Young adults are turning to so-called legal highs as they seek alternatives to poor-quality cocaine on the streets, a report has said.
The number of 18 to 24-year-olds being treated for addiction in 2009-10 fell sharply for every drug except cannabis as young adults turned away from class A drugs, the NHS's National Treatment Agency for Substance Misuse (NTA) said.
But it warned there was "evidence that legal highs have emerged as an alternative to low-quality cocaine".
"Despite this apparent step away from the most harmful street drugs, there is some evidence of a corresponding move towards new synthetic compounds (sometimes known as legal highs) such as mephedrone," the NTA drug treatment report said.
"The nature of the legal highs market means that new substances are continually emerging, bringing with them renewed concerns about their actual chemical composition and the potential harmful effects.
"Treatment data suggests few people so far have needed help for these new drugs. It could take some time for those using legal highs to develop problems that would call for formal treatment.
"So it's too early to tell if there is an emerging treatment need, although reports from A&E units suggest these new drugs do cause significant harm."
Peter Kelsey, the team leader for Lifeline Redcar and Cleveland, which aims to relieve poverty, sickness and distress among drug addicts, said: "People hear the word legal and they think safe. Yet it's anything but.
"We're seeing a big rise in people coming to us because of legal highs, which we think may be down to the poor quality and price of coke and the legal aspect."
Mephedrone - also known as Meow, Bubbles and M-Cat - was banned and made a class B drug in April after it was linked to the deaths of two teenagers, Louis Wainwright, 18, and Nicholas Smith, 19. Toxicology reports later showed the pair had not taken the drug.