Warnings have been issued by a west London council and nightclubs after two people died in suspected drug overdoses from highly potent synthetic opioids.
An investigation is under way into the "sudden deaths" of a 28-year-old man and a 20-year-old woman in Southall on 26 May, the Metropolitan Police said.
"This is following reports that the individuals allegedly passed away after taking an illicit substance in the form of a green pill," a spokesperson for the force added.
Drug testing charity The Loop has said the pills contain synthetic opioids called Nitazenes, which can be "50 to 500 times stronger than heroin".
Ealing Council said the two people who died are understood to have taken green pills with the number 80 on one face and possibly an "OP" on the other.
The police said no arrests have been made in connection with the deaths.
East London nightclub The Cause posted about the pills on their Instagram account, stating there had been "several hospitalisations across multiple London venues".
BBC News has contacted the London Ambulance Service (LAS) to verify this.
The Loop said the pills looked like Oxycodone but it was not known if they were purchased under the belief it was this medication. According to the NHS Oxycodone is a prescription painkiller used to treat severe pain.
What are nitazenes?
Nitazenes are a type of high strength synthetic opioid – drugs which have a similar effect to heroin, but made in laboratories rather than from poppy seeds.
Originally developed by the pharmaceutical industry in the 1950s as a painkiller, clinical trials were abandoned amid concerns about their harmful effects.
These compounds are dangerous because they can be 50 to 100 times stronger than heroin, so drug users have no idea of the potency of what they are taking.
The drugs suppress the respiratory system, so people can die as they stop breathing but the symptoms of an overdose can be reversed by taking an antidote called naloxone.
