Tongue Piercing Infections
If tongue piercing MUST happen, it is recommended that you avoid studs made of metal which increases the risk of infection, at least according to a recent study in the Journal of Adolescent Health.
The study demonstrated that studs made of steel might promote the development of a bacterial biofilm leading to increased risk of infection whereas those made of polytetrafluoroethylene or polypropylene are inert to such bacterial colonization.
I should also add that metal studs may cause an allergic reaction leading to painful tongue swelling/itching, especially in those with metal allergies. This is similar to the skin reactions that some people experience with cheap metal jewelry.
Tongue piercing: the impact of material on microbiological findings. J Adol Health online, 2011