One man has died and four others are in critical condition after taking MDMA over the weekend, police said.
A 26-year-old man from Rochdale died in hospital while being treated.
A total of 11 people were hospitalised over the weekend after taking the drugs, four of whom are in intensive care, Manchester Police said.
All of the patients are believed to have taken the same MDMA-based drug known as locally as "pink champagne" or "magic".
Officers have warned members of the public that the substance could be fatal.
Officers have also warned people to seek urgent medical advice if they think they have taken the drug. In a statement, they said warning symptoms of an overdose which requires immediate hospital treatment could include:
- Rigid muscles
- Shallow breathing
- Fast racing pulse
- Hyper-aggression or mania
- Foaming at mouth
Chief Superintendent Neil Evans said the cases were "extremely worrying".
Anybody with information should contact police on 0161 856 9063 or phone Crimestoppers anonymously on 0800 555 111.