A form of high-grade white heroin is making a comeback in the UK, despite having virtually disappeared during the 1980s and 90s, officials have said.
Heroin hydrochloride was common in the 1970s but was replaced by more well-known "brown" heroin.
The Serious Organised Crime Agency (Soca) has warned of a UK resurgence in the drug smuggled in from Afghanistan.
Soca said there have so far only been a few small seizures of white heroin, but they wanted warn of its return.
Soca deputy director Steve Coates said: "We're not panicking but we have noticed the re-emergence of this white heroin which virtually disappeared in the 80s and 90s."
Mr Coates said there had been a "seismic change" in the origin of heroin supplied to the UK.
He said: "I would estimate over 92% of heroin used in the UK and probably throughout Europe comes from Afghanistan.
"Back in the 70s heroin in the UK was mainly Chinese and south-east Asian."
He said there had been seizures in Afghanistan and Turkey, as well as a haul last year of £5.5m of heroin, including white heroin, concealed in straws sewn into the weave of Afghan rugs.
Mr Coates said Soca's aim is to raise the alarm among police, drugs charities and users about the return of white heroin and its dangers.
He added: "We've identified this as a potential threat. We're not over-egging it. We simply want to let partners know we've registered this and to keep it on the radar.
"There's been a definitive change in the market and we can see the way this has moved on. One of our duties is to publicise that."
Mr Coates's view that the return of white heroin should not bring panic, but awareness, was reiterated by Gary Sutton, head of drug services at drugs charity Release.
He said the threat of cardiac arrest through taking white heroin, as depicted in the film Pulp Fiction, was possible because snorting white heroin carries a higher risk of overdose than smoking brown heroin.
Source - http://news.bbc.co.uk/1/hi/uk/7863352.stm