"Experts warn using mouthwash more than twice a day can give you cancer," the Daily Mirror reports. The news comes from a European study that examined the oral health and dental hygiene of people diagnosed with cancers of the mouth, throat, vocal chords or oesophagus (collectively called "upper aerodigestive cancers").
The researchers found that people with the poorest oral health (including wearing dentures and bleeding gums) had a more than doubled risk of these cancers compared with those with the best oral health.
Similarly, they found that those with the poorest dental care (including frequency of tooth brushing and visiting the dentist) had a more than double risk compared with those with the best dental care.
Importantly, these associations remained after adjustment for smoking and alcohol consumption – established risk factors for these cancers – and for other factors that may influence risk, such as socioeconomic status.
But despite the Mirror's headline, the link between oral cancer and mouthwash is less clear. The association was only significant when looking at very frequent use (three times a day).
Very few people used mouthwash this frequently, which decreases the reliability of this risk estimate. There is certainly no credible evidence that mouthwash "can give you cancer".
Even if there is a true link, it is unclear whether it is mouthwash itself (the alcohol content) or the reasons it is being used, such as poor oral hygiene, that are responsible for the association.
The results do suggest a link between poor dental hygiene and oral cancers, however, and reinforce the importance of maintaining good dental health.
Where did the story come from?
This was multicentre research conducted by numerous academic institutions across Europe and the US.
The study was supported by the European Community Fifth Framework Programme, the University of Athens Medical School, the Bureau of Epidemiologic Research Academy of Athens, Padova University, Compagnia di San Paolo, Associazione Italiana per la Ricerca sul Cancro (AIRC), the Piedmont Region, targeted financing from the Estonian government through the European Regional Development Fund in the frame of Centre of Excellence in Genomics, and the 7FP Project ECOGENE.
It was published in the peer-reviewed Journal of Clinical Oncology.
The quality of the UK's media reporting on the study was mixed. BBC News rightly focused on the link between poor dental hygiene and oral cancer.
But the Daily Mirror incorrectly states in its headline that, "Experts warn using mouthwash more than twice a day can give you cancer". In fact, the researchers specifically go out of the way in their conclusion to state that their data does not provide proof that excessive mouthwash increases cancer risk.
What kind of research was this?
This was a case-control study that included a group of people diagnosed with cancer of the mouth, throat, vocal chords or food pipe (oesophagus). They were then matched with a group of people without these cancers (the controls) and were interviewed about their oral health, dental care and lifestyle.
The researchers aimed to see whether oral health and dental care – in particular, the use of mouthwash – may be associated with these cancers. As a group, these cancers are sometimes called "upper aerodigestive cancers" as they involve the upper parts of the respiratory and digestive system.
These cancers are said to account for around 129,000 new cancer cases in the European Union, making them the fourth most common cancers for men and the tenth for women.
Alcohol and smoking are widely known to be risk factors for these cancers. Other research has also associated the cancers with lower fruit and vegetable consumption, and found that they are more common among lower socioeconomic status groups.
Additional research has also suggested that poorer dental and oral health may be associated with increased risk, independent of alcohol and smoking behaviour.
It is also speculated that frequent use of mouthwash could be a risk factor as a result of the ethanol (alcohol) it contains. However, there is limited evidence proving that there is an increased risk associated with mouthwash containing alcohol.
This study aimed to examine whether mouthwash and wider oral health and dental care are associated with the risk of upper aerodigestive cancers, importantly adjusting for the potential confounders of smoking and alcohol.
What did the research involve?
This study used information from the multicentre alcohol-related cancers and genetic susceptibility in Europe (ARCAGE) case-control study, which was conducted across 13 centres in nine European countries.
The study included 1,963 people newly diagnosed with cancers of the mouth, throat, vocal chords or oesophagus between 2002 and 2005 (cases). They were matched by age and sex to 1,993 people without cancer, who were randomly selected from people attending the same medical centres or hospitals as the cases for other health reasons.
All participants were interviewed about a range of health and lifestyle measures:
- sociodemographic characteristics (number of years of full-time education was used as the main indicator of socioeconomic status)
- smoking history (lifetime smoking history was used to calculate "pack years")
- alcohol consumption (lifetime consumption of number of drinks per day was assessed for all categories of alcoholic drinks)
- weekly consumption of fruits and vegetables (recorded by food frequency questionnaire)
- employment history
- body measurements
- medical and dental history, including oral hygiene habits
Oral health was assessed using the following scoring system, where a maximum total score of 7 would indicate poorest oral health:
- wearing of dentures (none = 0; partial denture in upper or lower jaw = 1; partial denture in both jaws = 2; complete denture in one jaw = 3; complete denture in both jaws = 4)
- age at starting to wear dentures (no denture = 0; denture at age 55 years or older = 1; denture at age 35-54 years = 2; denture at age below 35 years = 3)
- frequency of gum bleeding from brushing teeth (sometimes or never = 0; always or almost always = 1; 0 in subjects wearing complete dentures in both jaws)
Similarly, dental care was assessed as follows, where a maximum total score of 8 would indicate poorest dental care:
- frequency of tooth cleaning (at least twice per day = 0; once per day = 1; 1-4 times per week = 2; less often or never = 3)
- use of toothbrush, toothpaste or dental floss (two or three of these = 0; only one of these three = 1; none of these = 2)
- frequency of visiting a dentist (at least once per year = 0; every 2-5 years = 1; less than every 5 years = 2; never = 3)
Participants were asked about their use of mouthwash in a separate question, but this was not included in these scores.
