They do not. Professionally fitted mouthguards are the absolute best guard that money can buy. One size fits all mouth guard types are subpar to custom-made sports guards. To maximize the protection of your teeth, you will need to use the custom-fit mouth guard because you can lose up to 70% of the protection if you use the boil and bite mouth guard. Stock guards (cheap, unfitted guards) offer some protection but are in danger of falling out or getting knocked out easily.