Why your gums bleed when you floss (and the two-week rule)

The most common sentence I heard in thirty-one years, usually said with a little embarrassment while I was setting up the tray, was some version of: "I stopped flossing because it made my gums bleed." Every time, I put the tray down and explained the same thing, and I am going to explain it here once, carefully, so that it is written down somewhere.

Floss does not make healthy gums bleed

Healthy gum tissue is firm, pale pink, and sits tight against the tooth. You can run floss along it, down under the edge where the gum meets the tooth, and nothing happens. I have done this to my own mouth every day for forty years and I cannot remember the last time I saw blood.

Gums bleed when they are inflamed. Inflammation is the body sending extra blood and immune cells to a spot where it has detected bacteria. The bacteria are in the plaque sitting along the gumline and just under it, in the little groove the floss is cleaning. The tissue gets swollen and fragile, and the blood vessels get close to the surface. When the floss touches it, it bleeds. The floss did not cause the inflammation. The floss found it.

This early, reversible stage is called gingivitis. The NIDCR page on gum disease describes it as the first stage, the one that can be fully turned around with cleaning at home and in the office. The later stage, periodontitis, is where the bone starts to go, and that one does not reverse. The whole game is keeping people in the first stage or out of both.

The two-week rule

Here is what I told every patient, and what I would ask you to try before you decide anything. Floss once a day, properly, for fourteen days in a row. Not gently to avoid the blood. Properly. The bleeding will get worse for the first two or three days, because you are disturbing tissue that is already angry. Then it starts to ease. By about day ten most people see much less blood. By day fourteen, if the problem was ordinary gingivitis, it is usually gone or close to it.

If you get to two weeks of honest daily flossing and you are still bleeding, that is information. It means something else is going on, and that is when you book the appointment.

Properly means this. Take about eighteen inches of floss, wind most of it around one middle finger and the rest around the other, and hold a one-inch section tight between your thumbs and index fingers. Slide it between the teeth with a gentle sawing motion, then curve it into a C shape around one tooth and slide it down under the gumline until you feel resistance. Scrape upward along the side of the tooth. Then curve it the other way around the neighboring tooth and do the same. Move to a clean section of floss and go to the next gap. The NIDCR's oral hygiene page has a decent description if mine did not land.

The C shape is the part almost everyone skips. Snapping floss straight down and straight back up cleans the contact point between the teeth and misses the gumline, which is where the problem lives.

When bleeding means something more

I want to list the signs that, in my experience, meant the patient needed the dentist and not just a better flossing habit:

• Bleeding that continues past the two-week mark with honest daily flossing
• Bleeding when you brush, not just when you floss
• Gums that look purple or dark red rather than bright red
• A persistent bad taste or a smell that other people have mentioned
• Gums that have pulled back so that teeth look longer than they used to
• Any tooth that feels loose, or a change in how your teeth fit together when you bite
• Pus, which looks like a white or yellow bead at the gumline when you press

Any one of those, and particularly the last three, is a periodontal problem, not a hygiene problem. The American Academy of Periodontology's patient resources explain what a periodontist does and when you should see one instead of a general dentist. Do not wait for the six-month cleaning if a tooth moves.

Things that make gums bleed more easily

Some bleeding is not about plaque, or not only about plaque. The ones I saw most:

Blood thinners. Warfarin, the newer anticoagulants, and daily aspirin all make gums bleed more readily and longer. The inflammation is still the cause. The medication just makes it more visible. Do not stop the medication. Do tell your hygienist.

Pregnancy. Hormone changes in the second and third trimester make gum tissue react more strongly to the same amount of plaque. Pregnancy gingivitis is common and it is worth being extra careful about flossing then, not less.

Diabetes, when blood sugar is not well controlled, slows healing and increases inflammation everywhere, including the gums. I could often tell from a patient's gums that their last A1c had not been good.

Smoking does the opposite, which is worse. Nicotine constricts the blood vessels, so smokers' gums often bleed less than they should for how inflamed they are. That hides the problem until it is advanced.

Dry mouth, from medications or mouth breathing or age, removes the saliva that normally washes bacteria away and buffers acid. More bacteria, more inflammation.

Where oral probiotics come into this

In the last decade I was asked more and more about probiotic lozenges and tablets for gums. The research on the strain Lactobacillus reuteri in particular is real: there are randomized trials where people who used reuteri lozenges alongside normal brushing and flossing had lower bleeding and plaque scores than people on placebo. You can read through them yourself in a PubMed search for L. reuteri and gingivitis. The effects are modest and they go away when you stop.

What I told patients: an oral probiotic such as ProDentim is an addition, not a replacement. If you are flossing daily and your gums still bleed a little, a reuteri-containing product is a reasonable thing to try for a couple of months, and some of my patients reported less bleeding on it. If you are not flossing, the probiotic is a $59 way to avoid the thing that actually works. Do the two weeks of flossing first. Then decide whether you want to add anything.

I want to be careful about one more thing. No probiotic treats periodontitis. If you have bone loss, you need scaling and root planing and a dentist watching the numbers. The tablet can sit alongside that. It cannot stand in for it.

What I would do tonight

Floss, properly, with the C shape, all the way around, even the back teeth you cannot see. Expect some blood. Write today's date on a sticky note on the mirror. Floss every night for two weeks. If the blood is gone, keep going and you are done with this article. If it is not, call a dentist and bring the sticky note.

And if, after that, you want to add a probiotic for the stubborn last bit of bleeding, the ProDentim page lists what is in the tablet. Read the strain names, compare them to the trials, and decide with your eyes open. That is how I would want a patient to do it.