Two people have been arrested as police investigate the deaths of three men suspected of taking contaminated drugs.
Greater Manchester Police said the men, aged 33 and 34 were being questioned on suspicion of supplying a controlled drug.
The arrests come after two men died in Greater Manchester and another collapsed and died in Liverpool.
Up to six other people were admitted to hospital with symptoms including shaking, shivering and complaints of a fever.
Police in northwest England are warning drug users that a contaminated batch of ecstasy tablets could still be on the streets.
Detective Chief Inspector Howard Millington from Wigan CID, talking about the men who died in Greater Manchester, said: "We are very concerned at how these deaths of two apparently fit young men have occurred.
"It is possible that they are linked and this is something we are exploring as part of the investigation.
"Our main concern is that there may be a contaminated quantity of illegal drugs and if this goes unchecked it could result in further deaths.
"If you are suffering adverse effects after taking one of these tablets I would advise you to go to hospital for a check up.
"I would always urge people not to take illegal drugs and remind them that you do not know what they have been made up with. They can contain poisons and illicit chemicals that can have potentially fatal effects."
The ecstasy tablets are said to be heart-shaped in purple, green, yellow and blue.
Greater Manchester Police have urged anyone who may have purchased such pills to contact them.
Superintendent Andrea Jones, said the men are believed to have bought the drugs in the Wigan area.
"These deaths were closely linked, both had taken drugs prior to admitting themselves to hospital.
"We want to raise awareness in the community of the risks and dangers of taking illegal drugs. No one knows what poisons these drugs contain."
The men who died in Greater Manchester were 19-year-old Jordan Chambers, who was treated at the Royal Oldham Hospital on Sunday and 28-year-old Gareth Ashton, who died at Wigan Infirmary on Monday. Both had similar symptoms.
In Liverpool, a 26-year-old man collapsed and died in the early hours of Sunday. Merseyside Police believe it was also drugs related.
Greater Manchester Police are not currently linking his death to the two in their region, but said they could not discount a connection.
A spokesman for Merseyside Police said: "Emergency services were called at about 3.30am to a guest house in Tuebrook to a report that a male guest was unwell and had collapsed. Paramedics attended and the 26-year-old was taken to hospital where he unfortunately died a short time later.
"Although the investigation is in the early stages it appears that the deceased man visited Liverpool on Saturday and is believed to have taken tablets, possibly ecstasy somewhere in the city."
Anyone with information has been urged to phone police on 0161 856 7149 or the independent charity Crimestoppers , anonymously, on 0800 555 111.