The Special Investigations Unit (SIU) is probing an incident Wednesday night in which a London police officer used naloxone.
Shortly after 7:30 p.m., police responded to a medical assistance call at a home near Huron Street and Adelaide Street.
The SIU says an officer performed CPR on a 21-year-old man and gave him naloxone before paramedics arrived and took him to hospital.
Naloxone is a potentially life-saving drug, used to temporarily reverse the effects of opioid overdose.
As of early Thursday afternoon, the SIU said the man remains in hospital in life-threatening condition.
Two investigators and two forensic investigators have been assigned to the incident.
The SIU wants anyone with information to contact their lead investigator at 1-800-787-8529. Anyone who captured video of the incident is urged to upload it through the SIU website.
The SIU is an arm’s-length agency that investigates reports involving police in which there has been death, serious injury or allegations of sexual assault.