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, or bumping the lip causing blockage of the salivary glands.
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.
A thin, fluid-filled sac appears on the inside of the lip. The sac is bluish and clear. It is painless, but bothersome.
A mucous cyst often can be left alone; it usually will rupture spontaneously. If the cyst returns, it may need to be removed.