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A mucous cyst is a thin sac filled with fluid that usually grows in areas in or around the mouth. They are mostly commonly found near or on the lips, though they can also grow on the tongue or palate, the mouth floor, the gums, or the inside of the cheeks. Mucous cysts may also appear at the site of lip or tongue piercings. The cysts are painless, common, and tend to be more annoying than harmful.
An average mucous cyst can be diagnosed with a physical examination. They consist of a sac that is a small, shiny, domed bump, filled with a fluid that has a blue hue. If it is not treated, the cyst can leave a permanent, but harmless bump, although it is just as likely that they will eventually entirely disappear on their own. The cysts on the mouth floor are known as ranula, while cysts that grow on the gums are called epulis. It is commonly believed these cysts will grow as the result of parts of the mouth being pulled through the teeth via sucking.
The cysts tend to grow slowly and start deep in soft tissue. When they appear on the surface of the skin, they are usually firm to the touch. There are two primary kinds of mucous cysts: superficial and classic.
Superficial mucous cysts tend to be smaller and it is easier for them to pop under pressure, leaving a small ulcer. These cysts typically appear on the lower lip. Classic mucous cysts are larger and firmer to the touch. They are more likely to appear inside the mouth, in places such as the mouth floor, cheek, gums, or inner upper lip.
Most of these cysts will eventually burst without intervention. Occasionally they will heal faster and with less hardening of the area if a sterile needle is used to pop them. As there is a risk of infection and permanent tissue damage, any attempt to drain the cyst should be done by a qualified medical professional. Cysts that are particularly uncomfortable or that continue to return can also be removed by a doctor.
As it has not traditionally been fully understood why these cysts grow, there has been no widely-accepted way to prevent them. Evidence suggests that the cysts are likely the result of trauma in the area where they appear. For this reason, many doctors recommend avoiding trauma to the mouth area by sucking the tongue, lips, or cheeks so that they slide through the teeth.