Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Get emergency help if swelling under the tongue or in the mouth makes it difficult to breathe or swallow, pushes the tongue upward, or is rapidly increasing.
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Seek prompt medical advice if the area becomes increasingly painful, red, warm, or drains pus, especially if you also have a fever.
These changes may suggest infection.
Do not wait for a routine dental appointment when breathing is affected.
3
Does any of this apply?
Book an appointment
Arrange a routine appointment with a dentist or doctor if a mouth lump:
Lasts longer than about two to three weeks
Keeps bursting and returning
Is growing or becoming painful
Interferes with eating, speaking, or wearing dentures
Feels hard, fixed, or different from a typical fluid-filled bump
Develops an ulcer, unexplained bleeding, or colour changes
Every sign above is quoted from the sofidex article on Oral mucous cyst. If you are not sure, err upward — it is always reasonable to be checked.