Concerns have been raised about a "potentially contaminated batch of drugs" after two people died following a night out in Bolton.
Emma Johnson, 21, and Chris Goodwin, 30, were found dead at separate addresses in the town on Sunday.
Witness accounts suggest alcohol, amphetamine and a synthetic stimulant had been taken, police said.
Post-mortem examinations and toxicology tests are now due to take place. Police said five people in the group fell ill.
Emergency services discovered the body of Ms Johnson at a house in Buttermere Road, Farnworth ,at 13:30 BST on Sunday.
Mr Goodwin's body was found at a property in Littlebrooke Close, Tonge Moor, shortly after.
Supt Phil Davies, from Greater Manchester Police, said they were "fast-tracking blood tests from all of the people involved".
He said: "From the witness accounts that have been taken, we know they have taken a cocktail of alcohol, amphetamine and what they are calling bubble - a generic term for a synthetic stimulant.
"We don't know whether it was the cocktail of drugs or it was a bad batch of something in particular."
He added: "What is very important in this case is what is in the blood of the individuals who were involved, so we can understand what was in their bodies at the times the deaths took place."