A custom guard stays in place when the mouth is open, lets you shout to a team-mate and breathe hard, and absorbs and spreads a blow across the arch rather than transmitting it to one or two teeth. Laboratory testing and field studies consistently show fewer and less severe dental injuries in players wearing well-fitted guards than in those wearing shop guards or none, though no guard removes the risk entirely.
A guard lasts two to three seasons in an adult with normal wear; it is replaced sooner if it splits, thins over the front teeth, or if the teeth have moved or changed. Children usually need a new one every year or two as the mouth grows.