Police said "an urgent line of inquiry is to understand exactly what the substance taken is"
One person has died and four others are critical after taking a "highly potent" form of the drug MDMA, police in the UK have said.
The 26-year-old man was one of 11 people rushed to hospital in Oldham in Greater Manchester on Saturday night after taking the drug.
It is believed to come in crystal form and may be known locally as "magic" or "pink champagne".
It has been described as resembling brown sugar.
Many of those interviewed by police said this was the first time they had seen the drug.
Greater Manchester Police urged anyone who may have taken it to seek medical attention immediately.
Symptoms of taking the drug include rigid muscles, shallow breathing and a racing pulse. They may progress to aggression or mania, seizure, foaming at the mouth and unconsciousness.
Chief Superintendent Neil Evans said: "We have a full team working on the police investigation and we are appealing for anyone with any information about where this drug is coming from to get in touch as soon as possible.
"An urgent line of inquiry is to understand exactly what the substance taken is and steps are being taken in that regard.
"I would like to reiterate the dangers of taking this or any recreational drug. We have sadly had one person tragically die as a result of taking MDMA this weekend and others are fighting for their lives.
"We will be supporting their devastated families. I hope we don't have to support even more families. Cases like this are ones that we always hope we don't have to deal with."
A 26-year-old man was arrested on suspicion of supplying Class A drugs and later released under investigation.
One other person is in a stable condition and three have been released from hospital.