To Floss or Not to Floss? That’s not the question

Bioethics-in-the-News-logoThis post is a part of our Bioethics in the News series

By Robyn Bluhm, PhD

Should we stop flossing?
Earlier this month, the Associated Press published a report showing that the evidence in support of flossing is “‘weak, very unreliable,’ of ‘very low’ quality, and carries ‘a moderate to large potential for bias.’” So, should we stop flossing? The American Dental Association says no: “interdental cleaners such as floss are an essential part of taking care of your teeth and gums.”

The AP report and the ADA’s response illustrate interesting questions about evidence-based medicine (EBM), an approach to medical decision-making that has become central to medicine since its introduction in the early 1990s. EBM is often defined as “the conscientious explicit and judicious use of current best evidence in making decisions about the care of individual patients.” The “current best evidence” referred to in this definition is (at least in the case of studies examining treatments) supplied by well-designed randomized controlled trials (RCTs). Although RCTs have been conducted to investigate the effectiveness of flossing, the quality of the studies – and therefore of the evidence they provide – has been poor; this much the AP report and the ADA agree on.

What counts as good evidence? The ADA and EBM
In their response to the report, the ADA appeals to other sources of evidence, ones that EBM considers inferior to RCTs. EBM provides a hierarchy of evidence that ranks research methods according to the quality of evidence they are expected to generate (see, for example, Guyatt et al. 2014, p. 11). At the top of the hierarchy are RCTs, followed by controlled trials that are not randomized. These studies use epidemiological methods to assess treatment outcomes in large groups of patients. Below these study designs comes (1) evidence from physiological research that aims to elucidate the details of biological processes, including the effects of an intervention, and (2) evidence from clinicians’ unsystematic clinical experience, described, for example, in case reports. According to EBM, the first of these lower sources of evidence is problematic because knowledge of physiology does not accurately predict clinical outcomes, while the second source is too limited and small in scale to give generalizable information.

dental floss embroidered in green
Image description: a dental floss package has been embroidered with green thread onto a plain piece of fabric. Image source: Flickr Creative Commons

In fact, the ADA’s response to the report on flossing appeals to both of these “lower” forms of evidence. They begin with an appeal to clinical experience, saying that “As doctors of oral hygiene, dentists are in the best position to advise their patients on oral hygiene practices because they know their patient’s oral health status and history.” The report later endorses the traditional rationale for flossing, which is exactly the kind of physiological reasoning criticized by EBM: “Cleaning between the teeth removes plaque that can lead to cavities or gum disease from the areas where a toothbrush can’t reach. Interdental cleaning is proven to help remove debris between teeth that can contribute to plaque buildup.”

Is the ADA ignoring EBM’s recommended approach to evidence? Not necessarily. Early accounts of EBM seem to recommend strict adherence to the hierarchy of evidence, advising clinicians that “they should look for the highest available evidence from the hierarchy” (Guyatt et al., 2002, p. 8). More recently, this advice has changed to “they should seek the highest-quality evidence available” (Guyatt et al., 2014, p. 11). The problem is that, in the absence of the “best” evidence, well-conducted RCTs, EBM does not provide clear guidance as to how to assess – and act on – the evidence that’s actually available.

What EBM misses…
Despite all of these problems with assessing the evidence, the ADA says that we should just keep flossing. For that matter, (poor evidence alert!) everyone I’ve told about this blog post has said that they don’t plan to stop. And it’s here that a dimension of evidence becomes apparent that is missing in EBM, but is central to bioethics: in deciding whether to use a treatment, we need to consider the balance of benefits and harms. We also need to set our standards of evidence accordingly. Flossing is unlikely to be harmful, and (despite the lack of evidence) may have significant benefits. It is also easy to do and inexpensive, both factors that count on the “benefits” side of the scale. We could try to acquire better evidence to support the ADA’s recommendations, but given how unlikely it is that flossing is actually harmful, we already have enough evidence to justify flossing.

