The highly potent substance, also known as 'magic', has been circulating in the Greater Manchester area, where it has left seven people seriously ill in hospital. Two men remain in intensive care.
Reactions to the drug, which is sold as MDMA, include rigid muscles, shallow breathing, a fast racing pulse, hyper-aggression, seizure, foaming at the mouth and unconsciousness.
Detective Inspector Jim Faulkner of Greater Manchester Police 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.
"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.