Online chatrooms and bulletin boards have been alive with the discussion of two new substances in particular – mephedrone and methylone – which are said to mimic their illicit counterparts. Users say mephedrone, which is nicknamed "meph", is a cross between cocaine and MDMA with an almost instant relaxing, yet stimulated high, and can easily be bought online.

The stimulant is described as two molecular "tweaks" away from MDMA and crystal meth.

One user told drug centre's magazine Druglink: "I pretty much stopped buying coke and pills and crystal [MDMA] once I found meph.

"I'd just make a bulk order and send off the payment and the package would arrive a few days later.

"Many people are sceptical that something that's legal can actually work. But it does."

Mephedrone is legal in the UK because it is not derived from any of the banned Class A drugs.

It comes instead from a compound of cathinone, which is a Class C drug, and derivatives of this drug are not currently controlled.

Mephedrone is made in Chinese laboratories and imported legally to the UK where it is often marketed as plant food, but also appears on sites which sell drug paraphernalia.

One UK website claims it had just imported 2kg.

Although mephedrone is said to lower social inhibitions, increase talkativeness and create euphoria, other have reported strange side effects.

One user said: "I did eight grams of meph over the weekend. My heart is still beating strangely and my mouth has all the skin peeled off on the inside."

In December last year, an 18-year-old university student in Sweden was reported to have died after taking mephedrone and the country has now outlawed the chemical.

Dr John Ramsey, a toxicologist at St George's University London, is also director of Tic Tac Communications, a drug analysis body that studies recreational drugs.

He told Druglink that the use of new drugs such as mephedrone was a cause for concern.

"People end up in A&E because they take stimulants, and they end up in A&E because they take these chemicals. Their acute effects are pretty much indistinguishable from MDMA and amphetamines.

"But we have no information on their toxicity because they have never been used as drugs until recently, so there have not been any formal studies.

"The only real reports we get are from people who either write them up in chatrooms or discussion groups, or end up in A&E."

Dr Ramsay said the economic benefit for sellers was enormous: "People are making absolutely huge amounts of money out of these drugs.

"They are selling at higher prices than tablets of ecstasy at the moment, and their manufacture is straightforward.

"There will always be something on the horizon that is falling outside the legislation. The law cannot keep up with what is happening on the street."

The Home Office said mephedrone was currently legal. "It has not been reported as a problem in the UK to the best of our knowledge.

"The Advisory Council on the Misuse of Drugs keep these matters under constant review and advises the Government accordingly."