Chipped teeth
Teeth will break if you bite down on something that's harder than them — chipped teeth is by far the most common risk for oral piercings (primarily tongue piercings). It's a simple common sense fact that if you bite down hard on a piece of metal, like a steel bead on the end of a steel barbell, your teeth will break before the metal does.
Chipped teeth, on top of being sensitive, painful, and ugly, put you at a heightened risk of tooth infection, leading to far greater problems, and can be costly to fix properly (several hundred dollars per tooth).
The chance of chipping your teeth can be minimized by wearing properly sized jewelry. In a tongue piercing, the bar should fit snugly with the tongue — if you want to guarantee that you'll chip your teeth, wear a long barbell (like the one you got pierced with). If you can annoy everyone by playing with the barbell over your teeth, it's probably too long.
The 3/4" (or so) barbell that you first get your tongue pierced with is long to accommodate any swelling. You MUST go back three to five weeks after the piercing and get a shorter bar. If you do not, you quite likely will chip your teeth at some point in the future.
Additionally, make sure that the beads are properly affixed to any oral jewelry — if it comes off while you're eating, it can chip your teeth easily as well.
Other than that, you can minimize this risk by choosing softer non-metal beads (or soft gold beads), but these are not really in common circulation.
It should be noted that even minor impact, such as clicking the jewelry on your teeth, can cause tooth fractures.
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