Water Flosser vs String Floss

On the published evidence, string floss has the stronger case — but the margin is small, the certainty behind all of it is low, and the device you will genuinely use every night beats the one with the better paperwork. That is the whole answer, and everything below is why it is not more decisive than that.

What the review actually says about each

The 2019 Cochrane review of home interdental cleaning devices covered 35 trials and 3,929 adults, and it gives each device a sentence. For floss: using it in addition to toothbrushing "may reduce gingivitis in the short and medium term". For oral irrigators, which is what a water flosser is in the literature: toothbrushing plus oral irrigation "may reduce gingivitis in the short term, but there was no evidence for this in the medium term".

That is the whole of the difference. Floss's sentence reaches further — into the medium term — and the water flosser's stops short. Fifteen of the trials looked at floss; five looked at oral irrigators.

Then the certainty grade, which applies to both and is the sentence that should end most arguments about this: "The evidence is low to very low-certainty. The effects observed may not be clinically important." The reviewers add that outcomes were mostly measured over short periods, in participants who mostly started with a low level of gum inflammation — people who did not have much room to improve.

Incidentally, the review's best-supported device is neither of the two in this entry. Interdental brushes "may reduce gingivitis and plaque in the short term", and the American Dental Association's own summary of the same literature is that "interdental brushes may be more effective than floss". If you came here to choose between two things, there is a third worth pricing.

The trial the machines are sold on

Against that background sits a specific, named, four-week trial that the marketing leans on heavily, and it deserves to be read properly rather than dismissed.

Mancinelli-Lyle and colleagues, published in 2023, randomised 105 people into three groups: a water flosser plus a manual brush, a water flosser with air microbubbles plus a manual brush, and dental floss plus a manual brush. All three groups showed statistically significant reductions in bleeding, inflammation and plaque at four weeks. On whole-mouth bleeding the water flosser group came out ahead of the floss group. The authors' conclusion is modest and accurate: "A manual toothbrush and water flosser, with or without microbubbles, is an effective oral care regimen for controlling gingivitis over four weeks."

Now the declaration, in the authors' own words: "The sponsor supplied the Waterpik® water flossers", and one author was "an employee of Water Pik, Inc." during the study.

That is not a scandal. Declared industry support is the ordinary condition of device research and the declaration is the honest part of it. But it means what you are looking at is a four-week, single-centre, manufacturer-supported comparison — and that when a manufacturer's page converts it into "85% more effective than string floss at reducing gingivitis", a great deal of context has been left on the cutting-room floor. Four weeks is not a year. "Up to" is usually carrying the friendliest subgroup.

The argument the evidence cannot settle

Here is the thing neither the review nor the trial measures, and it is probably the decisive factor for most households: whether you do it.

The ADA's practical conclusion is not a ranking at all. Its position is that people should clean between their teeth daily, and that "the best method for any given patient will be the one that they will regularly perform". It also notes, about floss specifically, that "It's generally not what type of floss is used, but how and when it's used".

Read that as permission. If floss has lived unopened in your bathroom drawer for two years and a water flosser is the thing you would genuinely use before bed, the machine is doing more for your gums than the better-evidenced method you are not performing. If you already floss every night without thinking about it, the evidence gives you no reason to change, and the HSE's guidance — "Floss your teeth daily if you can" — is describing what you already do.

Cost, which nobody frames honestly

Floss costs a few pounds a year and fits in a pocket. A water flosser in our comparison of what the makers publish lists between $69.99 and $99.99, needs counter space or charging, and has a running cost the makers do publish: Waterpik's own instruction is to replace tips every three to six months and to flush the machine monthly.

That gap is not an argument against the machine. It is an argument for being clear about what the money buys. It buys a method you may find easier to sustain and an easier time around braces, bridges and implants. It does not, on any evidence we could find, buy a better outcome than diligent flossing.

Where neither one helps

Both of these are for soft deposits. Neither removes hardened calculus below the gumline, and neither treats established gum disease: that is instruments in a surgery, which the HSE describes as a scale and polish or, deeper, root planing. If your gums bleed routinely, have receded, or your teeth feel loose, the choice between these two devices is not the decision in front of you — what gum disease is and who treats it is the more useful page, and then an appointment.

If you want the short version

Floss, or better still interdental brushes, if you will use them. A water flosser if that is what gets used instead of nothing. Both are a way of performing the same daily step, and the step is the part every source agrees on. If you want to see how thin the research underneath all of this really is, the entry on what the trials found takes it apart study by study.