You'll get a dry mouth and bad breath. Also might need nasal polyps sorted or hayfever meds or something if your nose is constantly blocked. Also not a good look if your resting state is a slack jaw appearance.

Yes, your nose is the preferred apparatus for mammals to breathe through. Ours becoming so small has led to health issues, but it's still the 'correct' way to breathe.

I used to be allergic to cats, and my family had cats growing up, so I was a mouth breather for most of my childhood until my body finally figured out that cat hair isn't some secret bacterial plot to destroy me.

I had a bunch of sores in my mouth from it getting too dry, and I believe my teeth suffered a bit too, but I was lucky that they were all baby teeth.

It also just took a decent amount of time for me to learn how to rely on my nose for breathing after not using it for so long. I eventually learned, but it took effort for me not to just default to the breathing method that didn't require concentration at first.

If you are a child like anon's nephew, mouth breathing will mess up your craniofacial development.

I have a severe deviated septum, excessive mucus and I have to most of the time. I don't recommend it, makes your teeth damaged easier and your breath can smell. Sleeping this way is hard and you don't sleep as well.

Yes, it’s terrible for your long term oral health and if you do it enough your teeth will even get fucked up

I finally got adult braces to fix my open mouth breathing teeth and I’ve had about a dozen sinus surgeries but I breathe through my nose now and sleep and feel so much better

If you start shoving food up your nose, then yes.

Dries out your mouth & makes your breath stink. And leads to snoring and sleep apnea, which leads to bad sleep, it's a vicious cycle. One of the unexpected gains I got from yoga is a clearer nose to breathe through, I always felt like it was slightly blocked or small but the breathing exercises do open the airway and I guess tone up the muscles in there.

My guess is that it's more sanitary to breathe through your nose if you're in an enclosed room full of people with suboptimal ventilation. That way you're not all chilling in a sauna of each other's saliva.

Other than that though it seems kinda stigmatizing towards people with seasonal allergies, which is kinda shit.

Oh and George Washington apparently wrote a book on etiquette that includes not breathing through your mouth. Weird facts one remembers from homeschool, amirite?

This isn't necessarily a direct answer to your question, but it is tangentially related and I wanted to share this somewhere :)

I had a combination septoplasty+turbinoplasty about two years ago and it has been the single most significant quality-of-life improvement of anything in my life. Before the surgery, I could not breathe through my nose, and it's hard to explain just how much better things feel now that I'm able to breathe through my nose most of the time. My mouth is no longer constantly dry, my lips are no longer permanently chapped and cracked and peeling. I no longer have permanently bad breath, which was something people in my family complained about a lot (doubly bad because i couldn't smell it). I actually have a sense of smell now. My extremely picky eating is pretty much a thing of the past, because food doesn't all taste like cardboard with a slimy texture. I used to have constant trouble with insomnia, now that I'm not constantly fighting to get air into my lungs I can sleep through the night almost every night.

Many trans people say that colors appear more intense and vivid after they started taking the correct hormones; that the grass is literally greener. I can confirm that from firsthand experience. I can also say with confidence that this is nothing compared to the sheer joy of unlocking an entirely new sense which you didn't have for the first 21 years of your life.

I have an anon who comes to visit three days a week. We play basketball. Eat chicken fingers with ranch. Teach him how to breathe through his nose and not walk on his knuckles. Now we on Reddit. Thanks for the gold kind stranger.

Sometimes Anon is wholesome to keep us guessing. It won't last long.

It's also full of fake stories so there's that

As always, there are exceptions that prove the rule