Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Contact your local poison centre or emergency service.
Call emergency services immediately if the person:
Contact emergency services or a local crisis service.
Immediate support is appropriate even if the person now says they feel better.
Routine follow-up is appropriate only after emergency clinicians have assessed and cleared the overdose.
This may include reviewing the prescription, medicine storage, emotional wellbeing, and any ongoing symptoms.
And for anyone, someone who:
Any suspected imipramine overdose needs urgent medical advice now , even if there are no symptoms.
Do not wait to see whether the person becomes unwell, and do not let them drive themselves.
And for anyone, someone who:
The article also says
If it is safe, take the medicine container or a photograph of its label to the hospital.
Share what may have been taken, when it happened, and whether alcohol or other medicines were involved.
Do not delay leaving to search for this information.
If the overdose may have been intentional, stay with the person if you can do so safely and remove nearby medicines or weapons.
Every sign above is quoted from the sofidex article on Imipramine overdose. If you are not sure, err upward — it is always reasonable to be checked.