A drug for epilepsy and anxiety is increasingly being misused by young people in the UK, researchers say.
Phenazepam was developed in the 1970s for the treatment of epilepsy, alcohol withdrawal syndrome, insomnia and anxiety.
It is a prescription-only drug in several former Soviet Bloc countries, and is not controlled in the UK, most of Europe, or the US.
As a result, people have been buying it legally over the internet, leading to reports of misuse in the UK, Sweden, Finland and the US.
In a letter to the British Medical Journal (BMJ), researchers from the University of Dundee said phenazepam was being used as a substitute for illegal drugs.
They have identified nine UK cases since January in which post-mortem blood samples contained phenazepam.
It is not known if the drug is directly responsible for any deaths.
Dr Peter Maskell, a lecturer in forensic toxicology at Dundee University, said: "This many cases suggests that the use of phenazepam by drug misusers in the UK is on the rise.
"Phenazepam can be obtained legally on the internet so it could become more widely used as a substitute for controlled benzodiazepines or designer drugs.
"It is important to note there is no evidence that people are taking more drugs as a result of the availability of phenazepam.
"Rather, 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 dealers are selling it 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."
In 2010, three people in the East Midlands and six people in Scotland were admitted to hospital after overdosing on phenazepam.
In each of the nine most recent cases identified in Dundee, there was a history of drug misuse. All cases involved men and women aged 31 to 45.
The cause of death was listed as adverse effects of opiates in seven cases, and non-drug related causes in two.
Dr Maskell said: "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."