Ecstasy tablets contaminated with highly toxic chemicals have been seized by police in Northern Ireland.
The mixture dubbed "Green Rolexes" could be particularly hazardous to health and has been linked with numerous deaths worldwide, the force said.
The warning came as investigators await the results of forensic tests in connection with eight recent unexplained deaths where drugs are suspected.
PSNI detective inspector Andy Dunlop said: "Both dealers and users may be unaware that what they believe to be Ecstasy actually contains PMMA (paramethoxymethamphetamine).
"PMMA pills are slower to take effect. This may cause the user to take more - which can lead to seizures, convulsions, heart attack and ultimately death."
The predominant symptom of fatalities around the globe has been over-heating of the body.
Mr Dunlop added: "They have been directly linked to death. There is no such thing as a safe drug or a safe dose."