Ten people were hospitalised at the weekend after taking an illegal drug known as "rocket fuel" at parties in Hertfordshire.
Police issued a warning about the hallucinogenic, similar to LSD and ecstasy, after incidents near Croxley Green and Berkhamsted.
Some people remain in hospital although others have now been released.
Three people from Hertfordshire were arrested on suspicion of supplying drugs and were released on bail.
The 10 people were taken ill over a 12-hour period on Friday night and Saturday morning.
Hertfordshire Police arrested an 18-year-old man from Watford, a 20-year-old man from Croxley Green and a 20-year-old woman from Leavesden on Saturday.
They have been bailed until 8 June.
Det Insp Paul Doran said anyone who had taken the drug and felt unwell should go to a hospital.
He added that people who took the substance were "dicing with death" and could suffer "life-changing internal injuries".
BBC News Online, 26th March 2013