The researchers also took blood samples to look at whether people had four variations in genes that code for proteins involved in breaking down alcohol (ethanol).
The researchers previously found these variations to be associated with risk of upper aerodigestive cancers, with one particularly associated among heavy drinkers.
As many brands of mouthwash contain alcohol, the researchers wanted to test whether a person who had these variants influenced the potential link between mouthwash and upper aerodigestive cancers.
What were the basic results?
Participants were aged 60 years on average. Almost half of cases had mouth cancer (48%), followed by cancer of the lower throat or vocal chords as the next most common cancer (36%).
After adjustment for all other measured health and lifestyle factors, the risk of upper aerodigestive cancers increased with poorer dental care. People with the worst dental care (scores of 5-8) had the highest risk, more than double the cancer risk for people with the best dental care (a score of 0; odds ratio [OR] 2.36, 95% confidence interval [CI] 1.51 to 3.67).
Looking at oral health, people with the poorest oral health (score of 5, 6 or 7) had an increased risk compared with those with the best oral health (score of 0). People with the highest oral health score of 7 had a more than doubled risk compared with those with a score of 0 (OR 2.22, 95% CI 1.45 to 3.41). Those with moderate oral health – a score of 1-4 – were not at increased risk compared with those with the best oral health.
Reported use of mouthwash of more than three times per day was associated with tripled risk of upper aerodigestive cancers (OR 3.23, 95% CI 1.68 to 6.19). Importantly, the researchers say that although this effect was strong, only 1.8% of cases and 0.8% of controls reported such frequent use.
These relatively small numbers reduce confidence that these estimates of risk are correct. There was also no link between less frequent use of mouthwash (less than three times a day) and risk.
Looking at the four gene variants, certain variants associated with faster ethanol metabolism were associated with a decreased risk of these cancers, while a variant associated with slower ethanol metabolism was associated with increased risk.
One particular variant associated with faster ethanol metabolism was found to be less common in mouthwash users compared with "never users".
How did the researchers interpret the results?
The researchers concluded that poor oral health and dental care seem to be independent risk factors for upper aerodigestive cancers, even after adjusting for potential confounders such as smoking and alcohol use.
They say that, "Whether mouthwash use may entail some risk through the alcohol content in most formulations on the market remains to be fully clarified."
This multicentre study conducted across nine European countries has many strengths, including its large sample size. Most importantly, it adjusted for smoking and alcohol consumption, which are well-established risk factors for these cancers and could otherwise influence the association between oral health and dental hygiene and these cancers.
The researchers also adjusted for other potential risk factors, such as socioeconomic status and how much fruit and vegetables people ate.
However, there are some potential limitations. Although the researchers have made every effort to adjust for these confounders, as the researchers themselves acknowledge, the questions asked about these lifestyle factors may not fully capture a person's smoking habits, alcohol use and diet, so there is still the possibility that they have some effect.
Also, the questions asked around oral health and dental hygiene may not have given a full representation of the person's mouth care. These self-reported measures were not checked against dental records.
The study asked people to rate their current oral health and dental hygiene, and in the people with cancer this was after their diagnosis. This may not reflect their lifelong oral health or care before their diagnosis. An independent assessment provided by a dentist, or examination of dental records, may have been more reliable.
Nevertheless, the study does support an independent link between oral health and dental hygiene and aerodigestive cancers. The link seems biologically plausible and further study could also assess why these links might exist. Previous studies have suggested similar links, and ideally a systematic review would be able to look at this new study alongside the other available evidence. Such a review may provide new insights into potential risk factors.
Despite reports to the contrary, the link between mouthwash and cancer is less clear. Although using mouthwash more than three times a day was more common among cases than controls, very few people used mouthwash this frequently – only 1.8% of cases and 0.8% of controls. Risk calculations involving such small numbers of people are less reliable than those including larger samples.
The possible link between mouthwash and mouth and throat cancers needs to be clarified. If there is a link, it is currently unclear whether it could be related to the alcohol contained in mouthwash, or whether the link is caused by poor oral health and not a direct effect of mouthwash at all. It could be that poor oral health or dental hygiene increases the risk, and people with poorer health are also more likely to use mouthwash.