Police carried out the largest seizure of synthtic opioids in a series of raids which lead to 11 arrests.
Around 150,000 tablets of a drug called Nitazene were found in Waltham Forest, east London, in a factory set up to make the pills, police said.
Detectives believe the drug was being sold on the dark web.
They also found various other class A and B drugs, a gun, a pill pressing machine, more than £60,000 in cash and £8,000 in cryptocurrency stored on hard drives.
A series of raids were carried out in Enfield, north London, on October 24.
Some 11 people were arrested between August 21 and November 21 and all charged with conspiracy to supply class A and B drugs.
Detective Superintendent Helen Rance, leading the investigation, said: ‘Synthetic opioids have been detected in batches of heroin found in London and across the UK.
‘They substantially raise the risk of incredibly serious harm to the user and are believed to be linked to a number of deaths.
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‘We are working closely with partners to monitor and proactively tackle this issue, provide advice and remove the availability of these dangerous drugs from our streets.’
Police say anyone who has taken synthetic opioids should get medical treatment urgently.
They have taken the unusual step of highlighting signs of someone who may have taken one of these drug.
These include having small, narrowed pupils; reduced or loss of consciousness; dizziness or drowsiness; difficulty breathing; nausea or vomiting; cold or clammy skin; blue or grey lips and fingernails; low blood pressure or decreased heart rate.
Ms Rance added: ‘The public health advice remains that illegal drugs should not be consumed.
‘There are support services available for people who need help. Anyone who has consumed synthetic opioids and experiences the symptoms described should seek urgent medical treatment.’
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