Students tempted to try out legal highs at the start of the new term were warned today that they could be even more dangerous than illegal drugs.
Substances such as Meow Meow and Ivory Wave are often packed with harmful substances and there is no way of knowing whether they are safe, a Government scientist said.
It comes after the Home Office announced plans for year-long bans that could be put in place quickly to take new drugs off the market while a comprehensive review of their potential harm was carried out.
Last month, the legal high Ivory Wave was blamed for the death of chef Michael Bishton, 24, whose body was found in the sea in Whitecliff Bay, near Bembridge, Isle of Wight.
The drug is sold for about £15 a packet and is advertised as relaxing bath salts, but the product has become popular as a legal alternative to illicit drugs.
Mr Bishton's girlfriend Sammy Betts, 21, said he had started to become paranoid at his mother's home after taking the substance.
Today, speaking at the launch of the Crazy Chemist campaign at Roehampton University in south-west London, crime prevention minister James Brokenshire warned that the legal highs market was changing.
"Unscrupulous drug dealers constantly try to get around the law by peddling chemicals, which are often harmful, to young people," he said.
"Through this campaign, we want to send a clear message to anyone tempted to try a new drug, that just because something is advertised as 'legal' does not mean it is safe and it may already be banned.
"There is increasing evidence that substances sold as 'legal highs' often contain harmful illegal drugs."
Dr Phil Yates, of the Forensic Science Service, added: "Over the last year or so, we've seen a huge increase in these types of legal highs which are now mostly illegal highs being sent into the forensic laboratories for analysis.
"These new drugs, they're such an unknown, it could be seen as much more risky to take them."
He said the term "legal high" did not mean that the Government had sanctioned them, but just that it had not been tested.
"These are completely unknown quantities," he said.
"They're not intended for human use, you really don't know what you're getting."
Ben Whittaker, of the National Union of Students, added that the term "legal highs" was misleading and could leave students in a "vulnerable position".
The proposed ban would send a clear message to users that these substances carried a risk and would prevent new chemicals becoming widely available, the Home Office said.
Under the plans, police would be able to confiscate suspected substances and the UK Border Agency would seize shipments entering the country.
Anyone caught supplying a banned substance would face a maximum 14-year jail sentence and an unlimited fine.
But possession for personal use would not be deemed a criminal offence in a bid "to prevent the unnecessary criminalisation of young people", the Home Office said.
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