Two men and a woman, aged in their 20s, died in separate incidents while a fourth man remains seriously ill.
Syringes and drugs paraphernalia were recovered near the victims at both addresses in Hackney, east London.
"There could be a batch currently in circulation which may pose a potentially lethal threat," said Det Insp Daniel Smith.
'Be careful'
Two men were found dead by police on Tuesday in Arcola Street. They have yet to be formally identified.
On Saturday, police found two people, a man in his 30s and a woman in her 20s, collapsed in a property in Whiston Road.
Both were taken to Homerton Hospital where the woman died. The man remains in a serious condition.
Det Insp Smith said: "We are warning drug users in Hackney to be especially careful.
"If you have taken drugs and feel unwell I'd urge you to seek urgent medical attention."
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