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 now if a bite or sting is followed by:
Difficulty breathing, wheezing or a tight throat
Swelling of the tongue, lips or throat
Faintness, collapse, confusion or marked weakness
Widespread hives with breathing or circulation symptoms
A sting inside the mouth or throat
Many stings followed by significant symptoms
Use a prescribed adrenaline auto-injector without delay if your emergency plan says 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 anaphylaxis settles by itself.
Seek prompt medical advice for worsening pain, spreading warmth, pus, red streaks, fever or feeling generally unwell.
Also get advice for severe swelling, a sting near the eye, symptoms that are not improving, or a bite acquired where insect-borne infection is a concern.
And for anyone, someone who:
has a wound that is hot, spreading red, or leaking pus
3
Does any of this apply?
Book an appointment
Routine follow-up is appropriate if bites keep appearing, the cause is unclear, or reactions repeatedly disrupt sleep or daily activities.
A non-urgent allergy assessment may be appropriate after a large reaction or any previous whole-body reaction to a sting.
Every sign above is quoted from the sofidex article on Insect bites and stings. If you are not sure, err upward — it is always reasonable to be checked.