Everyone’s teeth are different. A dentist will take an impression of your teeth and order a mouth guard that fits your mouth and protects your particular teeth. The result is a mouth guard that fits snugly over your teeth and gums and is comfortable to wear. They stay in the mouth better and protect the teeth better. These custom mouth guards also help treat painful illnesses such as TMD and TMJ.
Some companies offer a mouth guard that is made from a plastic material. You boil the mouth guard and then put the softened mouth guard in your mouth and try to shape it with your finger or tongue. Though this is better than a stock mouth guard it is still not as good as a custom mouth guard made by your dentist.
Generally, a mouth guard is only made for the top teeth. But, if you have braces or other devices, the dentist may prescribe a guard for the lower teeth as well.