Why do dentists always recommend to brush 2 times for 2-3 minutes?

i find it inconvenient to do that every day, so i brush once a month for 2 hours.
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I think it's an average estimate, not a hard rule.
The exact time doesn't matter, but it's an approximate reference for how long you could aim for
some people don't brush enough
some people brush too much (harms enamel and damages gums)
- In a sense, I guess op is right though — I recently read that ~70% of people in a study were brushing ineffectively, no matter how long they were told to brush. Their brushing only improved after being told to make sure to brush every every "sector" of their teeth.
  
  When I was growing up they used to say "brush after every meal" and then it became three times per day, now it's "please just brush twice at some point".

My electric toothbrush does 30 seconds per quadrant, 2 minutes total. I'm 38 have zero cavities and my teeth are fine. I only brush once a day.
- There’s other factors than just brushing your teeth but brushing is probably the easiest factor that most people can reasonably take control of.
  Genetics, obviously you can’t do much about. But you can avoid sweets and decide not to get pregnant (hormonal changes during pregnancy can cause mouths to get more acidic and make plaque harder to remove, and can also soften the gums and bones that hold teeth in, or even weaken the teeth directly). Also, if you want nice teeth, it’s especially important to abstain from smoking crystal meth. And that’s even more important during pregnancy.
  But telling everybody “brush twice a day for two minutes” is a small ask with huge returns.
  
  Idk. I’ve been able to maintain a pretty consistent balance of crystal and healthy teeth. Every one of them that’ve fallen out have been nice, white, and cavity free.
- I've always brushed my teeth twice a day and I've had like 6-8 cavities, a root canal, and an implant. There's more to it than brushing. Some of it is biological.
  
  Do you drink a lot of sugary drinks? Haven't had a cavity since I quit drinking (non-sugar free) sodas.
  
  What kind of toothpaste do you use? Do you floss, or use a fluoride mouthwash?
- Sonicare or oral b?
  I wish I had bought my sonicare much earlier than I did. Seems like the best way to keep your teeth healthy outside the dentist's office.
  Seems expensive until you have a dental bill that is much more expensive. I highly recommend getting a sonicare.

The whole point is to not leave starches/sugars in places long enough for a biofilm of plaque to form.
The problem is, teeth are poorly shaped to clean and the interface of the gums is down right hard to get to.
So what they're trying to do is to get you to clean often enough that you manage to get some of the hard to get areas covered but not so often that you're sanding the enamel of your teeth.

Because it should take you about 2 minutes to brush your teeth.
If you're actually brushing for 6 minutes straight, you're overdoing it and damaging your enamel and gums.
Brush gently, floss thoroughly.
- Adding to this, also make sure to floss gently.
  My hygienist told me I was flossing too hard lol.
- One reason why I enjoy my electric toothbrush, as it vibrates every 30 seconds to tell you to switch to the next quadrant, up to 2 minutes. No guesswork, and it brushes better than I ever could using a regular toothbrush.
- Seriously, I have no idea how one could brush teeth in 2 minutes. I brush gently, use the simple swipe away from the gum technique and just work systematic chewing surfaces, inner surfaces, outer surfaces. Each part gets 2-3 swipes. I had a professional dental cleaner teach this technique to me, and she also told me that she couldn't finish within 3 minutes. (Do not misunderstand me; I would happily get away with 2 minutes.) BTW flossing is another interesting topic, AFAIK there is no study which can show that flossing helps your teeth/gum. (I floss daily, but I just cannot understand why there is no study which supports this practice.)
  
  I only floss when I can't get something out from my teeth, but I hate the feeling of things in my teeth so I often take a drink of water and aggressively rinse and like force the water through my teeth. Never had any dental issues, so 🤷
  
  https://jdh.adha.org/content/96/3/8
  https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1601-5037.2008.00336.x
  https://aap.onlinelibrary.wiley.com/doi/abs/10.1902/jop.2006.050362
- No, it's not. I have never brushed my teeth too hard and have always used very soft toothbrushes, but I used to brush for 6-7 minutes, and my gums have suffered for it. Way too much of my teeth are now exposed, which is both ugly and causes sensitivity.
  In the long run, if one keeps this up, teeth will start falling out.
  Don't overbrush.

Brushing for longer has diminishing returns and hurts your gums. I'm just speculating, but it's mostly about removing debris from your mouth and applying flouride to your teeth.

My electric toothbrush works in four 30-second increments. Each 30 seconds is plenty of time for me to cover a quadrant. I slowly go over the outside, inside, top/bottom, gums in each quadrant. Maybe if I was meticulously brushing each tooth one by one, I could see the issue, but that's not necessary, is it?

So I've had something like 25-30 cavities filled in my life. I haven't had a single cavity in 10 years, though. I brush only once a day, and floss. The trick is flouride rinse afterwards. That's the secret.
- but the fluoride is gonna turn me into a government sleeper agent!1!!?!
  
  Only if it gets in your water

I'm also like you with my timing but most people I know say I take ages with my teeth. Apparently most people are done with their brushing in less than one minute. Therefore, dentists recommend spending at least two or three. I don't think you need to brush for less time than what you already are.
- What's your cavity situation like
  
  Zero, why?
- Thanks, good to hear, I am not the only one. :-)

I think it’s a game of diminishing returns.
Let’s say 6 minutes gives you perfect 100% cleaning.
Well 30 seconds probably already gets you 50% or more of the total benefit just by getting fluoride on your teeth and rinsing your mouth a bit so getting people to 3 minutes is probably approaching perfection anyway and if you start asking people to do 6 minutes then they’ll say fuck it I won’t bother at all so settle for the 80% win.
I assume.
- I like the internal logic of this argument. I’m going to assume this is fact now, with zero research.

