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
Contact your local poison centre or emergency medical service immediately.
Call emergency services now if the person:
Is difficult to wake, unconscious, or behaving very unusually
Has slow, shallow, irregular, or stopped breathing
Has a seizure, collapses, or develops severe muscle stiffness
Has blue, grey, or unusually pale lips or skin
Has taken promethazine intentionally
May have combined it with alcohol, opioids, or other sedatives
If they are not breathing normally, begin cardiopulmonary resuscitation if you know how and follow the emergency operator’s instructions.
Do not leave them alone.
And for anyone, someone who:
is unconscious, or cannot be woken
you believe has taken an overdose or swallowed something poisonous
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 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
2
Is any of this happening?
Be seen today
Treat any suspected promethazine overdose as urgent.
Do not wait for symptoms to appear.
Contact the follow-up service sooner for persistent confusion, breathing symptoms, fainting, abnormal movements, reduced urination, or worsening weakness.
3
Does any of this apply?
Book an appointment
Routine follow-up is appropriate after discharge if symptoms are improving as expected.
And for anyone, someone who:
is having side effects from a medicine, or is struggling to take it as prescribed
The article also says
Place an unconscious but breathing person on their side if it is safe to do so.
Every sign above is quoted from the sofidex article on Promethazine overdose. If you are not sure, err upward — it is always reasonable to be checked.