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.