Why I'll keep on flossing, thank-you.

You should floss your teeth everyday.

Knowing how much importance I place on evidence, and given the recent media hype spawned by a story by the Associated Press about the paucity of good quality evidence supporting that recommendation, you might be tempted to call me a hypocrite.

But you’d be wrong.

If you’ve been reading this blog, you’d also know that I fully recognize that we most often have to make decisions in the absence of complete or even good information (see here). And besides, as nice as it would be, we don’t need the highest quality evidence - multiple, consistent, well-conducted randomized controlled trials (RCTs) -  to provide a reasonable answer to every question.

So what is the state of the evidence regarding daily flossing? You could find that out for yourself – no media craze required – at the Cochrane Collaboration (which I’ve written about before here) who wrote:

"Twelve trials were included in this review which reported data on two outcomes (dental plaque and gum disease). Trials were of poor quality and conclusions must be viewed as unreliable. The review showed that people who brush and floss regularly have less gum bleeding compared to toothbrushing alone. There was weak, very unreliable evidence of a possible small reduction in plaque. There was no information on other measurements such as tooth decay because the trials were not long enough and detecting early stage decay between teeth is difficult."

And here's what they concluded:

"There is some evidence from twelve studies that flossing in addition to toothbrushing reduces gingivitis compared to toothbrushing alone. There is weak, very unreliable evidence from 10 studies that flossing plus toothbrushing may be associated with a small reduction in plaque at 1 and 3 months. No studies reported the effectiveness of flossing plus toothbrushing for preventing dental caries."

It should be no surprise that there are no randomized trials reporting a reduction in cavities (dental carries) among flossers compared to non-flossers. Such a study would require that large numbers of subjects be randomized and followed for years (since it takes years for carries to develop). Large, long-term studies are costly and labor intensive, and that's why they haven't been done. Who's going to pay millions of dollars to do those studies? Are you willing to cough up some money for the cause?

Perhaps the industry that profits from the manufacture of dental floss should cover the costs, but why would they when the available evidence is enough to reasonably conclude that regular flossing prevents gum disease, which is really enough to recommend it? Even if that's all that it does, you should be flossing. Never mind that there isn't evidence showing that it prevents cavities.

This is an especially important question among people like me with valvular heart disease (VHD). Gum bleeding that permits oral microbes access to the bloodstream is an important cause of infectious endocarditis: a life threatening infection on heart valve tissue. Among the most common causative microbes are oral bacteria that get into the bloodstream by way of gum injury and micro bleeding that occurs throughout regular daily life (1,2).

When I was diagnosed with VHD about 4 years ago, I started flossing every day. Prior to that, I was like Margaret Wente (whose inflammatory piece in the Globe and Mail sparked my desire to write this post), which is to say that I only flossed a day or two before seeing my dentist, and when I did, my gums always bled. But since flossing daily, my gums hardly ever bleed, and haven’t bled at all at my last 2 dental visits. I’m living proof that flossing prevents gingivitis and there is plenty of other anecdotal evidence like mine to add to the already existent and reasonably good higher-quality evidence that it does.

Another widely reported anecdote that I can speak to: despite brushing twice a day, that string used to have quite a smell when I was finished, but after a few weeks in the habit, the foul scent was gone. Preventing bad breath is another terrific reason to floss your teeth daily, don't you think?

Now, one caveat: all of my comments refer to regular (ie. daily) flossing. Intermittently flossing and stopping will likely lead to more gum bleeding because the gingivitis that causes the underlying bleeding just returns in between flossing stints. That could actually be harmful, since everybody is at risk of endocarditis (though not as at high a risk as people with VHD). But other than that, the potential risk of daily flossing is almost certainly astronomically low.

Just as we don’t need a RCT of parachutes for jumping out of airplanes, we also don’t need anymore RCTs of flossing. As tempting as it may be - because I know how unpopular the idea of regular flossing is to most people - you should not mistake an absence of evidence (or a paucity of evidence, in this case) for evidence of absence. I'm sufficiently convinced that daily flossing is having a positive impact on my oral hygiene, and perhaps even to my overall health, and you should be too.

The stakes may not be as high for you as for me, and you may reasonably decide that a reduction in gingivitis is not worth daily flossing for you, which would be fine because you'd be making a decision based upon a reasonable and well-informed interpretation of the evidence. But please, do not make the mistake that Margaret Wente and others in the media have made, which is to conclude, because it's the conclusion that they like and/or that they think their readers will like, that we should all stop flossing because there is evidence that it doesn't do any good.

That would not be fine at all.


