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:
Trouble breathing, wheezing, or a tight throat
Swelling of the tongue or inside the mouth
Difficulty swallowing or a sudden change in voice
Fainting, severe dizziness, confusion, or collapse
Rapidly worsening symptoms after a food, medicine, or insect sting
Use prescribed adrenaline immediately if your 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 settle.
Seek urgent medical advice for rapidly increasing facial swelling, significant lip swelling, or a first episode of angioedema, even if breathing is currently normal.
Swelling linked to certain blood pressure medicines may need prompt assessment.
3
Does any of this apply?
Book an appointment
Arrange routine medical care if the rash lasts for six weeks, repeatedly returns, disrupts daily life, or does not improve with recommended antihistamines.
Also seek assessment if individual lesions remain longer than 24 hours, are painful, leave bruising, or occur with fever, joint pain, or feeling generally unwell.
Every sign above is quoted from the sofidex article on Urticaria. If you are not sure, err upward — it is always reasonable to be checked.