I practice karate, and spar in class as well as in tournaments. I used to use on of those boil and bite type mouth guards. They worked great. There are cheap ones, but I usually used one of the better quality mouth guards made by Shock Doctor. When you boil the traditional boil-and-bite mouthguards, they become soft, then mold to your teeth when you bite down on them.
But when I got my braces put on, I couldn’t use those mouth guards anymore. The problem is that when you bite down on the softened warm plastic, they will stick your your braces. Shock Doctor makes a mouth guard for braces, which I bought at the local sporting goods store. You don’t boil it. It is this big bulky soft plastic thing. I used it for lack of anything else. It wasn’t very comfortable. It didn’t fit to the teeth, so it was difficult to talk. When I kiai’d while sparring, I have actually had it come flying out of my mouth.
So I continued searching for an alternative. I found a mouth guard for braces made by a company named Shield. It is called Extreme Braces Mouth Guard. If you do a Google search for “shield extreme braces mouth guard”, you find various places to but it. The mouth guard is made of two different types of plastic. On of the plastics gets softened when placed in boiled water, the other type doesn’t. I boiled some water in a hotel coffee maker, and placed the mouth guard into the hot water for the 35 seconds according to the directions. I pulled it out, put it into my mouth, and bit down. Then I ran it under cold water. It fit great! It fit as well as the traditional boil-and-bite mouth guards, but worked with my braces! I used it for the karate tournament. It was great!