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
Call emergency services if lip swelling occurs with trouble breathing or swallowing, tongue or throat swelling, faintness, or severe dizziness.
These can be signs of a serious allergic reaction.
And for anyone, someone who:
has lips or skin that look pale, blue, grey or blotchy
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 you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Seek same-day medical advice for rapid new lip swelling, even if breathing is currently normal.
You should also get prompt advice if redness is spreading into the surrounding skin, there is pus, you have a fever, or pain and swelling are quickly worsening.
3
Does any of this apply?
Book an appointment
Arrange a routine appointment with a doctor or dentist if:
Your lips are not improving after about two weeks of gentle care
The problem repeatedly returns
Cracks at the mouth corners do not heal
Eating or drinking is difficult
You develop recurring blisters or ulcers
A lump, ulcer, discoloured area, or rough patch persists
You think a medicine or dental product may be responsible
The article also says
A lasting scaly area on a sun-exposed lip should be assessed, but it is not usually an emergency.
Every sign above is quoted from the sofidex article on Sore or dry lips. If you are not sure, err upward — it is always reasonable to be checked.