should you be using it?

It’s well known that antibiotic use can upset the gut microbiome – the community of microbes, including bacteria, that play 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 – the diverse microbes in our mouths – and specifically, how using mouthwash could wipe out some of the ‘good’ 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 – nitric oxide effectively tells our blood vessels to relax, keeping our blood pressure nice and low.

 ‘A number of studies have now 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 to less frequent users.

And an earlier study, in 2017, by the same Puerto Rico-based research group found that over-weight 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 to 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 patients who have been hospitalised may increase their risk of death from sepsis – this is a life-threatening condition that occurs when the body’s immune system overreacts to an infection, leading to organ failure which can be fatal.

Scientists aren’t entirely sure what is behind the increased risk but suggest that killing the oral bacteria responsible for nitric acid production stops the body being able to absorb 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 anti-septic chlorhexidine (alongside toothbrushing) they reduce the plaque that causes tooth decay and early gum disease,’ says Dr Brookes.

‘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 other problems such as sepsis and also contribute to the wider problem of antibiotic resistance.’

So should we ditch 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 see 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 they can use instead.

‘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.’

In a statement to MailOnline, Kenvue, which makes Listerine, said: ‘We continuously evaluate the latest science. There is no evidence that Listerine causes cancer.’

WHAT’S IN YOUR MOUTHWASH?

The lowdown on antibacterial ingredients

ALCOHOL

This is sometimes added to mouthwashes as it functions as an antiseptic, but it can wipe out good and bad bacteria.

Previous studies linked mouthwashes containing alcohol with an increased risk of mouth cancer, but a 2020 review by Barcelona University found this was only if those using it were also smokers because alcohol makes the mouth’s cell walls more permeable to the carcinogens in cigarette smoke.

And although this latest study by the Institute of Tropical Medicine in Belgium found that daily use of alcohol-containing mouthwash increased numbers of strains of bacteria linked to cancer, according to Dr Zoe Brookes, an associate professor of dental education and research at the University of Plymouth, ‘meta-analysis on large numbers of studies has not consistently shown a link between alcohol-containing mouthwash and oral cancer’.

CHLORHEXIDINE

An antimicrobial found in high-strength mouthwashes for people with gum disease, this is highly effective at killing bacteria that causes gum disease.

But recent research links it to destroying the ‘good’ bacteria that help prevent heart disease and other health conditions. It can also stain teeth if used for more than a few days.

CETYLPYRIDINIUM CHLORIDE

Another antiseptic that reduces oral bacteria, lowering your risk of developing gum disease and tooth decay, but potentially also disrupting the important oral microbiome (the colony of microbes in the mouth).

ZINC CHLORIDE

A weaker antibacterial than chlorhexidine or cetylpyridinium chloride, but it can still kill off good bacteria in your mouth.

FLUORIDE

A mineral added to toothpaste and many mouthwashes and shown to strengthen tooth enamel and fight cavities.

The good news is studies on fluoride use show it causes little or no changes to the oral microbiome.

ESSENTIAL OILS

Plant oils such as eucalyptus, thyme and menthol are now commonly used in mouthwashes – a 2015 study, by the University of Santiago de Compostela in Spain, found they were as effective anti-plaque agents as chlorhexidine.

However, this result means regular use is also likely to upset your oral microbiome by killing off the good bacteria with the bad.