A man is thought to have become the first person in the UK arrested on suspicion of possessing mephedrone since it was banned on Friday.
The arrest, at an address in Bewdley, Worcestershire, came just hours after the party drug became illegal.
West Mercia Police said the man, believed to be in his 20s, had not been charged and was released on bail.
Inspector Clive Eastwood said: "We think this may be the first arrest since mephedrone was banned."
They must have been waiting outside the House.