Young adults are turning to so-called legal highs as they seek alternatives to poor-quality cocaine on the streets, a report has found.

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 said.

But it warned there was "evidence that legal highs have emerged as an alternative to low-quality cocaine".

"It could take some time for those using legal highs to develop problems that would call for formal treatment. It's too early to tell if there is an emerging treatment need, although reports from A&E units suggest these drugs do cause significant harm."