You are harming your teeth if you brush them that much, that’s why. Overbrushing is just as bad as brushing too little.

Buy high fluorine toothpaste and put a TINY (I mean, miniscule amount) on your toothbrush. There should be only a few speckles of the stuff on there. Then brush just enough to ensure each and every nook and cranny is covered in the stuff, rinse and spit. If the next time you go to brush you still taste the toothpaste, you put on too much and therefore don't need to reapply it. Just put on a little less next time.
Never had a cavity since I started doing that, and I'm not even a 1/16th through the tube. Best money I ever spent.
::: spoiler EDIT: A few other dental tricks that will help dramatically:
You don't need to brush twice a day, once is plenty and preferably before bed
Don't brush right after eating/drinking especially if it was acidic, that tears your enamel apart
Use only a pea-sized blob of toothpaste, if you use it like they show you in the commercials, you're an idiot
The primary goal of brushing is to apply fluoride, the secondary goal is to get shit out of your gums
Tooth whitening is a scam designed to poke at your insecurities. If you buy into it, you're an idiot
Dr. Fortnite says floss every night. Use a water flosser or pay attention to how your dentist flosses as their technique is immaculate
All floss is blood-flavored, do not try to push back against this universal truth for it is absolute
There is no sensation more elating than your dentist actually being impressed at your dental health
Some toothpastes and flosses cause cancer, research which ones and stay away from them
A Colgate tee shirt looks almost exactly like a supreme one and costs a fraction of the price

Have you asked your dentist?
- Are you aware some of us can’t afford that?
  
  Since OP has firm views about what dentists says, I'm going to assume that they visit the dentist on occasion.
  
  This, so much!
  It really makes me angry, that there is no free dental hygiene training for children/teenagers.
  I life in a comparatively rich society (Germany), but our retarded health care system pumps billions every year in preventable diseases (like most dental problems) and exactly zero in prophylaxes.
  
  Are you aware that the question was not directed at you?
- Fair and good question. I literally ask my dentist on every single visit the following points
  is my dental hygiene ok?
  do I damage my gums with my brushing?
  what can I improve in my dental hygiene?
  The dentist always tells me it is okay and I cannot do more, than I am doing.

- In case you are unaware current advice is to avoid brushing for a while after an acidic food or drink
  
  "after every meal" does not mean "immediately after every meal"
  
  Guidance for preschools around me is for them to brush kids teeth after every meal that’s served at school.
  That was recently reinstated after being suspended for a couple years and the teachers are practically (as much as they legally can) begging the parents to sign the waivers to opt their kids out of it. I don’t blame them. It seems excessive and it’d take a ton of time for two teachers to scrub a dozen or so sets of toddler teeth, while also controlling said toddlers while they wait for everyone to finish.

That’s how much time it takes for the fluoride to take effect?

Surely part of your 6 minutes is flossing and mouthwash though.
I systematically clean each surface but my electric toothbrush has an automatic timer that buzzes at the 2 min mark and its surprising how close it is. I normally only have one or two teeth to go.

There are two things with brushing your teeth: first, the mechanical action of the brush and the microparticuls in the toothpaste will clean the tooth. Second, the fluorine in the toothpaste will consolidate the enamel of the tooth, but it needs a bit of time to be absorbed.
This is why it's important to use real toothpaste and not the natural shit you can do yourself at home.
So 2-3min is the right time to brush the tooth and allow the fluorine to be absorbed. Some dentists will also tell you to not rinse the mouth, only spit the saliva and paste, I imagine so that fluorine has more time to work.

What's really strange to me, is that for all the expertise around dentistry, AFAIK there has never been an actual evidence-based study for how long and how often you should brush and floss.
Its literally the most important piece of dental advice they could study, and no research has been done. How much plaque builds up on average when only brushing once per week, once per month? It makes me question the whole discipline and wonder if they don't just say "brush 10 times a day" in order to sell more dental products.
Maybe they'd discover that actually you only need to brush twice a week, and floss twice a week. Maybe it ends up being flossing every day, and brushing once a month... who knows.
- AFAIK there has never been an actual evidence-based study for how long and how often you should brush and floss.
  The National Institute of Health has a ton of public-paid studies. Did you even bother to search it before making your astounding claim? https://www.nih.gov
  Just one search for nih brush time shows several studies. Let me just link the top two...
  https://pubmed.ncbi.nlm.nih.gov/19723429/ -> "This study was undertaken to measure plaque removal during untutored brushing over timed periods between 30 and 180 seconds with"
  https://pubmed.ncbi.nlm.nih.gov/16355646/ -> "This review shows that there is consensus in the literature that (meticulous) tooth brushing once per day is sufficient to maintain oral health and to prevent caries and periodontal diseases. Tooth brushing is also regarded as an important vehicle for application of anti-caries agents, such as fluorides. However, most patients are not able to achieve sufficient plaque removal by performing oral hygiene measures at home. Therefore, tooth brushing twice daily is recommended by most of the dentists in order to improve plaque control."
  OP poses one question with two parts. The first study answers the time (2-3 minutes) part. The second link answers the other part (twice daily).
  
  Is there any way to actually see those articles? All that's given is an abstract, and the articles don't appear to be linked.
- my mouth feels nasty after half a meal so uh... twice a day or more is good for me thanks
- From someone with a science background: There are a lot of expenses with that type of clinical trial.
  In particular, if you're going to assign someone to a group that is known to be unhealthy (brushing once a month) you need to pay for any dental or medical problems that arise from them not brushing.
  
  For sure, and they should def stop if there were any serious long-term affects, like any medical trial.