Police have been told to crack down on mephedrone dealers rather than users as the party drug became illegal.
The Association of Chief Police Officers (Acpo) has urged forces around the country to concentrate efforts on those selling the drug, adding that it had "no intention" of criminalising young people in possession.
But it said that users of mephedrone still run the risk of arrest and prosecution after its reclassification as a class B drug, with the decision being made on a case-to-case basis.
From midnight on Thursday night, those caught in possession of the party drug could face conviction and a possible five years in prison. The maximum sentence for dealing or trafficking the substance is 14 years.
In the days leading up to the ban, retailers of the drug - which had been legally sold over the internet as plant food - were urged to hand in their stocks to police.
But efforts to clamp down on its distribution will be increased after its reclassification comes into effect today.
A spokeswoman for Acpo said it maintained the position that forces should "concentrate their efforts on those people who sell and traffic" the drug.
She added: "We have no intention of criminalising young people who may be in possession of this drug. However, that does not mean that anyone caught with this drug in their possession for their own use will be ignored by police. They do, of course, still run the risk of being arrested and prosecuted. This will be dealt with at a local level by forces."
The move to ban mephedrone came despite the resignation of two Government drugs advisers over the issue.
Earlier this month, Eric Carlin followed Dr Polly Taylor in stepping down from the Advisory Council on the Misuse of Drugs (ACMD). In a letter of resignation, Mr Carlin said the decision to ban the drug was made despite there being "little or no discussion about how our recommendation to classify this drug would be likely to impact on young people's behaviour".
