An urgent warning has been issued after seven people who took what is thought to be a type of MDMA became seriously ill.
Believed to be known locally as "pink champagne" or "magic", which comes in a crystal form, Greater Manchester Police said the drug is highly potent.
On Saturday four men became seriously ill and had to be taken to hospital. Two of them have since been released while the other two remain in intensive care.
A further three people were hospitalised on Sunday. A woman and a man are in a critical condition while another man is in a serious condition.
The force said signs to watch for include rigid muscles, shallow breathing, a fast racing pulse, hyper-aggression, seizure, foaming at the mouth and unconsciousness.
Detective Inspector Jim Faulkner of GMP's Oldham Borough said the increasing number of people affected is causing the force "no end of concern".
"Please, if you believe that you or somebody you know has taken the drug and begins showing these symptoms then get yourselves to the hospital immediately," he said.
"We have arrested one man in connection with the supply of the drugs, however our enquiries and operational activity will be continuing."
The 26-year-old man arrested on suspicion of supplying Class A drugs remains in custody for questioning, GMP said.
Anybody with information that could assist the investigation should contact police on 0161 856 9063 or the independent charity Crimestoppers, anonymously, on 0800 555 111.