AS Mephedrone was officially classified as a class B drug, Herts Police sent out a strong warning to users.
The drug - branded as a legal high before national outrage - was made illegal as of Friday.
Also know as Meph, Meth, MCat, Miaow and Bounce, amongst others, users could now face up to five years in jail for possession.
Anyone found supplying the drugs faces up to 14 years behind bars.
Detective chief inspector Julie Wheatley said: “We want to ensure that people are well-informed on the penalties they could face if found in possession or supply of Mephedrone, as well as the ways anyone with concerns about the drug - or indeed any other illegal substance - can find the advice and support they need.
“Mephedrone has had a high national profile, and I’d like to reassure Hertfordshire residents that we will be taking robust action against anyone found with this harmful substance and, as always, our top priority is to work with our partners to help keep communities across the county safe.”
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