looking at thingiverse, people sure are eager to 3d print things that shouldn't be 3d printed
(a mouth guard?!)
FDM printing with PLA has the problems that it's inherently not a good material to have near your body (it will partially melt and leak nasty chemicals at you) but also it's completely un-sterilizable. There's all kinda of little tiny voids that will fill up with bacteria.
also, your hot end might be leaving lead in the print.
@foone At least it isn't as bad as UV-cured resin prints
...which people have probably put those in their mouths too, against *any* notion of suitability, outdated or otherwise oh god.