A man has been arrested on suspicion of possessing mephedrone on the day it became illegal - in what is thought to be the first case of its kind.
The man in his 20s and a quantity of the drug were found at a property on Load Street in Bewdley, Worcestershire.
The drug was listed as an illegal Class B substance from Friday after a law was rushed through Parliament.
Insp Clive Eastwood of West Mercia Police said: "We think this may well be the first arrest since it was banned."
No conclusive proof
The suspect has since been released on police bail.
Officers said ecstasy tablets and dried cannabis were also found at the house.
They executed a warrant after receiving information that mephedrone was being dealt from the address.
Mephedrone has a similar effect to amphetamines, ecstasy or cocaine but also causes nausea, palpitations and vomiting.
It has been linked to several deaths but there has been no conclusive scientific proof yet that it has been responsible for any of them on its own.