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
These concerns usually need dental follow-up rather than emergency hospital care.
Go to an emergency department or call emergency services if you have:
Difficulty breathing or swallowing
Rapidly increasing swelling of the mouth, face, or neck
Swelling beneath the tongue or in the floor of the mouth
Swelling around the eye, trouble opening the eye, or vision changes
Confusion, faintness, or severe weakness
These signs may mean the infection is affecting the airway, eye area, or the rest of the body.
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
Arrange an urgent dental appointment if you have possible symptoms of an abscess, including persistent toothache, a swollen gum, facial swelling, pus, or pain when biting.
Do not wait for a routine check-up.
Contact a dentist promptly if symptoms return after treatment, a temporary filling comes out, or swelling and pain are not improving as expected.
Seek urgent same-day medical or dental help if you develop fever, feel increasingly unwell, have spreading facial swelling, or cannot open your mouth normally.
Every sign above is quoted from the sofidex article on Tooth abscess. If you are not sure, err upward — it is always reasonable to be checked.