A boil-and-bite mouthguard gets its shape from being softened in hot water and bitten into while the material is still soft, and that same softening process is what wears the fit down a little more with every reboil. Most boxers mold one boxing mouthguard on day one and never touch it again, but the material keeps a kind of memory, and by the third or fourth reheat the edges stop gripping the gumline the way they did at the start. A guard that shifts mid-exchange isn't doing its job even if it looked fine in the mirror that morning. Replace it on a schedule tied to how often you spar, not on how it looks, because cracks usually show up after the fit has already failed. The most common mistake buyers make is grabbing the cheapest single-layer guard for full sparring and expecting sparring-level protection out of a piece built for pad work.
Layer count matters more than most buyers assume walking in. Single-layer guards, sometimes still sold under the older UK term gum shield, are thinner, easier to breathe and call out combos through, and fine for pad work or light bag rounds where contact to the jaw is rare. Double or triple-layer builds trade some of that breathability for a gel or EVA core that spreads impact across a wider surface, which is the difference that shows up the first time a check hook lands clean. Choose single-layer if you're doing technical drilling or coaching sessions where nobody is throwing full power. Choose double-layer if live sparring is a regular part of your week, because the extra shock absorption is doing real work by then, not sitting there for looks.
Strap or tether style is a rule question before it's a preference. USA Boxing and most amateur sanctioning bodies require the mouthguard to attach to the headgear strap during competition, and gyms running fight-style sparring often carry the same rule into the room. A tethered mouthguard isn't a performance upgrade over a standalone piece, it's a compliance requirement that also happens to stop you from losing the thing across the ring mid-round. If your gym doesn't run competition rules, a standalone guard works fine and is easier to pull out and rinse between sessions.
Material comes down to a short list once boil-and-bite is separated out: EVA foam is the standard for cost and flexibility, silicone shows up in premium double-layer guards for its bounce-back after repeated bites, and thermoplastic dental-lab guards exist for boxers who need a permanent, exact mold instead of a home reboil every few months. A boxer who grinds their teeth heavily at night, or has had jaw pain after hard sparring rounds before, is often better served by a dentist-fitted option than another round of stovetop boiling, since a lab mold won't loosen the way a home-boiled one eventually does. A heavy night-grinder is one of the few buyers this whole category isn't really built for, and that's worth saying plainly instead of upselling them a thicker retail guard that solves a different problem.
Sizing splits into adult, youth, and sometimes a mid-size bracket for smaller-jawed adults, and getting it wrong shows up fast: too small and it pinches the back molars until a boxer starts avoiding biting down at all, too large and it triggers a gag reflex that makes three-minute rounds miserable. Braces change the picture entirely. A standard boil-and-bite guard doesn't seat properly over wires and brackets, so anyone in orthodontic treatment needs an ortho-specific guard built with extra clearance, not just a smaller version of the same product.
Ventilation is a smaller but real factor for anyone doing extended sparring or cardio-heavy rounds. A guard with a molded breathing channel at the front cuts down on the dry-mouth, can't-get-air feeling that makes boxers spit theirs out mid-round, though it does trade a sliver of protection at that exact spot for the airflow. Storage gets ignored until it becomes a problem: a mouthpiece case with actual ventilation holes matters more than people give it credit for, since a boxing mouthguard left wet in a gym bag pocket is how bacteria and a permanent odor set in within a couple of weeks. A rigid case also keeps the shape from warping in a hot car, which is a faster way to ruin a fit than a season of normal use.