Deaths linked to legal highs may just be the first signs of the damage they do, according to a medical expert.
In total 43 people in the UK died after taking now-outlawed methcathinones in 2010, the National Programme on Substance Abuse Deaths said.
This compared with five deaths in 2009, its report said.
But Dr Owen Bowden-Jones, founder of the Club Drug Clinic at Chelsea and Westminster Hospital, said: “We may just be seeing the tip of the iceberg.”
A BBC investigation revealed untested chemicals were being sold in London.
“The concern I have is that in two or three years, we’ll begin to see the consequences of the harm that’s going on now,” Mr Jones added.
Dr John Ramsey, a toxicologist at St George’s Hospital, described how there are dozens of types of untested and sometimes deadly chemicals that are being sold legally in London.
Dr John Ramsey said users could not know the risks
“We put a urinal in Wardour Street in Soho and found 60 different drugs in there,” he said.
“I think these are probably the first people who have taken these compounds.
“They’ve never been evaluated as drugs anywhere in the world before,” he added.
“They can’t possibly know the risks of the compounds they’re taken.”
He described how “these things are made in China, shipped over here, a kid buys a gram and takes it”.
As part of its investigation, BBC Inside Out legally ordered £640 of chemicals from China, packaged in a bag labelled: “Harmful if swallowed. May damage an unborn child. Avoid breathing dust and fumes. If exposed, call a poison centre”.
Experts told the programme these are the type of chemicals that are sold in 1g bags for £15 each in London – meaning the £640 of chemicals from China could have a London street value of £15,000.
Association of Chief Police Officers (Acpo) drugs spokesman Tim Hollis said: “Kids are sending around party invites with a link on where to buy your drugs.
“The Home Office and police find that extremely difficult to get our heads around and we are flat footed.”
Baroness Molly Meacher, of the Parliamentary Drug Policy Group, said: “The UK Border Agency has got great hangars full of little packets of white powder.
“They just simply haven’t got the technology, they haven’t got the money, they haven’t got the resources.
“They don’t know what’s in all those packages.”
The Metropolitan Police and UK Border Agency (UKBA) declined to be interviewed by BBC Inside Out.
But Dr Ramsey said: “Controlling substances… all it does is spawn the production of one that isn’t controlled.
“We’re damned if we do and damned if we don’t.”
“I went in to Tesco naked and assaulted a police officer – they asked me what are you on? ”
Katie Wilson, Former Benzo Fury user
Katie 19, from east London, described her experience of taking now banned drug Benzo Fury.
“I got a lot of euphoria, happiness, I presumed like ecstasy,” she said. “After the Benzo had worn off, I swore I was going to die.”
“I went in to Tesco naked and assaulted a police officer.
“They asked me what sort are you on? Heroin? Crack? Crystal meth?”
Maryon Stewart’s daughter Hester died after taking the now banned substance GBL.
Ms Stewart has since set up the Angelas Foundation, which highlights the risks of legal highs.
“There are kids dying every week. The youngest I’ve heard of is 14 years old,” she said.