Flossing removes bacteria and food debris from the 40% of your tooth surfaces that a toothbrush can’t reach. These are the tight spaces between teeth and just below the gumline, where cavities and gum disease most often start. Beyond simple cleaning, flossing disrupts colonies of bacteria that organize into a sticky film called plaque, preventing them from maturing into the kind of buildup that causes real damage.
Why Brushing Alone Isn’t Enough
Your toothbrush does a solid job on the front, back, and chewing surfaces of your teeth. But bristles physically cannot slip into the narrow gaps between teeth. That leaves roughly 40% of total tooth surface untouched, no matter how thorough your brushing technique. The spaces between teeth are warm, moist, and sheltered from saliva flow, making them ideal environments for bacteria to thrive.
Plaque in these gaps isn’t just a thin coating. It’s a structured community of bacteria that anchors itself to enamel and gum tissue. Left undisturbed, it hardens into tarite (calculus) within about 48 hours, at which point only a dental professional can remove it. Flossing works by physically shearing this bacterial film off the tooth surface before it has a chance to calcify. The thread slides along each side of every tooth, scraping plaque loose from areas where decay and inflammation are most likely to develop.
What It Does for Your Gums
The most immediate benefit of flossing is reducing gum inflammation. When plaque sits along the gumline between teeth, your immune system responds to the bacteria by sending more blood flow to the area. That’s what causes the redness, puffiness, and bleeding you might notice when you first start flossing. It’s not the floss injuring your gums; it’s your gums reacting to an infection that was already there.
A large network meta-analysis found that flossing produces a statistically significant reduction in bleeding on probing compared to brushing alone, though the effect size is relatively modest, roughly a 5% to 10% additional reduction. Interdental brushes (the tiny bottle-brush-shaped picks) and water flossers actually performed better in clinical trials, with water flossers achieving about a 19% reduction in bleeding and interdental brushes producing the largest drops in gum inflammation scores overall. That said, any interdental cleaning is substantially better than none.
If your gums currently bleed when you floss, you can generally expect the bleeding to decrease within a few weeks to a month of daily use. That timeline reflects how long it takes for inflamed tissue to heal once the bacterial irritant is consistently removed.
How It Affects Cavities Between Teeth
The cavity-prevention story is more nuanced than most people expect. A systematic review of six controlled trials found that flossed tooth surfaces had a lower risk of developing cavities, with about a 14% to 21% risk reduction overall. But the strength of that benefit depended heavily on who was doing the flossing.
When dental professionals flossed children’s teeth on school days, the cavity risk dropped by 40%, a large and statistically significant effect. When people flossed on their own, however, the studies found essentially no measurable reduction in cavities. The likely explanation isn’t that flossing doesn’t work; it’s that most people don’t do it well enough or consistently enough to get the full benefit. Technique matters. You need to curve the floss into a C-shape around each tooth and slide it below the gumline, not just snap it between teeth and pull it back out.
Changes to Your Oral Bacteria
Regular flossing doesn’t just remove bacteria temporarily. It shifts the overall composition of the bacterial community in your mouth. A population-based study found a dose-response relationship between flossing frequency and the diversity of oral microbes: daily flossers had measurably lower bacterial richness and diversity compared to non-flossers.
Lower diversity might sound like a bad thing, but in this context it’s actually a healthy sign. In gum disease, bacterial diversity tends to increase as harmful species like those that cause periodontal infections move in and multiply. By disrupting plaque daily, flossing keeps these disease-associated bacteria from gaining a foothold. Studies have specifically shown that consistent oral hygiene reduces the abundance of key periodontal pathogens, the species most strongly linked to progressive gum disease and bone loss around teeth.
String Floss vs. Water Flossers
Traditional string floss can remove up to 80% of plaque between teeth when used correctly. Water flossers, which shoot a pressurized stream into interproximal spaces, have shown competitive or even superior results in some head-to-head comparisons. One study found water flossers reduced plaque between teeth by about 82%, compared to 63% for string floss. Whole-mouth plaque reduction followed a similar pattern: 74% for water flossers versus 58% for string floss.
Water flossers work through a different mechanism. Instead of mechanically scraping plaque with a thread, they generate high shear forces from the water stream that exceed the adhesion strength of the bacterial film, essentially blasting it off the tooth surface. This can be especially useful for people with braces, dental implants, or bridges where threading traditional floss is difficult or impossible. Interdental brushes remain the top performer for gum inflammation in clinical trials, but they require enough space between teeth to fit the brush through.
The best interdental tool is whichever one you’ll actually use every day. If string floss sits in your drawer untouched, a water flosser you enjoy using will deliver far better results in practice.
What Happens When You Skip It
When plaque between teeth goes undisturbed day after day, the consequences unfold in a predictable sequence. First, the gums become inflamed, a reversible condition called gingivitis. If the plaque continues to accumulate, the inflammation progresses deeper, eventually destroying the bone and connective tissue that hold your teeth in place. This advanced stage, periodontitis, is irreversible and is the leading cause of tooth loss in adults.
Cavities between teeth are also notoriously sneaky. They develop on surfaces you can’t see or feel until they’re large enough to cause sensitivity or pain. By that point, they often require more extensive treatment than a cavity on an accessible surface would. Interproximal cavities are among the most common findings on dental X-rays, and they form precisely in the zones that flossing is designed to protect.
How to Get the Most Out of Flossing
The American Dental Association recommends cleaning between your teeth once daily. Time of day doesn’t matter, though flossing before brushing may allow fluoride from toothpaste to better reach the surfaces you’ve just cleaned.
Use about 18 inches of floss, winding most of it around your middle fingers so you can advance to a clean section for each tooth. Guide the floss gently between teeth using a back-and-forth motion rather than snapping it straight down into the gums. Once the floss is between two teeth, curve it into a C-shape against one tooth and slide it up and down, going slightly below the gumline. Then curve it against the adjacent tooth and repeat. Every tooth has two interproximal surfaces, and each one needs attention.
If you find string floss difficult to maneuver, floss picks, water flossers, and interdental brushes are all viable alternatives. The clinical evidence supports all of them over doing nothing, and the performance differences between tools matter far less than whether you clean between your teeth consistently.