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A ban on the drug mephedrone has come into force across the UK.Legislation placing the synthetic stimulant and other related compounds in the Class B category alongside cannabis was rushed through Parliament.
The government's drugs advisors - the Advisory Council for the Misuse of Drugs (ACMD) - found evidence the substances were "likely to be harmful".
Possession carries a maximum sentence of five years imprisonment, while dealers could receive up to 14 years.
The Association of Chief Police Officers says it has "no intention" of "criminalising" young people caught with mephedrone, also known as "plant food" or "meow".
Police say they will focus on suppliers and ahead of the ban, a number of internet sites selling the so called legal high were closed down.
Advisor resignations
But BBC home affairs correspondent Danny Shaw says one online supplier has claimed to be liaising with the world's "leading research chemists" to develop replacement substances.
And the ACMD, in its report to the government, said an illegal market in mephedrone could develop as users stockpiled the drug before the ban.
This week, retailers were asked to hand over stocks of mephedrone by council officials in England and Wales.
The Local Government Association (LGA) said shops had been given chance of voluntarily handing over stocks of mephedrone before its sale became an offence.
The move to ban the drug came in the wake of several deaths linked to the substance.
Two government drugs advisers resigned over the move.
In his resignation letter, one adviser, Eric Carlin, said the decision 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".This article is from the BBC News website. ? British Broadcasting Corporation, The BBC is not responsible for the content of external internet sites.