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 hives or swelling occur with:
Difficulty breathing, wheezing, or noisy breathing
Tongue or throat swelling
A hoarse voice or trouble swallowing
Faintness, collapse, confusion, or marked weakness
Rapidly developing symptoms affecting several body systems
Use a prescribed adrenaline auto-injector immediately if your emergency plan tells you to do so.
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
Do not wait to see whether an antihistamine will control breathing or circulation symptoms.
Seek prompt medical advice for swelling that is worsening, widespread hives after starting a new medicine, or hives accompanied by fever and feeling significantly unwell.
3
Does any of this apply?
Book an appointment
Arrange routine medical follow-up if hives keep returning, continue for six weeks, disturb sleep, or have no clear response to usual antihistamine treatment.
Also arrange an assessment if individual marks stay in the same place for more than 24 hours, are painful, or leave bruising.
Every sign above is quoted from the sofidex article on Giant urticaria. If you are not sure, err upward — it is always reasonable to be checked.