Drug addicts craving heroin are increasingly turning to the tranquilliser once known as Valium for a fix, it has been claimed.
Drug information charity DrugScope said larger numbers of people are using diazepam as an alternative to heroin.
The tranquilliser, once known as "mother's little helper", used to be 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 towns and cities across the UK covered by the research. A 10mg 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."
"Recently I have noted that some individuals on methadone maintenance are using diazapam, particularly during the stabilisation phase. This seems to be on analysis due to the fact that it is percieved as a harmless alternative to heroin or bupronorphine. It is an area which needs addressing as part of their substance misuse treatment" – Cathy Hardy, HMP Stocken