A government adviser has quit the Advisory Council on the Misuse of Drugs over the criminalisation of mephedrone.
Eric Carlin said the council's decision taken earlier this week was "unduly based on media and political pressure".
Mr Carlin is the fourth member of the body to resign following the sacking of former chairman Professor David Nutt.
The council has recommended that the substance, linked to at least four UK deaths, and other so-called "legal highs" be classified as Class B drugs.
Mr Carlin, 47, said he was increasingly disillusioned with the lack of attention paid by politicians and the media to drug prevention and early intervention among young people.
He also criticised the Advisory Council on the Misuse of Drugs's (ACMD) "apparent lack of interest in the subject".
In his resignation letter to Home Secretary Alan Johnson, Mr Carlin wrote: "We had little or no discussion about how our recommendation to classify this drug would be likely to impact on young people's behaviour.
"Our decision was unduly based on media and political pressure."
He added: "As well as being extremely unhappy with how the ACMD operates, I am not prepared to continue to be part of a body which, as its main activity, works to facilitate the potential criminalisation of increasing numbers of young people."
Mr Carlin's resignation comes just days after another adviser, Dr Polly Taylor, quit the ACMD for similar reasons.
Professor Nutt, who was sacked by Mr Johnson in October 2009, said he was not surprised by Mr Carlin's resignation.
"There's not been proper consideration given to the broader justice and political aspects of making a drug class-B and criminalising maybe tens of thousands of young people," he said.
"I'm not surprised that people think it's all been done for political reasons rather than scientific or health reasons."
The Home Office called Mr Carlin's resignation "regrettable".
A spokesman said: "However it does not impact on our plans to ban mephedrone and the other substances as soon as parliamentary time allows.
"The home secretary has full confidence in [chairman] Professor [Les] Iversen and the rest of the ACMD committee."
Shadow home secretary Chris Grayling said the relationship between the government and the ACMD had become "utterly shambolic".
He said: "After all the chaos of the last few months, it finally looked as if Alan Johnson might be getting things back into shape again.
"The decision on mephedrone was the right one, but this latest resignation suggests pretty clearly that the home secretary has been completely unable to restore his relationship with the experts who advise him."
Mr Johnson will ban mephedrone and the other substances within weeks, following the advice of the ACMD.
Class-B drugs, which include cannabis and amphetamine sulphate, carry a maximum sentence of five years in prison for possession or 14 years for supply.