1. Lockhart PB, Brennan MT, Sasser HC, Fox PC, Paster BJ, Bahrani-Mougeot FK. Bacteremia associated with toothbrushing and dental extraction. Circulation 2008;117:3118–3125

2. Veloso TR, Amiguet M, Rousson V, Giddey M, Vouillamoz J, Moreillon P, Entenza JM. Induction of experimental endocarditis by continuous low-grade bacteremia mimicking spontaneous bacteremia in humans. Infect Immun2011;79:2006–2011.

Common sense is too common

The snow has finally stopped falling and you're pleased to be inside, driveway all shovelled neatly (first one on the block!), the aroma of a fresh pot of coffee permeating your home. Too bad for you that the squeezing sensation in the centre of your chest that kind of niggled while you were outside has just returned, but this time with a vengeance. Your wife calls 911 because the worried look on your face is actually worse than hers. What's the first thing paramedics will do when they arrive and learn that you're having chest pain? What's the first thing you probably want them to do?

Before even giving you an aspirin to chew, they'll probably strap on nasal prongs and give you some oxygen. After all, if you're having a heart attack, the problem is that the supply of blood, and therefore of good ol' oxygen, to your heart muscle is being choked off by a blood clot in a heart artery.

What could go wrong by giving a supplement of a natural substance that all our cells need to survive and of which those cells are being deprived?

Lots, apparently. Just click here to see the results of a clinical trial that randomly assigned 638 patients with chest pain to receive supplemental oxygen or not. Four hundred and forty one of them turned out to be having heart attacks and during followup, compared to those who just breathed ambient air, those who got the oxygen had larger heart attacks, more recurrent heart attacks, and more cardiac arrhythmias.

Traditional Chinese Medicine Store

Now just think, if oxygen turns out to be bad for you in the midst of a heart attack - a natural and "common sense" treatment given a basic understanding of the situation - what other supplements that are natural and appeal to common sense might be bad for you, too?

Supplement Store

The available data make a strong enough case to stop routinely providing supplemental oxygen to patients having chest pain and heart attacks. A time honoured treatment should no longer be employed. There is a long list of medical therapies established by common sense that are no longer offered because they've been shown in clinical trials to be ineffective or harmful (and it's an amazing list that you really should click on and see so here's another chance if you missed it before).

Tell me this: when was the last time that a supplement, herbal, or homeopathic treatment was tested by those practitioners in a clinical trial and pulled from the shelves?

Homeopathy Store

Another timely example of the failure of common sense is screening for cancer. If you have a test that can detect cancer, applying it to populations should reduce premature deaths. Simple, right? But the situation for screening is much more complicated than it seems and, in fact, screening the general population for cancer, including breast, lung, colon and prostate cancer, is probably a huge waste of time, effort, worry, and money (with the possible exception of pap smears for cervical cancer). Not only that, it probably causes more harm than good, and it's a practice that should stop. The same is true for the annual physical examination, which is going the way of the Dodo. Why? Because skeptical people put these common sense notions to the real test of randomized trials and the results weren't what common sense had predicted.

Common sense tells us that because something seems like it should work, it does work, but it's time to get humble, to became skeptical of common sense. It's time to admit that many things are far more complicated than they seem. I know that may make some people feel small and insignificant, but that's really not necessary. Evidence makes us powerful. Experts aren't necessarily smarter than you. Real experts just know the evidence better than others. By learning how to find and assess the evidence, you can become powerful, too. An easy way to get to the best understanding of the available health-related evidence is to use Cochrane, a global non-profit, non-governmental organization that you can read all about at Wikipedia here. Search your health query and Cochrane is your friend.

Voltaire said that common sense is not common. On the other hand, it seems far too common to me. In everything from philanthropy to economics, from politics to education, and beyond, what we need is less common sense, and more high quality evidence.

What doctors and anti-vaxxers have in common: Part 2

When I openly pose the post-vaccine fever scenario, most people consider it more likely than not that the vaccine caused the fever. That is, most people think the chance that the vaccine caused the fever is > 50%. Almost every medical student in a group of 20 that I was teaching a few weeks ago thought that. After all, fever is a well-recognized side effect of vaccine injections and your child was perfectly fine until a few days after the injection. Who wouldn’t reasonably conclude that the vaccine likely caused the fever?

Notice that by focusing responses on 1-49%, I set the question up to give you a better chance at being correct than they were but still, nobody got the question right. Don't worry. You are in good company. For now, just remember the feeling you had when you made your choice.

Back in the 80’s, investigators were concerned that combining mumps, measles, and rubella vaccines into one (MMR) might increase the risk of side effects, so some doctors in Finland did an ingenious study to determine what was not just temporally associated with vaccine injection, but caused by vaccine injection.

