One man is dead and four are critically ill in hospital after taking a "highly potent" form of the drug MDMA, police said.
The 26-year-old from Rochdale was one of a number thought to have taken a drug known locally as "pink champagne" or "magic" who were rushed to casualty over the weekend, Greater Manchester Police said.
Another 26-year-old man was arrested and later bailed on suspicion of supplying Class A drugs.
Detectives believe the drug comes in a crystal form, with "many of the people police have spoken to saying that this is the first time they have seen or heard of the drug".
Chief Superintendent Neil Evans said: "This is incredibly worrying and, with our colleagues from the NHS, we are doing all that we can to warn people against taking the drug and help those who may have taken it.
"So please, if you believe that you or somebody you know has taken the drug and begins showing symptoms, then get yourselves to the hospital immediately."