A thin, fluid-filled sac appears on the inside of the lip. The sac is bluish and clear. It is painless, but bothersome.
The sac can also occur on the tongue, palate, inside the cheeks, the floor of the mouth, or around tongue or lip piercings.
Your doctor can usually diagnose a mucous cyst simply by looking at it.
Maheu-Robert LF, Andrian E, Grenier D. Overview of complications secondary to tongue and lip piercings. J Can Dent Assoc. 2007; 73(4):327-331.