The Metropolitan Police said 33 people were taken ill after taking synthetic opioids in Camden in early March.
The investigation is being carried out with co-operation from partners including Camden Council.
According to the force, there have been no deaths, but officers are monitoring the situation.
Synthetic opioids are man-made drugs that mimic the effects of natural opioids such as opium or heroin.
Scotland Yard previously said that people should not buy, sell or consume illegal drugs – emphasising that there are “very serious high risks”.
Detective Chief Inspector Chris Soole, who is leading the Met’s operation, said: “The activities will continue until police and partner organisations are satisfied that the substances found to be in circulation are no longer causing a risk to users.
“This may be a long-term undertaking; but rest assured that my team and I, who are based in Camden, are committed to ensuring that those who supplied or are supplying these substances are identified, arrested and brought to justice.
“If you have information about the people who profit from the misery caused by addiction to illegal drugs, please call police on 101 or contact Crimestoppers.”
