Cochrane's 2019 systematic review of home interdental cleaning found that cleaning between the teeth, on top of brushing, reduces gingivitis: gum inflammation, the earliest stage of gum disease. Cochrane graded the certainty of that finding low to moderate.1 The review does not show that interdental cleaning prevents periodontitis, the advanced, tissue-destroying stage.
Who is Cochrane, and why does their verdict matter?
Cochrane is the reference researchers reach for when they want to know what clinical trials collectively show about a treatment, which is why its verdict carries weight. It's an independent, international network that gathers every qualifying randomized trial on a question, grades the quality of each one, and publishes a conclusion, along with an unusually candid statement of how confident you should be in it. Cochrane reviewers are famous for withholding endorsement; "insufficient evidence" is one of their most-used phrases.
So it's worth paying attention to what their review of home interdental cleaning (Worthington and colleagues, 2019, covering trials of floss and other between-the-teeth tools used alongside a toothbrush) actually concluded.1
What did Cochrane find?
Cleaning between the teeth, on top of brushing, reduces gingivitis: gum inflammation, the earliest stage of gum disease.
That's the core result: in randomized trials, people who added interdental cleaning to brushing had less gum inflammation than people who brushed alone. Gingivitis is the stage where gum disease is still fully reversible: inflamed, often bleeding gums, before any permanent damage to the structures holding your teeth. Reducing it is exactly what a daily prevention habit is supposed to do, and the trials show interdental cleaning does it. The direction of the evidence is consistent.
The certainty, in Cochrane's own grading, is low to moderate. And rather than bury that phrase, this article exists to explain it, because understanding it is the difference between reading evidence and reading marketing.
What does "low-to-moderate certainty" actually mean?
Certainty grades describe how confident reviewers are that the measured effect is close to the true effect. "Moderate" means the finding would probably survive further research roughly intact; "low" means new, better trials could shift it. It does not mean the finding is likely wrong: it means the trials that exist have real limitations. For home oral hygiene, those limitations are structural, and it's worth seeing why:
The trials are short. Most run weeks to a few months. That's long enough to measure changes in gum inflammation, but nowhere near long enough to watch a decades-long disease process play out under randomized conditions. Nobody has funded a 20-year trial randomly assigning thousands of people to floss or not floss, and realistically, nobody ever will.
The trials can't be blinded. In a drug trial, a placebo pill hides who's getting the treatment. There is no placebo for flossing: every participant knows exactly which group they're in, and so does anyone watching them. Unblinded trials are automatically graded down, no matter how carefully they're run.
In other words: modest certainty is what honest evidence in this category looks like. A product ad will tell you the science is settled. Cochrane tells you what the trials show, how sure we can be, and why. When you see a home-hygiene claim delivered with absolute confidence, the confidence itself is the red flag.
What does the review not show?
It does not show that interdental cleaning prevents periodontitis, the advanced, tissue-destroying stage of gum disease, only that it reduces gum inflammation, gingivitis. That's the line this library holds, because the evidence draws it. Proving prevention would require the long randomized trials that don't exist. So we will never tell you flossing "prevents," "stops," or "cures" periodontitis, and you should be skeptical of anyone who does.
What fills in the long-term picture is a different kind of evidence: large observational studies. In national CDC survey data, adults who clean between their teeth regularly have measurably less gum disease, with the most frequent cleaners showing the least severe disease: an association, not proof, but a consistent one. We cover that data in regular interdental cleaning and lower odds of gum disease. Trials supply the short-term cause and effect; surveys supply the long-term correlation. Neither alone is the whole story. Together they point the same way.
One more note for completeness: the same Cochrane review also looked at specific tool types, and the evidence there is thinner. Adding a water flosser to brushing, for instance, has been shown in some controlled studies to reduce early gum inflammation, though researchers note the evidence is still limited.1 The between-the-teeth habit itself is where the evidence is strongest: the tool matters less than the doing.
That's the honest summary of the strongest clinical evidence on earth for this habit: it works on the earliest, reversible stage of gum disease; the certainty is modest for structural reasons that will never fully go away; and the long-term case rests on association, clearly labeled. If you want to see how we grade every source on this site the same way, that standard lives at how we source.
Common questions
Does Cochrane say flossing works?
Cochrane's 2019 review found that cleaning between the teeth, in addition to brushing, reduces gingivitis, the earliest and still-reversible stage of gum disease.1 The direction of the evidence is consistent. The certainty grade is low to moderate, so "works" should be read as reduces gum inflammation rather than prevents gum disease.
Why is the certainty only low to moderate?
For structural reasons that won't go away. The trials are short (weeks to a few months) because nobody has funded a decades-long randomized trial of flossing, and realistically nobody will. And the trials can't be blinded: there is no placebo for flossing, so every participant knows which group they're in, and unblinded trials are automatically graded down.1
Does low certainty mean the finding is probably wrong?
No. Certainty grades describe how confident reviewers are that the measured effect is close to the true effect. "Moderate" means the finding would probably survive further research roughly intact; "low" means new, better trials could shift it. It reflects the limitations of the trials that exist, not evidence against the result.
What does the review say about water flossers specifically?
Less than people assume. The same review looked at specific tool types and the evidence there is thinner: adding a water flosser to brushing has been shown in some controlled studies to reduce early gum inflammation, though researchers note the evidence is still limited.1 The between-the-teeth habit is where the evidence is strongest. We compare the tools in string floss vs. water flosser.
If the trials are short, what covers the long term?
Large observational studies. In national CDC survey data, adults who clean between their teeth regularly have measurably less gum disease, with the most frequent cleaners showing the least severe disease. That is an association, not proof.2 Trials supply the short-term cause and effect; surveys supply the long-term correlation. We cover that data in regular interdental cleaning and lower odds of gum disease.
- Worthington HV, et al. Home use of interdental cleaning devices, in addition to toothbrushing, for preventing and controlling periodontal diseases and dental caries. Cochrane Database of Systematic Reviews, 2019, CD012018. Randomized-trial evidence; low-to-moderate certainty for reduced gingivitis; water-flosser evidence limited.
- Marchesan JT, et al. Interdental cleaning is associated with decreased oral disease prevalence. Journal of Dental Research, 2018. PMC6728587. Observational national survey data; association with dose-response.