Synthetic cannabis substitutes, including Spice, are as harmful as cannabis and should be made illegal, government drugs advisers have said.
Spice, a mix of herbs and man-made cannabinoids, is sold on the high street and online for about £20.
But the Advisory Council on the Misuse of Drugs warns it can cause paranoia and panic attacks.
It wants all types of cannabinoids banned. The Home Office is expected to legislate later this year.
The advisory council's chairman Professor David Nutt has called on the government to enforce a general ban on cannabinoids, because suppliers can make new versions of the chemical in the laboratory.
He said: "Spice and other synthetic cannabinoid products are being sold legally as harmless 'herbal legal highs'.
These are not harmless herbal alternatives and have been found to cause paranoia and panic attacks
Professor David Nutt, Advisory Council on the Misuse of Drugs
"However, the herbal content is coated in one or more dangerous chemical compounds that mimic the effects of cannabis.
"These are not harmless herbal alternatives and have been found to cause paranoia and panic attacks."
Later this year the Home Secretary, Alan Johnson, is expected to change the law to ban Spice and similar products. They are likely to be made a class B drug, alongside cannabis.
The move to classify Spice with cannabis was welcomed by the charity, Drugscope.
However its chief executive, Martin Barnes, said it would be hard for the law to keep pace with new synthetic drugs.
"It is consistent with the Misuse of Drugs Act to control substances which have the potential to cause significant harms - 'Spice' falls into this category," he said.
"While we support this pre-emptive move, it does highlight important questions about the ability of the legislation to cover so called 'legal highs' and to keep up with the development of new synthetic drugs."
A Home Office spokesman said: "We are determined to crack down on those 'legal highs' that pose a significant health risk.
"We will publish our response shortly, along with the proposed controls for a range of other substances."
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