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 occur with:
Trouble breathing, wheezing, or a tight throat
Swelling of the tongue or inside the mouth
Difficulty swallowing or speaking
Fainting, severe dizziness, confusion, or collapse
Sudden symptoms affecting several body systems after a likely allergen
Use a prescribed adrenaline auto-injector immediately if your personal emergency plan tells you to.
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 symptoms improve.
Seek prompt medical advice for rapidly worsening facial swelling, severe widespread hives, or symptoms that began after starting a new medicine.
Swelling without hives can have other causes and may need separate assessment.
3
Does any of this apply?
Book an appointment
Arrange routine medical follow-up if hives last for six weeks, keep returning, disturb sleep or daily activities, or are not controlled by usual treatment.
You should also arrange an assessment if individual marks remain longer than 24 hours, are painful, leave bruises, or occur with fever, joint pain, or feeling unwell.
Every sign above is quoted from the sofidex article on Hives. If you are not sure, err upward — it is always reasonable to be checked.