They studied 581 pairs of twins and randomly gave the MMR injection to one of the twins and a placebo injection to the other. Then, so as to not deprive the children that got a placebo injection of the vaccine's proven benefits, three weeks later, they gave each twin the opposite of what they had earlier received. Parents, who were blinded (they didn’t know when the kids were getting the placebo or the MMR injection), were instructed to vigilantly check for fever and a variety of other potential side effects after each injection.

In the first 6 days after placebo injection, 17% of children had a fever. That’s the background frequency of fever in vaccine-age children, and it's pretty high, huh? On the other hand, in the first 6 days after MMR injection, 17.2% of children got a fever. The difference - 0.2% - is what can be causally attributed to the active ingredients in the vaccine.

Conclusion: >99% of fevers that occur in the first week after MMR injection have nothing to do with the vaccine at all. 

However, we are very likely to erroneously attribute the fever to the vaccine. We do this because we have a powerful intuition that leads us to identify a causal relationship when 2 events that could possibly be causally related follow each other in time. We are mistaking what is merely possible for what is probable. Unfortunately, children develop mysterious illnesses like MS, autism, epilepsy, hepatitis, arthritis, etc. with regular frequency. Sometimes, those illnesses will appear fairly soon after getting a vaccination, and that’s when that intuition of ours can do some serious damage. That same intuition does damage when mysterious conditions like these disappear -as they often spontaneously do- after interventions that have no effect on the disease like prayer, acupuncture, chiropractic neck manipulations, homeopathic remedies, etc. Only randomized controlled trials can sort this stuff out.

Imagine once again your son who got a fever early after the MMR injection, only now, he is a healthy 18-year-old man who was spared the ravages of mumps, measles, and rubella. At any point in his upbringing, would you have ever had cause to stop and be thankful for the vaccine? How could you? You’d have had no way of knowing whether your child would have contracted one of these illness had he not been vaccinated!

See what’s happening here? The way that we experience life offers us no way to identify the situation where a vaccine prevents a horrible illness or death. On the other hand, we have a tendency to erroneously attribute adverse events to vaccines when they follow each other in time. Remember how you felt about your child's fever after MMR vaccine? Parents considering whether to vaccinate their children can anticipate the regret that they will experience if their child does develop an illness like autism afterwards, but they cannot anticipate the relief that they cannot experience when their child is spared a vaccine-preventable illness. Anti-vaxxers are much more likely to anticipate the former type of regret than the latter because they tend to overestimate the risks of vaccines and underestimate their benefits. Our intuitions erroneously set vaccines up to be unattractive. 

And so it is with physicians and oral anticoagulants (OAC’s). Bleeding events are fairly common. Whenever somebody experiences a bleeding event on an OAC, they and the doctors tasked with treating the bleeding blame the OAC. The reality is that most bleeding events that happen while on an OAC would have happened otherwise: just as only 0.2% of fevers early after the MMR injection were caused by it, only 0.2-0.3% of bleeds /yr are caused by the OAC. But physicians can anticipate the regret that they will experience when they prescribe an OAC to a person with atrial fibrillation (AF) who then develops a bleeding event. The finger of blame will point to the OAC and the physician who prescribed it. Unfortunately, nobody ever returns to the prescribing physician to pat them on the back and thank them for the stroke that they and the OAC prevented because, just like the case of mumps that your son avoided, there is no way to recognize a stroke that would otherwise have happened. Doctors' intuitions erroneously set OACs up to be unattractive.

Just as there is evidence that anti-vaxxers overestimate the risk of vaccines, so is there evidence that physicians overestimate the risks of OACs, exaggerating  their anticipated regret over prescribing one.

Our ancestors evolved on an African Savannah with no pressures to select for intuitions that address the types of complicated primary prevention questions posed by vaccination and OAC use in AF. It seems that we did evolve intuitions that lead us to readily identify patterns and infer causal relations where there often isn't one. On the whole, this rudimentary heuristic has done well to protect us from certain kinds of dangers, but, as Sam Harris has written, “we have flown the perch built for us by evolution”. If we are to make advances with complex questions, we simply have to recognize when our intuitions lead us astray, and they regularly do, whether we are specialized physicians or lay people. None of us are spared the consequences of irrationality. It is a struggle we all must recognize and participate in.

If you are faced with complex decisions like vaccinating or taking an OAC, I hope that you are most heavily weighing the evidence from RCTs. If you aren't, then I hope that you are seeking the involvement of people who are. Otherwise, you're just being irrational.