An Australian professor of oral medicine has stirred up new debate over whether regular use of alcohol-containing mouthwashes can increase the risk of oral cancer.
In the January issue of the Australian Dental Journal (ADJ), Michael McCullough, D.D.S., Ph.D., of the University of Melbourne dental school, claims that 25 years' worth of clinical and epidemiological research leaves no other conclusion. But dental experts worldwide disagree -- and it isn't the first time.
Dr. McCullough and his co-author, Camile Farah, M.D.Sc., Ph.D., G.C.Ed., F.R.A.C.D.S., the director of research at the University of Queensland School of Dentistry, used the well-established correlation between smoking, alcohol consumption, and oral cancer as a jumping off point for their review of nearly 30 epidemiological and in vitro studies to determine if a similar relationship exists between alcohol-containing mouthwashes and oral cancer.
Among the research included in their analysis is a 2001 study in Puerto Rico of 342 patients with oral and pharyngeal cancer and 521 population-based controls that found that the use of alcohol-containing mouthwash caused an elevated risk for oral cancer among patients who neither smoked cigarettes nor drank alcohol (Cancer Causes and Control, June 2001, Vol. 12:5, pp. 419-429).
A more recent multicenter controlled study of 3,210 patients in Latin America and central Europe with head and neck cancer concluded that self-reported daily mouthwash use "is a significant risk factor for the development of head and neck cancer independent of tobacco use and other alcohol consumption" (American Journal of Epidemiology, November 15, 2007, Vol. 166:10, pp. 1159-1173).
"There is now sufficient evidence to accept the proposition that developing oral cancer is increased or contributed to by the use of alcohol-containing mouthwashes," Dr. McCullough and Dr. Farah concluded in their ADJ article.
But some experts say these conclusions are unfounded because the article itself -- and several of the studies cited in it -- do not meet the Cochrane Database criteria of evidence-based medicine. In the 2007 study, for example, the authors didn't ask the participants what kind of mouthwash they used, how often they used it, and if it contained alcohol, according to Sol Silverman, D.D.S., a professor of oral medicine at the University of California, San Francisco School of Dentistry and the ADA spokesperson on oral cancer.
"Back in the late 1990s, the FDA was about to put a label on all alcohol-containing mouth rinses talking about the risks of oral cancer because of some of the articles and research that had been done," Dr. Silverman said. "But those articles didn't pass the evidence-based criteria, so it was decided that there was not evidence to put this label on the products. Now all of a sudden we have this article from Australia, which even the Australian Dental Society does not endorse."
Dr. McCullough defends the article's findings, particularly in light of previous research that has consistently demonstrated the relationship between alcohol consumption and oral cancer.
"90% to 95% of oral cancer patients have a history of some form of tobacco use, and also with drinking of alcohol," he said. "A lot of criticism [of the ADJ article] is coming from people associated with the companies that manufacture these mouthwashes or places that have approved or supported these products in the past."
|Alcohol content of some commercial mouthwashes (v/v = volume/volume; w/v = weight/volume)
|Data courtesy of Dr. Michael McCullough.|
Dr. McCullough also believes that the amount of alcohol in some of the leading mouthwashes -- notably Listerine -- is significant enough to warrant further consideration.
"The first breakdown product of alcohol is acetaldehyde, which is a known human carcinogen, and there is evidence that this can occur in the human oral cavity," he said. In addition, the ethanol in mouthwash -- which is used primarily as a solvent for other ingredients -- is thought to allow cancer-causing substances to permeate the lining of the mouth more easily and thus cause harm.
"We think there is epidemiological evidence now to support this association, and definitely biological reasons behind the topical affect of alcohol mouthwashes," Dr. McCullough said.
A leading cancer researcher disagrees. Carlo La Vecchia, M.D., M.Sc., the head of the Laboratory of General Epidemiology at the Istituto di Richerche Farmacologiche in Milan, Italy, published a separate literature review last year on mouthwash and oral cancer risk. His conclusion? That a link between alcohol-containing mouthwash and oral cancer "is not supported by epidemiological evidence" (Oral Oncology, October 25, 2008).
"We know that moderate alcohol use is not associated with oral cancer, so I would be inclined to conclude that the amount of alcohol in the oral cavity from mouthwash should not exceed the amount you would get from drinking one to two drinks per day," Dr. La Vecchia said. "A daily mouthwash user is inclined to be drinking about the same [amount of alcohol], so you can conclude a strong association but not much more."
Despite his insistence that a more direct link exists between oral cancer and the topical effects of regular use of alcohol-containing mouthwashes -- including the fact that many smokers and drinkers use mouthwash to improve their breath -- Dr. McCullough said he does not believe these mouthwashes should be banned or even made "prescription only," as some news stories have suggested.
"There is good reason to use these mouthwashes, particularly after periodontal surgery," Dr. McCullough said. "We are not saying alcohol-containing mouthwashes don't have a role at all -- but not in long-term use, such as twice daily over five years. We are trying to raise the awareness of this in the dental community. Dentists should be the ones telling patients when to use them and what is in them."
A number of alternatives work just as well, he noted.
"More and more in the last few years, there are mouthwashes without alcohol," Dr. McCullough said. "For example, chlorhexidine mouthwashes, which are traditionally recommended by dentists, have alcohol levels of 15% or less. It is the ones with 20% or more alcohol [higher than in beer or wine] and using these over several years that is the problem. Normal oral hygiene should just be about toothbrushing and flossing and visiting your dentist every six months or so."