Tongue piercing
Tongue piercing.
Playing with the barbell which is pierced in the tongue can result in a space between the anterior teeth (the incisors).
A case report published in the Journal of Clinical Orthodontics (Midline Diastema Caused by Tongue Piercing JCO_2010-07-426) reveals that

Interincisive diastema caused by a barbel
those who have pierced tongues tend to push the barbell against their teeth, causing a space between teeth due to the incisors being shifted forward.
Among our clientele, we have noticed that having a pierced tongue can make it easier for the lingual retainers, which are bonded to the back of the anterior teeth, to detach. The observation made by the JCO’s study about the fact that the patient pushes with his tongue makes me say that it is somewhat logical. I conclude that if a barbell and a tongue are powerful enough to move teeth, the barbell is strong enough to wear away the glue that covers the lingual retainer.
A study convened by Mayo Clinic on the prevalence of body art (body piercing and tattooing) in university undergraduates and the incidence of medical complications was published in 2002 (Prevalence of Body Art (Body Piercing and Tattooing) in University Undergraduates and Incidence of Medical Complications). Four hundred fifty-four students (218 males, 236 females) answered a detailed survey questionnaire.
Among male students, 83 (38%) had pierced ears and 15 of these students had removed their piercings. Ten (4%) had pierced tongues, 5 had removed the piercing. 7 had pierced nipples, 5 had removed the piercing.
Among female students, 67 (29%) had pierced ears, 4 had removed the piercings by the time of the survey. Thirty-seven (16%) had pierced tongues, 10 had removed the piercing. Fourteen (6%) had pierced nipples, 2 had removed the piercing and 74 (32%) had pierced navels, 7 had removed the piercing.
22% of males and 26% of females were tattooed.
Medical complication
Among the 229 pierced students, 3% suffered local trauma, 4.5% reported bleeding, 9% had bacterial infection. Among the 47 individuals with pierced tongues, 3 (10%) reported subsequent oral or dental injuries.
Conclusion
Oral piercing, and more particularly tongue piercing, is not without risks and severe consequences. Tongue piercing is associated with bleeding, pain, edema (swelling), tooth fractures (individuals bite the barbell and break off a piece of tooth), gingival trauma, difficulties eating and speaking.
The study convened by Mayo Clinic shows a possible bias in their method. The students, who were recruited for the study, do not necessarily represent the general population and the university clientele is not necessarily a target and representative population who has body piercings. Prevalence and complication rate could be different in a population issued from another socioeconomic environment.
An oral surgeon working at the Hôpital de l’Enfant-Jésus told me that some patients come to the hospital emergency department with a severe infection in the site of the tongue piercing. This causes septicemia (blood infection). Septicemia causes a drop in blood pressure and necessitates the administration of vasoconstrictors to increase the blood pressure. Blood vessels in the extremities (fingers and toes) can collapse and can produce necrosis of these extremities. This results in amputation of the affected fingers or toes. I could not believe my ears when the surgeon told me that, but I have known him long enough to know that he was not talking to me about an urban legend. It is true that young people, in Quebec City, develop an infection caused by their tongue barbell and leave the hospital with less fingers or toes.

Lingual piercing hole
The right picture represents a view from above and from below the tongue. The top hole (blue arrow #1) is the entry point of the hole which comes out under the tongue (green arrow #2). The hole never heals completely and communication with the inside of the body is established. Notice the nearby sublingual vein which brings the blood circulation toward the heart. This is why the slightest infection in this piercing hole can have extremely severe consequences.

Tongue barbell
The left picture shows how a patient can play with his barbell between his teeth.

Tongue piercing and gingival recession
The right picture shows the presence of gingival recession (green arrows) in the central incisors. When the tongue is at rest against the lower teeth, the ball pushes against the gum and teeth (black circle) causing a slight pressure or rubbing. This pressure or rubbing is sufficient to wear away the thin layer of gum tissue covering the roots. The piercing has been there for 10 months only. After being informed of the severity of the situation, the patient quickly removed her piercing.
Lip piercing

Lower lip piercing and gingival recession
Lip piercing generally causes friction on the marginal gum of the tooth and gingival recession results. In effect, the base of the bead that holds the jewellery rubs against the edge of the gum, which causes recession and exposes the dental root. A gum graft will then be necessary to prevent the progression of the recession. This is without saying that the piercing jewellery must not be worn any longer.

Upper lip piercing and gingival recession
The right image represents another eloquent demonstration of gingival recession (red arrows) which exposes the root of a canine (green arrows) because the base of the piercing bead rubs against the gum (blue arrow).
