There are many types of mouthpiece on the market, but they all do the same thing, keeps your jaws together by biting onto the mouth piece during sleep; known as jaw retention or mandibular position appliances (There are also a few that emphasize on the tongue retention). Mouthpiece can either be bought off the shelf, or they can be fully custom-made for you by dentists who specialize in sleep treatment with oral appliances. An off the shelf mouthpiece typically cost a hundred dollars or less, and last between six months to one and half an year depending on the material and built quality. Off the shelf mouthpiece are made out of medical grade polymers; before your first usage the manual would instruct you to immerse your mouthpiece in a hot water for the mouthpiece to soften, and then you will place the mouthpiece into your mouth and bite on it for the softened mouthpiece reshape based on the shape of your teeth, and once the mouth piece is cooled it’s now shaped to accustom your bite. Well, to an extent. If you would like a mouthpiece that can fit much better to your bite, then you should consult a sleep dentist for a custom-made oral appliance. Depending on the detail of the oral appliance and who your dentist is, the cost can easily be in the thousands; and there may be recurring cost too, the mouthpiece don’t last a life time.
Several years ago I evaluated the importation of an off the shelf mouthpiece from an English manufacture, so I tried the mouthpiece myself. I have worn mouth guards before for playing rugby, so wearing the sleep mouthpiece is not something I have never experienced as they are somewhat similar. However, to put a plastic thing in my mouth for an hour of tensed exercise was nothing like to have it in my mouth for an entire night. Biting on the mouthpiece often times create tension in your jaw which could lead to muscle pain on your face. Which I found contradicting as you want to be relaxed going into sleep, yet wearing the mouthpiece would actually require work from you. Another complaints people have is simply the sensation of having a plastic block stuffed in your mouth for the entire night, it’s not pleasant and it makes swallowing saliva rather difficult; also, your tongue would be tasting the plastic the entire night. A mouthpiece if not used properly for a long period of time, could also create improper alignment of jaw, creates under-bite or overbite, in the long run.