Most mouth pieces have to have the backs trimmed slightly so as to fit comfortably and not cause the gag reflex response. The trick is to cut off just enough but not so much that your back teeth still have good protection. That being said cut just a fraction off and then try the mouthpiece out and if you are still gagging then cut off a little more. You also may not have got a good fit when forming it the first time and you can sometimes try to form it again if its not too deformed now. When formed correctly it should protect all your teeth as well as those in back and stay affixed to your upper jaw just from the suction that's created and not cause you to gag.
Make sure you leave it long enough in hot boiling water and then when you remove it with some tongs, shake it off of any excess water and then place it in your mouth on your upper jaw and close your jaw gently while pushing your tongue up into the roof of your mouth while sucking air through your teeth.
Getting a good form and fit can be a little tricky if you have never done this before so buy cheap ones till you get more experience with this. I have read that the more expensive ones don't really give you that much more protection than the cheaper ones so don't blow $35.00 on one. Stick to the five dollar ones and learn how to form them and the most I ever paid for a good one was about $15.00. Getting a good fit is key to any of them working well and protecting your teeth along with wearing them when you should.
Yes you can, if you're referring to what i call a mouth guard.
hahaha yeah we call them mouth guards here haha, yes you can.
I've just bought a gum shield and just moulded it, it's digging into the back of my mouth and it's very uncomfortable and I couldn't fight with it like that, could I cut a bit off each side of it or will it ruin it?
