There has been a dramatic decrease in the amount of heroin in the UK, BBC News has learned.
The Serious Organised Crime Agency claimed the drop was due to supply routes being cut off and said the street price of the drug had doubled.
Europe's biggest drug testing company, Concateno, said the number of addicts testing positively for heroin had halved in the past six months.
However, it warned that suppliers were diluting heroin with other drugs.
If heroin purity returned to normal levels, addicts would face an increased risk of overdose.
There are an estimated 300,000 users of heroin, which comes mainly from Afghanistan and Pakistan, in the UK.
Concateno takes swabs from anyone on a court testing order or in drug treatment for heroin.
Its figures show positive tests occurring in up to 48% of cases over recent years, but last December the rate fell to 22%.
The suggestion of an interruption in the supply is backed up by police reports of shortages in several places in the UK.
Officers from the Serious Organised Crime Agency (Soca) point to both the efforts of law enforcement agencies and environmental factors such as the floods in Pakistan.
And they say exaggerated perceptions among traffickers of a poppy blight may have led dealers to hold their supplies back and create higher prices on the street.
Relationships between Soca and the Turkish police are also said to be strengthening, leading to pressure on what has been a key supply route.
The drug is now said to command £40,000 a kilo - double the price of last year.