A range of 'herbal highs' could be banned by the summer under a fast track scheme to cut the number of powerful but legal drugs on sale in the UK.
The government's drug advisers are warning that many are freely available powders laced with chemicals closely linked to illegal drugs.
They're available over the internet, at festivals and in specialist shops.
A list of banned chemicals could be drawn up as early as July with a ban coming into force soon afterwards.
"We are worried about it," said Professor Les Iversen from Oxford University, who sits on the Advisory Council for the Misuse of Drugs.
"There is a considerable risk of adverse effects and particularly overdosing.
"We are actively looking at the problem now to see what can be done."
Hundreds of internet sites and head shops around the UK now sell a bewildering variety of legal highs.
Most are little more than vitamin pills laced with high levels of caffeine or herbs containing the chemical ephedrine.
I was on the way home from a friend's house and suddenly everything was so intense. It wasn't good at all
'Sarah', 17, had a bad experience with a legal high
But a new generation of legal highs started to emerge a couple of years ago, many with effects more closely associated with stronger drugs like cannabis, ecstasy and cocaine.
For a while, some contained the amphetamine-like drug BZP, an artificial chemical originally used as worming tablet for cows.
But the government recently said it would classify BZP as an illegal, class C substance along with the industrial solvent GBL after two accidental deaths connected to the drugs.
Then, six months ago scientists in Germany found the legal 'Chinese herbal powder' called Spice they were testing was not herbal at all but laced with a chemical linked to the active ingredient in cannabis.
More than 150 'cannabinoid' chemicals were discovered back in the 1970s by drug companies hoping to sell products with some of the same effects as cannabis.
That work was eventually scrapped. But the scientific research is now being used by some makers of legal highs to sidestep the law, according to drug advisors.
"This is an entirely new situation. We only found out what Spice really was late last year," said Professor Iversen.
"From what we know now about the components in it, we see no reason why it should be any different from cannabis with the added hazard of not knowing what dose you are taking."
Drug researchers think the active ingredient in Spice could be ten times stronger than the THC in standard cannabis, although users may take less of it as a result.
Seventeen-year-old 'Sarah', not her real name, smoked Spice with her friends when cannabis wasn't available.
"We thought it was much safer and we weren't going to get paranoid but it had the same effects," she said.
"It's definitely stronger. It was more trippy than the weed on the street.
"It was borderlining on magic mushrooms which I didn't like personally.
"I was on the way home from a friend's house and suddenly everything was so intense. I got home and had to get my mum to look after me. It wasn't good at all."
Sarah says she never had a problem buying it even though she was under 18.
"There should be more information about it out there. I think they should just ban these shops because they are not really helping anybody."
Spice and similar products like it officially sell as 'herbal incense' that is not for human consumption.
The manufacturers did not return Newsbeat's request for an interview.
Newsbeat also contacted dozens of internet and high street suppliers stocking the product.
Most refused to talk to us publically but claimed off the record that Spice is weaker than cannabis and users will just stop taking the drug when they've had enough, making an overdose unlikely.
The active ingredient in standard Spice has already been banned in France, Germany and Austria.
But with hundreds of other similar chemicals out there, it is easy for the makers to switch to a legal alternative and continue producing a substance with the same effect.
It is likely the government will try to ban a wide variety of the 'cannabis-like' chemicals at the same time this summer in an attempt to stop the use of drugs like Spice.
But sceptics reckon the authorities will find it increasingly hard to catch up with the companies behind legal, synthetic versions of banned drug.
By Jim Reed
BBC Newsbeat reporter
Tuesday, 9 June 2009 07:32 UK
Original URL: http://news.bbc.co.uk/newsbeat/hi/health/newsid_8090000/8090025.stm