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
For any suspected overdose, contact a poison centre or emergency medical service immediately.
In the United States, Poison Control is available at 1-800-222-1222 .
Elsewhere, use your local poison information service.
Call emergency services now if the person:
Has a seizure
Collapses, faints, or cannot be awakened
Has trouble breathing
Is severely confused, agitated, or hallucinating
Has a very fast or irregular heartbeat
Develops very hot, flushed skin
Took diphenhydramine intentionally
May have taken a large or unknown amount
If they are not breathing normally, begin CPR if you know how and follow the emergency dispatcher’s instructions.
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
Do not wait for symptoms.
3
Does any of this apply?
Book an appointment
After recovery, arrange non-urgent follow-up for continuing concentration problems, new urinary symptoms, persistent muscle pain, or concerns about medication use.
Mental health follow-up remains important after intentional poisoning even when physical recovery is complete.
And for anyone, someone who:
is having side effects from a medicine, or is struggling to take it as prescribed
The article also says
If the person is unconscious but breathing, place them on their side if you can do so safely.
Every sign above is quoted from the sofidex article on Diphenhydramine overdose. If you are not sure, err upward — it is always reasonable to be checked.