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Police say a fatal batch of heroin, that has already claimed the lives of two men, nearly killed another victim.
The woman was taken seriously ill on Friday in the Portsmouth area after using the class-A drug.
A member of the public called an ambulance and the woman was taken to hospital.
It is believed she may have died if she had not been treated.
Detective Inspector Christopher Parry, of Portsmouth CID, said: ''Protecting the community remains our priority during this investigation into stopping the supply of this batch of heroin.
Police are particularly grateful to drugs intervention workers and counsellors for delivering advice to some of the most vulnerable people in our communities.''
The two people died in the Portsmouth area this week and others have been taken ill.
A 33-year-old man from Fareham, who had been in a critical condition in hospital, died on Thursday.
A 36-year-old man taken ill at an address in Paulsgrove, Portsmouth, died on Wednesday.
Two other people have required hospital treatment after taking the drug but have recovered, police said.
Detectives are now making inquiries into the suspected link between all the people and the same batch of heroin while reaching out to an estimated 1,200 heroin and crack cocaine users in Portsmouth to warn them of the danger.
Post-mortem examinations are due to be held in the next few days to help establish the exact causes of death in both cases.
Police say two men aged 32 and 48 have been arrested on suspicion of supplying a class-A drugs.