Dealers of recreational drugs will barely be affected by the Home Office's crackdown on "legal highs" because manufacturers have already developed alternative substances that will escape prohibition.

The British Government has been so far behind other European countries in tackling drugs like GBL and BZP that revellers have already moved on to the next high, according to toxicology experts.

The Home Secretary, Jacqui Smith, announced on Thursday that shewas planning to classify BZP – a drug similar to ecstasy which is also known as "Benny" and was originally a worming treatment for cattle – as a class-C substance, outlawing its trade and possession in the UK.

But specialists say that manufacturers in China and on theContinent have already diverted their resources to producing large batches of mephedrone, a drug with effects similar to cocaine which will remain entirely legal.

Mephedrone, nicknamed "meph" and sometimes marketed as plant food, is sold for as little as £15 a gram online and in high street "head shops" (which sell bongs and rolling paraphernalia). It is a member of the methcathinone family of substances which is distantly related to the khat plant. Metabolism of cathinone produces chemicals structurally similar to amphetamine and adrenaline. "Legal highs" is the catch-all tag for an array of drugs that take users out of their bodies while staying within the law.

A coroner called for BZP to be banned after the death of 22-year-old mortgage broker Daniel Backhouse, who had mixed the substance with ecstasy powder, while last month in Brighton, Heather Stewart, 21, died after taking the party drug GBL, an industrial solvent.

"The UK is slow and behind the times in banning BZP," said Dr John Ramsey, a toxicologist at St George's University of London, who is also the director of Tic Tac Communications, a drug analysis body that studies recreational drugs. "We can't rely on the Home Office to play catch up when it comes to these drugs. We need a sensible debate in the media about their potential risks."

But it is mephedrone that is increasingly partygoers' legal high of choice. Users report intense euphoria, talkativeness and increased levels of energy. It is already illegal in Sweden, Denmark, Finland and Israel, but at present there are no plans to ban it in Britain.

The Swedish ban came after mephedrone was linked to the death of an 18-year-old woman in Stockholm last December.

Because the substances are legal, little is known about their side-effects or circulation. "Nobody is investigating them," said Dr Ramsey. "The problem is to try to collect the information. It's all under the radar, and people [who arrive] in A&E aren't very honest about what they have taken."

He continued: "No one has the remotest idea of what the long-term effects of taking these substances are – they could interact with legitimate medication such as contraceptive pills and HIV medication, or they could cause birth defects. But because they are not [illegal] drugs they're not tested by the pharmaceutical industry so we simply don't know how bad they are for you yet."

Prior to the development of legal synthetic drugs, the market was full of so-called "herbal highs" that were relatively benign products containing caffeine or ephedrine from the herbs guarana and ephedra whose effects were not much more powerful than a strong cup of coffee.

The popularity of legal highs grew sharply when BZP substances were introduced to the market in the late 1990s and it has taken governments across Europe a decade to classify it as illegal. In March 2008, the European Union decided that all member states should control the use of BZP by March 2009, but in the UK it remains legal, pending the change in the law. On YouTube, videos promoting meph are quickly "flagged" as offensive by other meph users, to try to keep the substance's profile low (and hence postpone any legislative action against it).

Dr Ramsey said: "I don't think your average 15-year-old would dream that he could buy a something legal from a 'head shop' on the high street that might kill him."

The user: 'It's easy to get – all you need is a debit card and internet access'.

Sebastian Scott, 26, lives in Clapham, south London. He is originally from Australia and works as a project manager in local government.

"I have tried ecstasy, acid, ketamine, weed, mushrooms, MDMA, and coke. Up to last year I used MDMA and coke on a regular basis when I went out clubbing. But then I heard about meph[edrone] through a friend who raved about it. All I had to do was Google it and lots of shops came up as selling it. I bought two grams for €40 (£35) and the postman delivered it to my front door two days later.

"By the end of the weekend it was all gone. It's very similar to the effect you get with coke but much cheaper, so now I only do meph. It's easier to get hold of because all you need is a debit card and internet access. It makes you talkative, happy, a bit spaced, energised and time seems to go faster. Because the high is short you end up using quite a lot. The comedown is worse than for any of the illegal stuff I have tried; it makes you really emotional and anti-social. But I feel safer using it and I know it's legal.

"I still hide the bag of meph in my shoe when I go out. To a bouncer it doesn't matter, they will assume it's coke. It would be frustrating to get thrown out when you are not on the wrong side of the law."

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