Russian epilepsy medication is being increasingly used in the UK as a substitute for illegal drugs, scientists have warned.
Phenazepam is a psychoactive also used to treat conditions like insomnia and alcohol withdrawal syndrome.
But researchers at Dundee University said they had detected a "trend of misuse".
The drug is not controlled in the UK, most of Europe or the US - so can be purchased legally over the internet.
Phenazepam is available on prescription in Russia and many other CIS states. Reports from Sweden, Finland, and the US suggest it is being used illicitly in place of similar drugs like diazepam.
The Dundee team said they had found nine cases since January 2011 where postmortem blood samples had contained phenazepam.
Dr Peter Maskell, a lecturer in forensic toxicology at Dundee, said the discovery suggested use of the drug was on the increase in the UK, but stressed that it could not be directly identified as the cause of death in any of the circumstances.
He said: "It would seem it is increasingly being used as a replacement for other drugs, most notably diazepam, because we are seeing more instances of its use.
"Whether that is actually a deliberate switch on the part of users or because it is what dealers are selling is unclear at this stage.
"Like other benzodiazepines, phenazepam can be addictive and mixing with other drugs such as heroin or alcohol increases the risk of drug interaction."
Concern over illicit use of the drug in the UK was raised in 2010, when three people in the East Midlands and six people in Scotland were admitted to hospital after phenazepam overdoses.
These cases and increased seizures of the drug by police led the Scottish government to issue warnings about phenazepam.
The Dundee team began screening postmortem blood samples for phenazepam from the end of January 2011. In each of the nine cases detected, the victims had a history of drug use.
Dr Maskell added, "Although we have detected use in nine cases, phenazepam cannot be directly identified as a cause of death in any of them. There is a key difference between this drug and other legally available substances which have hit the headlines in recent years.
"This is not a party drug likely to be consumed by casual users but is more likely to be seen in persons with a history of misuse, often with harder drugs such as heroin, methadone and other opiates."