It is important to clean mouth guards regularly. Given the risk of trauma, some clinicians advocate for required mouth guard use in many sports, not just hockey and football. Players should be counseled not to share mouth guards as they are not as effective (or as frequently used) if they are poorly fitted, and sharing mouth guards may transmit disease.
Custom made mouth guards are the most comfortable and therefore most likely to be consistently worn. They also provide the best protection. However, they are most expensive and require a dental visit for fitting.
Boil and bite mouth guards provide a reasonable compromise between cost and protection, but have much less rubber between the biting surfaces of teeth and therefore offer less protection.
Stock mouth guards, because of their poor fit, should not be recommended unless no other option is available. The most important message is that a mouth guard be used consistently.
Fernandes LM, Neto JC, Lima TF, Magno MB, Santiago BM, Cavalcanti YW, de Almeida LD. The use of mouthguards and prevalence of dento-alveolar trauma among athletes: a systematic review and meta-analysis. Dental Traumatology. 2019; 35(1):54-72.