The number of addicts testing positive for heroin halved in the past three months, Europe's biggest drug testing company said today.
Concateno said just 21% of drug addicts tested positive for heroin in the first three weeks of the year, compared with an average of 45% between January 2009 and October last year.
But the firm said a "heroin drought" meant suppliers were diluting the drug with other substances and warned that addicts would face an increased risk of overdose if heroin purity returned to higher levels in the future.
The lack of pure heroin "can be attributed to a number of issues including a fungus that has blighted this year's poppy crop in Afghanistan, and greater law enforcement", Concateno said.
Peter Akrill, the firm's head of oral fluids analytical services, said: "This drop in positivity rates corroborates the belief among substance misuse professionals that the UK is currently experiencing a widespread 'heroin drought'.
"Our figures indicate a disturbing trend - with the true drug in short supply, users move to more adulterated forms.
"This situation creates a worrying cycle: a shortage in drug availability at street level can mean it is more likely to be cut with higher proportions of other substances as bulking agents.
"This reduces the active component of the opiate, and a regular user therefore has to ingest a greater quantity of the drug to achieve the same 'hit'.
"From experience, we know that when heroin then becomes available, there is a real and likely chance of increased overdoses and fatalities."