Last year, the force warned people not to take tablets labelled AMT or 5-IT which have been connected with a number of deaths in Europe.
Superintendent Grahame Clarke said: "These substances are unreliable, unpredictable and potentially very dangerous - users may believe that they have taken ecstasy and it is very likely that they will suffer from a significant negative reaction.
"Symptoms reported recently include increased heart rate, elevated core temperature and seizures.
"These pills are not covered by any form of quality control and this is supported by past forensic analysis.
"Whether bought online or from a regular supplier, there is no guarantee the user will be safe."