More than 30 people were taken seriously unwell after taking dangerous fake heroin in Camden.
Police said at least 33 people had been taken unwell since Monday, 17 of whom were forced to attend hospital.
In one case on Wednesday evening, Met officers were forced to give emergency first aid to a man in cardiac arrest in Haverstock Road after taking what he thought was heroin. The force said it was likely that the actions of those officers saved his life.
Other cases have been reported around Hampstead Heath, Oval Road and Jamestown Road, as well as in other areas of the London borough.
Of those taken to hospital, police said they were not aware of any being life-threatening, but that they continued to investigate. No arrests have yet been made.
Met Detective Chief Inspector Chris Soole said: “We continue to work alongside our partners, including the local council and health professionals, to establish the full circumstances and identify everyone who has been affected.
“Analysis of substances recovered shows that what users believed to be heroin was in fact a dangerous synthetic opioid. It is clear that this remains in circulation, especially in the Camden area, and it causes an extreme reaction.”
DCI Soole urged anyone who is at risk to take precautions by speaking to health professionals.
“As I said on day one, my advice remains that people should not buy, sell or consume illegal drugs,” he said. “They are illegal and the trade is not regulated, so there are always very serious risks. Please seek help.”
Camden Council called the cases “extremely worrying” and said it had targeted outreach teams to the affected locations, who carry naloxone, which can reverse opioid overdoses.
Cllr Anna Wright, the council’s Cabinet Member for Health, Wellbeing and Adult Social Care, said: “Contaminated heroin continues to be an issue of national concern.
“In Camden, we are continuing to invest in our drug and alcohol services and raise awareness of how residents can access support including drop-in sessions in community venues, access to dedicated services for women, outreach prescription support and one-to-one support from Keyworkers.”
