What is the difference between a dentist custom made mouthguard and a chemist/sports store generic mouthguard?

Chemist /sport store generic mouthguards are not made for the uniqueness of your own mouth, whereas custom made mouth guards are constructed off a mould of your teeth which help them fit securely. The Australian Dental Association strongly recommend the use of custom fitted mouthguards as these give your teeth the best chance of protection from being hit by either another individual or sporting equipment.