Floss removes bacteria and food particles from the tight spaces between your teeth that a toothbrush can’t reach. These gaps make up roughly 40% of your tooth surfaces, and without some form of interdental cleaning, plaque builds up there undisturbed. That buildup is the starting point for cavities, gum disease, and the chronic inflammation that can affect your health well beyond your mouth.
How Floss Works on a Physical Level
Your teeth are coated in a sticky film called plaque, which is essentially a colony of bacteria bound together in a structure called a biofilm. This biofilm clings tightly to enamel and gum tissue, and it can’t be rinsed away with water or mouthwash alone. Floss works by applying two types of force simultaneously: compression (pressing against the tooth surface) and shear (scraping along it). Together, these forces physically break the biofilm apart and peel it off the tooth.
The reason this matters so much between teeth is geometry. Toothbrush bristles are too thick to reach into the narrow contact points where two teeth touch. Floss slides into that space, wraps against the curved surface of each tooth, and disrupts plaque right at the gumline and slightly below it, where the most damaging bacteria tend to settle.
What Happens to Bacteria You Don’t Remove
When plaque sits undisturbed between teeth for days or weeks, its composition changes. Early plaque is mostly harmless bacteria, but over time the community shifts toward anaerobic species, meaning bacteria that thrive in low-oxygen environments. The ones that cause the most damage include species that produce toxins and trigger intense immune responses in your gum tissue.
This bacterial shift is what drives periodontal disease. First comes gingivitis: your gums become inflamed, red, and prone to bleeding. Left untreated, the infection can progress to periodontitis, where the bone supporting your teeth begins to break down. The bacteria responsible for this progression, particularly certain spirochetes and gram-negative anaerobes, are the same ones that accumulate in neglected interproximal spaces.
Reducing Gum Inflammation and Bleeding
The most well-supported benefit of flossing is its effect on gum health. A randomized clinical trial published in BMC Oral Health found that adding floss to a brushing routine reduced interproximal bleeding on probing by about 62% compared to brushing alone. Overall gum bleeding dropped by roughly 56%. These improvements appeared within four weeks and held steady at twelve weeks.
A Cochrane systematic review, which pools data from multiple trials, confirmed that flossing in addition to brushing may reduce gingivitis in the short and medium term. The evidence for plaque reduction was less consistent, which highlights an important nuance: flossing’s clearest benefit is in calming gum inflammation rather than eliminating every trace of plaque. Even partial disruption of the biofilm is enough to prevent the bacterial shift toward the more harmful anaerobic species.
Does Flossing Prevent Cavities?
This is where the evidence gets more complicated. A systematic review of six trials found that professional flossing performed on school days over nearly two years reduced cavity risk between teeth by 40% in children. That’s a substantial effect, but there’s a catch: when children and adolescents flossed on their own, no cavity reduction was detected.
The likely explanation is technique. Professional flossing is thorough and consistent, while self-flossing, especially in younger people, tends to be rushed or incomplete. This doesn’t mean flossing can’t prevent cavities in adults who do it properly. It means the cavity prevention benefit depends heavily on how well and how consistently you do it. The gum health benefits, by contrast, show up even with average technique.
The Connection to Heart Disease and Overall Health
Periodontal disease isn’t just a mouth problem. A meta-analysis found that people with periodontal disease face a higher risk of cardiovascular disease, regardless of sex. The mechanism appears to center on systemic inflammation. When your gums are chronically infected, inflammatory markers rise in your bloodstream. Oral bacteria can also cross through damaged gum tissue into your blood vessels, where they may worsen the buildup of arterial plaques.
This doesn’t mean skipping floss causes heart attacks. It means that chronic gum disease is one contributor to the kind of low-grade, body-wide inflammation that raises cardiovascular risk over time. Keeping your gums healthy through interdental cleaning is one way to reduce that contributor.
String Floss vs. Water Flossers vs. Interdental Brushes
String floss is the most familiar option, but it’s not the only one. A systematic review comparing water flossers to string floss found that the majority of studies favored water flossers for plaque removal. In one trial, water flossing reduced whole-mouth plaque by about 74% compared to 58% with string floss, with an even larger gap in the spaces between teeth (82% vs. 63%). Several other trials showed similar advantages, though a couple found no significant difference.
Interdental brushes, the tiny bottle-brush-shaped picks, have also shown strong results. Studies in orthodontic patients found interdental brushes were significantly more effective than floss at both removing plaque and reducing gum inflammation. They work especially well when the gaps between your teeth are large enough to fit the brush without forcing it.
The best tool is the one you’ll actually use every day. If you find string floss awkward or painful, a water flosser or interdental brush is a perfectly valid alternative. The American Dental Association recommends cleaning between your teeth once a day with floss or another interdental cleaner, without specifying which type.
Getting the Most Out of Flossing
If you use string floss, the key details are: curve the floss into a C-shape around each tooth rather than snapping it straight up and down, and gently slide it just beneath the gumline on both sides of each gap. That sub-gumline contact is where floss does its most important work, reaching bacteria that are already starting to irritate your gum tissue. Use a clean section of floss for each tooth to avoid redistributing bacteria.
Timing doesn’t matter much. Flossing before or after brushing, morning or night, produces similar results. What matters is doing it consistently. The bacterial biofilm between your teeth begins reorganizing within 24 hours of being disrupted, which is why the once-a-day recommendation exists. If you floss sporadically, perhaps two or three times a week, you’re giving those anaerobic bacteria regular windows to re-establish themselves.
Bleeding when you first start flossing is normal and typically resolves within one to two weeks of daily use. If it persists beyond that, it’s a sign of established gum inflammation that may need professional attention.