So-called "legal high" drugs are popular with clubbers
A new family of drugs offering users a "legal high" has been found by researchers in London.
They are based on cathinone, the active ingredient in the plant khat, commonly used as a stimulant in East Africa.
Although cathinone is a controlled substance, scientists say these new compounds have been deliberately modified to circumvent the law.
Expert John Ramsey told the BBC: "Presumably [users] think they're safe, and they're probably not."
The new breed of drugs are being sold for between £3 and £6 a pill.
Mr Ramsey is director of TIC TAC Communications based at St George's Hospital in south London.
He analyses the contents of legal high pills and other substances found in nightclub "drug amnesty" bins.
"The very latest ones we have seen are cathinones," he said.
These individuals present with clinical features that are very similar to established recreational drugs such as ecstasy, amphetamine, cocaine and ketamine
Dr Paul Dargan Guy's and St Thomas' Poisons Unit
"They are being sold as a safer alternative to ecstasy and I guess that there are a group of people who really want to engage with the dance scene and engage with their friends and don't want to break the law.
"Presumably they think they're safe, and they're probably not."
There is a booming market for "legal high" drugs in the UK. They are sold in so-called "head shops" that deal in drug paraphernalia, as well as at music festivals and over the internet.
It is not known how many of the pills are sold in the UK, but one recently disbanded website supplying them told drug researchers they were shipping more than 3,500 doses into the country every day.
Most legal high pills are based on a group of drugs called piperazines, of which BZP is the most common.
These will be designated as controlled substances in the UK early next year under a European directive, but there is evidence that those who market them are already trying to avoid the attention of the law.
Pills analysed at the TIC TAC laboratory, which were purchased from a UK-based website, were marked "BZP and piperazine-free", but in fact when the contents were examined they were found to contain both substances.
And the latest discovery of cathinone-based pills suggests that manufacturers are aware of the coming changes to the law and are trying to find ways around it.
Doctors at Guy's and St Thomas' Poisons Unit have seen the effects that legal highs can have on some users.
Dr Paul Dargan told the BBC: "These are individuals who present with clinical features that are very similar to established recreational drugs such as ecstasy, amphetamine, cocaine and ketamine."
Those clinical features can include rapid heart rate, raised blood pressure, high temperatures and seizures.
Ahead of the classification of BZP, the Medicines and Healthcare Products Regulatory Agency said it was pursuing 12 cases involving piperazine-based pills.
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