Great question. Alas, surprisingly few studies address it directly. Based on existing evidence, flossing first isn’t necessarily better for oral health than the other way around.
Still, dentists have opinions on the matter. Dr. Edmond R. Hewlett, a spokesman for the American Dental Association and a professor of restorative dentistry at the University of California, Los Angeles, recommends flossing first. His rationale? Get the unpleasant task out of the way to avoid the temptation to not do it. “Let’s face human nature, if you’re going to skip one, which one will you skip?” he said.
By contrast, Dr. Philippe Hujoel, a professor of oral health sciences at the University of Washington in Seattle, advises his patients to brush with a fluoride toothpaste, then floss. That way your mouth will be awash with fluoride as you are maneuvering the floss, he said.
However, it turns out flossing is not a proven way to prevent cavities, even though some dentists and hygienists suggest it is.
Rather, flossing’s main benefit is stanching bloody gums and reducing the gum inflammation known as gingivitis.
“Gingivitis is the first step in losing your teeth,” Dr. Hewlett said. “The nice thing about catching inflammation when gums are bleeding is you can reverse it then, if that’s all that’s going on.” (Teeth brushing and flossing aren’t adequate to treat more advanced inflammation.)
A 2012 review of 12 randomized controlled trials found that people who brushed and flossed regularly had less gum bleeding than the brush-only camp, though the authors cautioned the quality of the evidence was “very low.”
That same report, in The Cochrane Database of Systematic Reviews, found only “very unreliable” evidence that flossing might reduce plaque at one and three months. And no studies reported on the effectiveness of flossing combined with teeth brushing for cavity prevention.
“Self-flossing clinical trials have failed to show a benefit in the reduction of dental decay,” said Dr. Hujoel, a periodontist.
There are practical reasons to floss, of course. It can dislodge raspberry seeds and other food debris you may or may not be able to feel, for example, and some overachievers prefer to attend meetings without spinach in their teeth.
As for technique, the American Dental Association recommends guiding the floss along the curve of the gum line at the base of each tooth, in addition to flossing up and down between teeth.
It could be that amateurs just don’t know how to floss correctly, because there is some evidence that professional flossing can reduce cavities in children who have had minimal exposure to fluoride.
One systematic review of six trials found that when professionals flossed the teeth of those children on school days for 1.7 years, there was a 40 percent reduction in the risk of cavities.
That may be good news for the children and spouses of dental hygienists. But for the rest of us who don’t have a professional at home to floss for us, we can choose whether we floss before or after brushing our teeth.