The House of Commons will today debate whether to rethink the war on drugs. While it is only a backbench business debate, and is therefore not binding, it still represents a step towards reviewing the UK’s drug laws. There is a simple reason why the UK’s drug policy is so expensive and ineffective: the law is so old. Policy is still dictated by the 1971 Misuse of Drugs Act, signed into law shortly after the 1971 UN Convention on Drugs. In 43 years since, the approach has failed catastrophically.

The evidence, both in Britain and the world, is becoming irrefutable. The war on drugs has not only been a failure but has been among the most self-defeating policies in history. Over $100bn a year is spent worldwide fighting the war on drugs. And to what end?

While worldwide crime is falling, the continued rise in drug crime is giving law enforcers plenty to do. Of seven types of crime studied by the United Nations Office on Drugs and Crime, drug possession and trafficking were the only two to have risen between 2003 and 2012. Up to 300m people used illicit drugs in 2012. In vast swathes of the world, it has never been easier to access drugs. Much of the danger comes from drugs being illegal and unregulated. In southwest Asia, 28.8 per cent of people who inject drugs have HIV. In total, around 180,000 people died in drug-related deaths in 2012, most of which were due to preventable overdose.

What is the definition of insanity? Spending over $1tn over 40 years period and arresting more than 45m people – while drug use remains unchanged – must come close. But that is what has happened in the United States since 1971. The war on drugs has led to the US penal population rising from 350,000 in 1972 to over two million today. The United States has the highest rate of imprisonment in the world. Egregious penalties, the result of the notorious "mandatory minimum" sentencing laws, mean that a black man in America today is more likely to be imprisoned than in apartheid South Africa. The percentage of all women incarcerated also increased by 757 percent between 1977 and 2004.

The UK is not immune to the destructiveness of the war on drugs. 2,955 people died from drug poisoning in 2013, despite more than £3bn being spent on tackling drug use. Crimes related to drugs cost the UK a total of £13.3bn every year. And, in a microcosm of worldwide trends, drug use has affected the poorest parts the most. The drug mortality rate is highest in the North East, the poorest region of Britain – over twice as high as in London.

But for all the focus on the deleterious impact of the War on Drugs in the West, its most significant impact has been to enrich Latin American drug dealers, devastating the countries in which they run amok. In Colombia alone, about 3,800 homicides per year are associated with the War on Drugs.

The policies of the West amount to a simple trade off: mass violence and corruption in large swathes of South America and Asia in exchange for a slight reduction in drug use in the West. As a report from the LSE Expert Group on the Economics of Drug Policy this year asserted, “prohibitionist drug policies are a transfer of the costs of the drug problem from consumer to producer and transit countries, where the latter are pushed to design and implement supply-reduction policies.” The six countries in the world with the highest murder rates are all in the global hub of drug trafficking in Latin America and the Caribbean. The drug trade has funded guerrilla insurgent groups like FARC in Colombia and the Shining Path in Peru.

Governments are beginning to realise the destruction wreaked by the War on Drugs. Uruguay led the way by last year legalizing the cultivation, sale and consumption of marijuana. Jamaica is currently drafting legislation to decriminalise marijuana. A host of political leaders, including Juan Manuel Santos, the President of Colombia, and Otto Pérez Molina, the President of Guatemala, have also railed against the destructive orthodoxy on drugs.

Parts of the West are doing so too. Two years ago Colorado and Washington voted to legalise the sale of marijuana. Fifteen other states have some form of marijuana decriminalisation law. “The trend towards cannabis regulation seems to be irreversible,” says Martin Jelsma of the Transnational Institute.

In 2001, Portugal decriminalised the personal possession of all drugs, making low-level drug possession as an administrative rather than criminal offence. Enough time has passed to judge the policy a resounding success. Money saved on enforcing the law has been invested in treatment for addicts. After 13 years, drug use in Portugal is less than the European average, and has actually declined among under-25s – those most at risk of addiction. Overall drug use among the population has ticked up slightly, but the health benefits have been dramatic. The number of newly diagnosed HIV cases among people who inject drugs fell from 1,016 to 56 between 2001 and 2012. The number of drug-related deaths has fallen from 80 in 2001, to 16 in 2012, while the government has saved huge sums from the reduced demand on prison places.

The claim of the 1998 UN Convention – “A drug-free world, we can do it!” – has never looked more absurd. As the LSE Expert Group report asserts, “Decades of evidence conclusively show that the supply and demand for illicit drugs are not something that can be eradicated. They can be managed, either well or badly.” If Portugal is a flagship of the more enlightened approach, the opposite remains true in the UK.

Yet at least this reality is being recognised. Today’s debate on drug policy is the result of a petition started by the Green MP Caroline Lucas, which attracted over 130,000 signatures. Nigel Farage also advocates the decriminalization of drugs, while 67 per cent of the UK population supports a review of drugs policy options.

For politicians in office, tackling the lunacy of the war on drugs suddenly becomes less appealing. “Privately, ministers and shadow ministers will all say the current policies are ridiculous and don't work,” says Baroness Meacher, the chair of the all-party parliamentary group on Drug Reform. “Politicians when in power lose courage.” She says that Nick Clegg has been “more clear than any other senior politician” on the need for reform, suggesting a coalition involving the Lib Dems might offer the most likely path to the UK’s drug policies being reviewed.

Politicians who judge that the expedient action is to do nothing should remember that the biggest beneficiaries of rethinking the approach to drugs would not be casual drug-users in the UK. It would be millions of the poorest people in the world, whose lives are decimated by the impact of the war on drugs.