…and why it matters
I suggest that the best way to understand the recent discussion over the evidence for flossing is not as an invitation to rethink our oral hygiene practices, but as an invitation to think more broadly about what kind of evidence, and how much, we should require before we change our health care practices – whether to begin to use a new therapy or to stop using an old one. The “low stakes” in this case may help to illuminate the issues and give us a simple case from which we can begin to think about more complicated questions about values and evidence. And there are plenty of these complicated questions, many of which have also appeared in recent media stories. Consider:

My first response to each of these stories is that I’d want better evidence than I would in the flossing case, in each case because the stakes are higher – both the risks and the potential benefits are greater. But I suspect that there’s no straightforward way to set evidentiary standards based on possible benefits and harms in these cases. I’d welcome readers’ comments on how we might think about the evidence required to decide about these and other complicated cases!

robyn-bluhm-photoRobyn Bluhm, PhD, is an Associate Professor in the Department of Philosophy and Lyman Briggs College at Michigan State University.

Join the discussion! Your comments and responses to this commentary are welcomed. The author will respond to all comments made by Thursday, September 8, 2016. With your participation, we hope to create discussions rich with insights from diverse perspectives.

You must provide your name and email address to leave a comment. Your email address will not be made public.


  1. American Dental Association (August 4, 2016) Federal government, ADA emphasize importance of flossing and interdental cleaners. Retrieved from
  2. Donn, J. (August 2, 2016) Medical benefits of dental floss unproven. Retrieved from
  3. Guyatt, G., Rennie, D., Meade, M.O., Cook, D.J. (2014) Users Guides to the Medical Literature 3rd Edition. New York: McGraw Hill.
  4. Kaplan, K. (April 20, 2015) To find breast cancer, more mammograms aren’t better, expert panel says. Retrieved from
  5. Kirkey, S. (April 5, 2016) Too precious to push – shame, fear and the spike in C-sections: “It’s not safer, but it’s believed to be safer.” Retrieved from
  6. Kolata, G. (August 3, 2016) Why “useless” surgery is still popular. Retrieved from
  7. Mayor, S. (2016) Intranasal vaccine provides similar protection to injected flu vaccine, finds study. BMJ 354: 354:i4481. Retrieved from
  8. Sackett, D.L., Rosenberg, W.M.C., Muir Gray, J.A., Haynes, R.B., and Richardson, W.S. (1996) Evidence based medicine: What it is and what it isn’t. BMJ 312:71-72. Retrieved from
  9. Scutti, S. (June 23, 2016) CDC panel recommends against using FluMist vaccine. Retrieved from

7 thoughts on “To Floss or Not to Floss? That’s not the question

  1. How could any physician be against something called “evidence based medicine.” Here is how:

    The “best quality” studies are randomized clinical trials, and generally the only entities that can fund randomized clinical trials are pharmaceutical companies.

    Several years ago, I noticed that symptoms for “chemobrain” are similar to symptoms of attention deficit disorder; so I proposed a study to Shire Pharmaceuticals of using Adderall (racemic amphetamine) to treat chemobrain. Shire showed some interest, but not for using Adderall, which was going off patent in a couple years (since racemic amphetamine was discovered in 1887, it is not quite clear to me why it is still on patent at all, but anyway). What they were interested in funding was a study using Vyvanse (Lisdexamphetamine, which is a substance metabolized into amphetamine once it has been swallowed). Vyvanse has more “patent life” left on it. Currently, a 30 supply of Vyvanse costs patients about $250, while a 30 day dose of Adderall costs about $25.

    For a variety of reasons, I did not do the chemobrain study; but if I had, Vyvanse would have had high quality evidence of effectiveness (assuming that my study had positive results, which I am pretty sure that it would have had), and Adderall would not have that level of evidence. There is no scientific reason to believe that the two drugs would have differential effectiveness.

    In addition, it recently come to light that nearly 90% (!) of oncologists on the panels that recommend guidelines have ties to the pharmaceutical industry.

