The tranquilliser once known as "mother's little helper" is growing in popularity among drug addicts as an alternative to heroin, it was claimed today.
Drug information charity DrugScope said larger numbers of people are abusing diazepam, which is better known under its defunct brand name Valium.
The tranquilliser - which was launched by Swiss pharmaceutical company Hoffmann-La Roche in 1963 - was once one of the world's most widely prescribed drugs, and was associated with the acceptable, suburban face of drug-taking.
Now addicts are combining it with strong alcohol or methadone in a potentially lethal cocktail, particularly to ease the effects of withdrawal from crack cocaine, DrugScope warned.
Diazepam - nick-named "blues" or "vallies" - may also be growing as a cheap alternative to heroin, because supplies of heroin have suffered a recent slump in purity, according to a survey by the charity.
Usage was rising in 15 out of 20 town and cities across the UK covered by the research.
A 10 milligram dose of diazepam costs just £1, the charity found.
DrugScope chief executive Martin Barnes said: "The rise in the use of illicitly imported diazepam is concerning, particularly as drug users face a high risk of overdose when using the drug in combination with other drugs such as methadone and alcohol.
"With the proliferation of counterfeit diazepam comes unpredictable quality and strength."
He added: "A heroin shortage might instinctively appear a positive development, especially as it can lead to more people entering treatment, but it can bring its own problems.
"Users may be more inclined to inject rather than smoke the drug during times of shortage or poor quality.
"And there is a higher risk of overdose when the heroin market readjusts to more normal levels of supply and quality."
The charity's annual Street Drug Trends Survey compiles information from 100 drug services, drug action teams and police forces to give a snapshot of the national picture.
Police and Customs seizures of diazepam have rocketed from 300,000 pills seized between July 2003 and June 2006 to two million between July 2006 and June 2008, DrugScope said.
National average street drug prices have remained "relatively stable" over the past 12 months, the survey found.
Skunk cannabis grown at home on a commercial scale dominated the cannabis market in many areas, with some users finding it hard to obtain lower-strength resin or imported herbal cannabis, it added.
The price of heroin per gram has risen from £46 in 2006 to £49 this year, it said, and cocaine has dipped by £1 a gram to £42.
Over the same period, the cost of an ecstasy pill has dipped from £3 to £2.30, while the veterinary tranquilliser ketamine - which has been growing in popularity on the drug scene for several years - is now £20 a gram compared with £28 two years ago.