Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services if someone:
These are signs of opioid overdose, not ordinary withdrawal.
Give naloxone if it is available, call emergency services, and provide rescue breathing or cardiopulmonary resuscitation if trained.
Stay with the person because more than one naloxone dose may be needed.
Prompt assessment is particularly important if you are pregnant, have a serious heart or lung condition, use alcohol or sedatives regularly, or have had severe withdrawal before.
Seek urgent medical care if you cannot keep fluids down, have persistent severe diarrhea, are passing very little urine, feel faint, become confused, or develop a fast or irregular heartbeat.
Seizures, hallucinations, chest pain, severe abdominal pain, or thoughts of self-harm also require urgent assessment.
Arrange medical advice before reducing or stopping an opioid that you use regularly.
Every sign above is quoted from the sofidex article on Opiate and opioid withdrawal. If you are not sure, err upward — it is always reasonable to be checked.