Since 1990, the street price of illegal drugs has fallen in real terms while the purity/potency of what's on offer has generally increased, both of which are indicators of availability.

The United Nations recently estimated that the illicit drug trade is worth at least US $350 billion every year. And needle sharing is one of the key drivers of blood borne infections, including HIV. The drug trade is also linked to high rates of violence.

Over the past several decades most national drug control strategies have focused on law enforcement to curb supply, despite calls to explore approaches, such as decriminalisation and strict legal regulation.

The researchers analysed data from seven international government-funded drug surveillance systems, which had at least 10 years of information on the price and purity of cannabis, cocaine and opiates, including heroin.

They also reviewed the number of seizures of illegal drugs in drug production regions and rates of consumption in markets where demand for illegal drugs is high.

Three of the seven surveillance systems reported on international data; three reported on US data; and one reported on data from Australia. In some cases the data went back as far as 1975, with the most recent data going back to 2001.

Three major trends emerged from the data analysis: the purity/potency of illegal drugs either generally remained stable or increased between 1990 and 2010; with few exceptions, the street price generally fell; and seizures of drugs increased in both the countries of major supply and demand.

In the US, after adjusting for inflation and purity, the average street price of heroin, cocaine and cannabis fell by 81%, 80%, and 86%, respectively, whereas the purity and/or potency of these drugs increased by 60%, 11%, and 161%, respectively.

Similar trends were observed in Europe where, during the same period, the average price of opiates and cocaine, adjusted for inflation and purity, decreased by 74% and 51%, respectively, and in Australia, where the price of cocaine fell by 14% and the price of heroin and cannabis dropped by 49%.

In the US seizures of cocaine roughly halved between 1990 and 2010, but those of cannabis and heroin rose by 465% and 29%, respectively; in Europe seizures of cocaine and cannabis fluctuated, but seizures of heroin had risen 380% by 2009.

On the basis of the data, the authors conclude, as previous studies have, "that the global supply of illicit drugs has likely not been reduced in the previous two decades."

They add: "In particular, the data presented in this study suggest that the supply of opiates and cannabis have increased, given the increasing potency and decreasing prices of these illegal commodities."

And they conclude: "These findings suggest that expanding efforts at controlling the global illegal drug market through law enforcement are failing."

"It is hoped that this study highlights the need to re-examine the effectiveness of national and international drug strategies that place a disproportionate emphasis on supply reduction at the expense of evidence based prevention and treatment of problematic illegal drug use," they add.

In an accompanying podcast, co-author Dan Werb says that other indicators of the effectiveness of illegal drug policies are needed, such as rates of HIV related transmission.

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