The United Nations general assembly has gathered in New York for a special session to discuss how the world’s nations can together combat the global drug problem. Deciding on an appropriate strategy will probably be a matter of fierce debate – but even the terminology to be used remains contentious.

In 2009, the Obama administration said that the term “war on drugs” was counterproductive because it made some people feel as though they were being targeted.

Since Richard Nixon declared the war on drugs in 1971, it seems as though people rather than products have been most directly affected. But lack of data makes it hard to understand the impact: like most illicit activities, drug production, trade and use is hard to measure accurately. And without knowing baseline values, it’s hard to understand the effect of any given policy – let alone comparing the impact of various policies.

However, where long-term data is available, it does point to systematic failures in drug policies. A study published in the British Medical Journal in 2013 found that despite efforts to limit the supply of these drugs, since 1990 prices have fallen while the purity of the drugs has increased. The trends were similar in the US and in Europe. The authors’ conclusion was clear: “These findings suggest that expanding efforts at controlling the global illegal drug market through law enforcement are failing.”

Price and purity of various drugs. Photograph: British Medical Journal

More specifically, the researchers found that the average inflation-adjusted (and purity-adjusted) prices of heroin, cocaine and cannabis fell by 81%, 80% and 86%, between 1990 and 2007 while average purity increased by 60%, 11% and 161%, respectively.

Internationally, the illegal drugs trade is larger than any other illegal activity. According to a 2011 report by Global Financial Integrity, illicit activities are worth $650bn worldwide, of which $320bn is due to drugs alone. Other high-value illegal trades were counterfeiting (worth $250bn) and human trafficking ($31.6bn). However, those statistics are severely limited.

It’s easy to assume that statistics published in the same year are comparable – tempting, but wrong. That estimate of the value of the international drugs trade comes from 2005 while the figure for counterfeiting comes from 2009. The truth is, no one knows the precise value of the international drugs trade, although it is clear that the returns are sufficiently lucrative that some individuals will want and need to accept the risks.

Those risks include punishment by the justice system. However, there too the efficacy of policy is highly questionable. The Brookings Institution published an overview of global counter-narcotics policy in 2008 which compared three different approaches: “the punishment model” employed in the US, which uses incarceration to deter use; “the depenalization model” used in Italy and Spain, which keeps narcotics illegal but doesn’t criminalize personal use within established maximum amounts; and, finally, “the decriminalization model” used in the Netherlands, which allows cannabis sales for personal use.

The Brookings analysis was most critical of the US punishment model where incarceration rates have exploded (from fewer than 50,000 people in 1980 to 210,200 in 2015) at huge cost to taxpayers despite the fact that few prisoners have access to any form of drug treatment.