Definition of Mucous cyst:

A mucous cyst is a painless, thin sac on the inner surface of the lips. It contains clear fluid.

Causes, incidence, and risk factors:

Mucous cysts are common. They are painless but can be bothersome because you are so aware of the bumps in your mouth. The cysts are thought to be caused by sucking the lip membranes between the teeth.

Mucous cysts are harmless. If left untreated, however, they can organize and form a permanent bump on the inner surface of the lip.

They are called ranula when on the floor of the mouth, and epulis when on the gums.

The sac may form around jewelry that has been inserted into the lips or tongue (“piercings”).