Using sterile instruments and jewelry reduces the incidence of infections. Some people develop an allergic reaction to the metal properties in the jewelry. If you are thinking of having your body pierced or are the parent of a would-be piercer, remember that the health risks are considerable. The procedure can be very painful. Piercings are done without anesthetics, since only a doctor can prescribe and administer anesthetics.
Infection is the main danger.
Speech impairment, nerve damage, eating difficulties, and metal allergies are the other long-term possibilities with any kind of mouth piercing. Inadvertently biting down on a metal stud in the tongue can also damage teeth. Nipple piercing for women might well permanently interfere with the ability to breast-feed, since milk ducts might be scarred. A tetanus shot in advance would be a wise idea. If you haven't had one in the past 10 years.