Yes, you need to clean between your teeth. Whether you use string floss, a tiny interdental brush, or a water flosser is less important than doing something to disrupt the bacteria that build up in the spaces your toothbrush can’t reach. The American Dental Association recommends cleaning between your teeth once a day, and the U.S. Department of Health and Human Services has reaffirmed flossing as “an important oral hygiene practice.”
That said, the evidence behind flossing is more nuanced than the simple advice suggests. Here’s what the research actually shows, what flossing does and doesn’t protect against, and which tools work best.
Why the Spaces Between Teeth Matter
Bacteria in your mouth attach to a thin layer of saliva molecules on your tooth surfaces, then multiply and build a sticky scaffold called biofilm. Other bacteria and fungi pile on, increasing the complexity of that colony over time. When this biofilm sits undisturbed, it hardens into tarite (calcite buildup your dentist scrapes off), eats into enamel, and triggers gum inflammation.
Your toothbrush handles the front, back, and chewing surfaces of each tooth reasonably well. But bristles don’t reach the tight contact points between teeth or dip below the gumline where two teeth sit side by side. That’s roughly 30 to 40 percent of your total tooth surface area left untouched. Without mechanical disruption in those gaps, biofilm matures and the conditions for cavities and gum disease set in.
The Flossing Controversy, Explained
In 2016, the U.S. Dietary Guidelines quietly dropped their longstanding recommendation to floss daily. The Associated Press had filed a Freedom of Information Act request asking for the supporting evidence, and a review of 25 studies found the research behind flossing was weak. Headlines declared flossing pointless.
The reality is less dramatic. The Department of Health and Human Services clarified that neither the 2010 nor 2015 advisory committees had actually reviewed the evidence on brushing and flossing, so the authors simply didn’t carry the recommendation forward. They weren’t saying flossing doesn’t work. The problem is that the type of rigorous, long-term clinical trials required for a federal dietary guideline are extremely difficult to conduct for something like flossing. You’d need to randomly assign thousands of people to floss or not floss for years and track their dental outcomes, and that study essentially doesn’t exist.
What Flossing Does for Your Gums
The strongest evidence for flossing is in gum health, not cavity prevention. A Cochrane review, the gold standard for evaluating medical evidence, found that flossing in addition to brushing reduces gingivitis (early gum inflammation) in the short and medium term. The effect was modest but consistent across multiple trials.
A large Korean population study found that people with 20 or more teeth who did not floss had roughly 41 percent higher odds of periodontitis, the advanced form of gum disease that destroys bone and leads to tooth loss. That association held even after adjusting for factors like smoking, income, and education. For people who had already lost many teeth (fewer than 20 remaining), flossing didn’t show a significant benefit, likely because the damage was already done or the remaining teeth had wider gaps better served by other tools.
Periodontitis isn’t just a dental concern. It’s a major reason adults lose teeth as they age, which affects chewing, speech, and nutrition. Preventing it is one of the clearest practical reasons to clean between your teeth.
What About Cavities?
This is where the evidence gets thin. You’d expect that removing plaque between teeth would prevent cavities in those spots, and logically it should. But systematic reviews have found only one study showing a clear reduction in cavities from flossing, and that study involved professional flossing (done by a dental worker, not the patient) on children’s baby teeth over 20 months.
Studies where people self-reported their own flossing habits showed no consistent benefit for cavity prevention, and a couple even found that frequent flossers had more cavities. That likely reflects a common pattern in health research: people who already have dental problems may start flossing more in response, making it look like flossing causes cavities when it doesn’t. The honest answer is that we don’t have strong proof flossing prevents cavities between teeth, even though the biological logic is sound.
Interdental Brushes May Work Better
String floss isn’t your only option, and it may not even be the best one. A network meta-analysis comparing 10 different interdental cleaning tools found that interdental brushes, those tiny bottle-brush-shaped picks, ranked first for reducing both plaque and gum inflammation. Floss ranked fourth or fifth depending on the outcome measured. In people with gum disease, floss removed about 50 percent less plaque than interdental brushes.
Water flossers also performed well, ranking second for reducing gum inflammation. Multiple systematic reviews have found water flossers more effective than string floss at reducing gingival bleeding. They’re particularly useful if you have braces, dental implants, or bridges that make threading floss difficult.
The catch with interdental brushes is that they need enough space between teeth to fit. If your teeth are tightly packed with no gaps, a brush won’t slide in without forcing it. For tight contacts, string floss or a water flosser is the practical choice. Many dentists now recommend using interdental brushes where they fit and floss where they don’t.
Proper Technique Matters More Than You Think
Flossing incorrectly can cause real damage. A published case report documented a 33-year-old patient with otherwise excellent oral hygiene who had gum clefting and significant bone loss at several sites, all traced to years of aggressive, improper flossing. The damage had gone undiagnosed for a long time because clinicians assumed good oral hygiene meant no self-inflicted harm.
The correct approach is to curve the floss into a C-shape around each tooth, hugging the side of the tooth rather than snapping it straight down into the gum. Slide it gently below the gumline, then move it up and down against the tooth surface. Do this on both sides of every gap. Sawing back and forth or forcing the floss down with a snap can cut into soft tissue and, over time, contribute to gum recession and bone loss.
If you find string floss difficult to use correctly, that’s another reason to consider alternatives. A water flosser or interdental brush is easier to use well, which means you’re more likely to actually do it consistently and less likely to injure your gums in the process.
How Often and When
Once a day is the standard recommendation. The timing doesn’t matter much. Before brushing, after brushing, before bed, in the morning: pick whatever makes you most likely to do it consistently. Some dentists suggest flossing before brushing so that fluoride from toothpaste can reach the newly cleaned surfaces between teeth, but there’s no strong evidence that the order makes a meaningful difference.
Research on biofilm formation shows that frequent mechanical disruption is what prevents bacteria from maturing into the organized colonies that cause harm. The key insight is that you’re not just removing food debris. You’re resetting the bacterial clock between your teeth, keeping colonies young and disorganized before they can cause inflammation or acid damage.