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 or go to an emergency department if you have trouble breathing, airway swelling, fainting, confusion, severe weakness, or signs of serious dehydration.
Very little urine, marked dizziness, or being unable to keep fluids down can indicate dehydration.
Emergency assessment is also appropriate for widespread blistering or peeling, severe eye pain, sudden vision changes, or extensive painful sores accompanied by significant illness.
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
Seek prompt medical assessment for a new target-shaped, painful, or blistering rash, especially if you recently started a medicine or have sores in your mouth, eyes, or genital area.
Contact a healthcare service the same day if:
Mouth pain makes eating or drinking difficult
Your eyes are red, painful, light-sensitive, or producing discharge
Passing urine is painful or difficult
The rash is spreading quickly or your skin is becoming painful
You develop fever, pus, increasing swelling, or worsening redness
These features may indicate severe erythema multiforme or another condition requiring urgent treatment.
Every sign above is quoted from the sofidex article on Erythema multiforme. If you are not sure, err upward — it is always reasonable to be checked.