    I am considering filing a use patent for “a filamentous substance which can be used to remove detritus from inter dental surfaces.” If the patent is issued, I plan to fund a high quality study to prove that it improves some measure of dental health. There will then be high quality evidence, and I will charge $275 for a one month supply.

    1. I fully support your attempt to “monetize” dental floss – it could be the next EpiPen! What could possibly be wrong with that?

      More seriously, critics of EBM have started to recognize the extent to which the evidence available is shaped by those who have a financial interest in which treatments are deemed to be “evidence-based” (see, for example, What to do about it, however, is another set of questions entirely.

  2. As a practicing emergency medicine physician, I agree with your analysis. Just because there are no high quality randomized trials on flossing, does not mean we should abandon the practice given the low cost in terms of the floss itself (fraction of a penny per floss) and the potential harm (occasional bleeding for those taking a daily aspirin). In fact, I would argue that it may be unethical for the NIH or other governmental agency to fund a RCT on flossing, given the waste of tax payer dollars on such a trivial matter. At the other extreme, our tax dollars should support adequately powered, high-quality RCTs on issues that are important to our populace, such as your example of C-section vs. vaginal delivery. Although it is often challenging to conduct shared decision making when evidence is weak, what other choice do we have? Ultimately, the patient must decide on treatment X or Y, so physicians must do their best to provide a summary of the evidence in terms of potential harm and benefit for each individual patient. The only other option is to roll the dice or pick a card, any card…

    1. I agree that the C-section vs. vaginal delivery case is important, but I’m not sure the problem would be solved by an RCT. We know that personal and social values strongly influence women’s choices (and physician recommendations) regarding planned C-sections, and the RCT would not simply provide “neutral” evidence. What would we do if it turned out that, for whatever outcomes we thought indicated a “better” treatment, C-sections are better than vaginal deliveries? I’m inclined to say that having evidence is clearly better than not having it, but that it can make a complicated situation even more complicated. (And, yes, I know this is very much a philosopher’s answer – not much help for immediate decision-making!)

  3. Thanks for an interesting article and great counter to the “evidence based” craze. I have been a fan of evidence based medicine, largely because I think it can lead to lower costs and increased efficiency as well as increased patient safety. Unfortunately I’m not sure it has delivered on that promise. In addition, several of my family and friends have had problems that “traditional medicine” (i.e. western, supposedly evidence-based medicine) has been unable to solve. As they have experienced help from integrative medicine and the like, we have found some evidence for dietary and supplement type treatments that doesn’t pass the “evidence based” muster, as well as some that does but most physicians are just not aware of it (no drug company promoting decreasing histamines in your diet, for example). Of all the interventions we have tried, expense has really been the only likely harm. Many of them have helped greatly! Yet traditional physicians are skeptical, and would rather order expensive invasive tests or imply that the problem is all in the patient’s head. I was in that boat up until several years ago. Many years of practice have made me more skeptical of what we “know” in modern medicine and more open to “non-traditional” methods of treatment, even if they don’t meet the “evidence based” standard, as long as they are unlikely to be harmful.

    1. Thanks for your comment. I’m ambivalent about EBM, too. On the one hand, evidence is good! On the other, EBM borrows its methods from epidemiology, in that compares outcomes in different groups in large groups of patients. But those effects can be really small, and even large effect sizes don’t guarantee that outcomes can be predicted in specific patients.

      I don’t know much about alternative/integrative medicine, but I understand that a large part of the successes it has are due to practitioners working closely with patients, to try to understand what works for each individual. Given that patients using “traditional medicine” _do_ often use supplements, dietary changes, etc. along with the treatments they’re prescribed, it seems only sensible that traditional medicine should take this into account. But RCTs are not going to help us to do this.

      1. RCTs “might” help us do this, but I doubt it. a pill is a pill and its effect can be compared on large groups of people. A holistic approach to a patient’s health is harder to duplicate exactly with every patient, and each small aspect of that approach may have a small, statistically insignificant, effect. Yet combined they may have a marked effect.

Comments are closed.