The government's chief drugs adviser says the UK is "floundering" in its attempts to control the online mephedrone market.
A BBC investigation has found that the illegal drug is still widely sold through websites despite the ban.
The Advisory Council on the Misuse of Drugs also said many new legal highs had flooded the market since mephedrone and naphyrone were banned this year.
Chairman Professor Les Iverson said it was difficult for these to be policed.
Naphyrone, which is sold as NRG1, was legal until being reclassified as a Class B drug after research from the ACMD showed it could be 10 times more potent than cocaine.
Responding to the BBC investigation into the online mephedrone trade, Professor Iverson said: "At the moment we're floundering. We haven't got adequate mechanisms to combat the internet crime. And it is internet crime if you're selling a banned substance."
Experts say it is now impossible to know what you are taking because as soon as one compound is outlawed, another slightly different one takes its place.
The ACMD has suggested a blanket ban on all chemicals similar to mephedrone and naphyrone.
He said: "I don't want to get to a situation where I have to go to the home secretary every month and ask for something else to be banned.
"It's a new highly profitable industry. It's a game between the chemical manufacturers who are obviously quite smart chemists, internet dealers and the law."
Critics say banning a substance could cause bigger problems.
Michael Linnell, from the drugs charity Lifeline said: "What we're in danger of is that nobody knows what the law is. You can't just ban your way out of a problem because it could result in far more dangerous chemicals coming onto the market.
"We're now in a situation where people are snorting white powder and they have no idea what it is and the people selling it don't know what it is either."
Most of the legal highs are manufactured in China and imported to the UK where they are sold as "research chemicals" or plant food. Dealers are able to get round the law by making sure they state substances are not for human consumption.
Experts say MDAI, a synthetic chemical that replicates the effects of ecstasy, will be the next legal high to take off.
In the investigation the BBC bought samples online and had them analysed by Dr John Ramsay, a toxicologist from St George's, University of London.
The samples did not contain MDAI, and were an unknown chemical that his team had never seen before.
He said: "This is now becoming very typical. Few of these chemicals are actually what they say they are and many also contain Class B drugs. So not only is there a health risk but you could also inadvertently be breaking the law.
"People need to realise these are chemicals and not drugs. They've not been tried or tested for human use in any way and nobody has any idea of the health consequences.
"In the short term you could get heart palpitations or even vascular collapse but there is also a risk that in years to come we could discover these have even caused birth defects. That's how dangerous they could be."
While the chemists are still one step ahead of the law, the challenge for the government is how to ban something when you don't know what it is.
A Home Office spokesperson said: "We need a new approach with our drug laws to move faster to protect the public. This is why we are introducing a system of temporary bans on new 'legal highs'. With the new temporary banning system, UK legislation will provide a robust model."
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