Detective Chief Superintendent Roy McComb said "no single bad pill" has been found responsible of the deaths of the victims, who were mainly in their 20s and 30s, the Belfast Telegraph reported Friday.
Three people have been detained in an investigation of the deaths in the last four weeks but were later released, McComb said.
Authorities are waiting for the results of forensic tests.
In a separate alert, police warned of tablets of ecstasy that have been laced with a bulking agent. The pills are more dangerous because users may take more thinking the tablets are less powerful.
Megyn Kelly: Santa Claus and Jesus are both white men
Wisconsin business offering 'therapeutic cuddling' forced to close