Diazepam, formerly known as Valium, is prescribed for insomnia and anxiety
An increasing number of drug users are turning to the tranquilliser diazepam, a drugs information charity claims.
DrugScope says the popularity of diazepam - formerly known as Valium - is rising among drug users in 15 out of 20 UK towns and cities it surveyed.
Diazepam is being used as a heroin substitute, and often taken alongside alcohol and methadone to ease the comedown from crack cocaine, it said.
But combining it with other drugs leads to a high risk of overdose, it warned.
The annual survey questioned 100 drug services and police forces in 20 towns and cities across the UK.
Diazepam is a Class C drug in the UK, prescribed to reduce anxiety and tension and legally available on prescription only.
On the street it is nicknamed "blues" or "vallies" and a 10mg tablet is available for £1, Drugscope found.
Its rising popularity is thought to be down partly to its cheapness, partly to it becoming more easily accessible - and partly because of a drop in the availability and quality of heroin.
There has been an unexpected heroin shortage - local fluctuations have occurred despite the bumper harvest in Afghanistan
Harry Shapiro, DrugScope
Diazepam from legitimate sources in Europe is being smuggled in, and it is also being bought over the internet from India, Pakistan and Thailand. Some of the latter is counterfeit, the charity warned.
While 300,000 diazepam pills were seized by police and customs between July 2003 and June 2006, in the period between July 2006 and June 2008 seizures rose to two million.
Harry Shapiro, DrugScope's director of communications, said there had also been an "unexpected heroin shortage".
"Local fluctuations have occurred despite the bumper harvest in Afghanistan, which illustrates the complexity of the illicit drug market," he said.
But mixing diazepam with other drugs, especially alcohol, is a "potentially lethal combination", he said.
The drug is also highly addictive, and withdrawal can take months.
'Complex' drug market
In other trends, DrugScope said "poly" drug use - taking combinations of drugs such as alcohol, cannabis, cocaine and ecstasy - was a significant problem among some young people.
Drugscope Chief Executive Martin Barnes on the drugs popularity
It said the price of street drugs remained relatively stable over the last 12 months.
It said cocaine was still popular, with a "two-tier" market in which low quality powder was sold for less than higher quality powder.
In the ecstasy market, there was an increase in the amphetamine-like drug BZP replacing MDMA as the ingredient in pills.
The Advisory Council on the Misuse of Drugs is considering the drug BZP in order to provide advice to the government on its classification.
And for cannabis, commercially home-grown skunk dominated the market in many areas, with users finding it hard to obtain lower strength resin or imported herbal cannabis.
Drugscope also found that in many areas, young Class A users who could not afford heroin or cocaine at any one time were turning to alcohol such as super-strength lager, cheap vodka and cheap white wine.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.