Drug addicts are reportedly swapping heroin for the tranquilliser diazepam or Valium because it is cheaper. Drug information charity DrugScope said larger numbers of people are abusing diazepam. It warned that addicts are combining it with strong alcohol or methadone in a potentially lethal cocktail, particularly to ease the effects of withdrawal from crack cocaine. 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 is 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 said. The tranquilliser - originally launched in 1963 - was once one of the world's most widely prescribed drugs, and was associated with the acceptable, suburban face of drug-taking. 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." 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.