I agree with David Cameron: not a statement you should become accustomed to gracing this column. Well, to clarify, the earlier incarnation of the prime minister, in any case. “I ask the Labour government not to return to retribution and war on drugs,” he pleaded back in 2002. “That has been tried and we all know that it does not work.” His wisdom was not restricted to word, but was evident in deed, too. As a member of the home affairs select committee on drug misuse, he voted to consider “the possibility of legalisation and regulation” of drugs.

Neither was this flirtation with reason and common sense short-lived. When he stood for the Tory leadership in 2005 – back in the days of fluffy huskies and hug-a-hoodie Conservatism – he sagely declared that “Politicians attempt to appeal to the lowest common denominator by posturing with tough policies and calling for crackdown after crackdown. Drugs policy has been failing for decades.”

In contrast to the nation’s deficit, Cameron’s rational, evidence-based approach to drugs has disappeared. The self-evidently catastrophic war on drugs – an unforgivable waste of life, wellbeing, treasure and time – is now to be intensified. A new crackdown on “legal highs” will now mean the criminalisation of, among other substances, poppers, a substance particularly popular among gay men interested in enhancing their sex lives.

Now, the private lives of politicians should be exactly that, unless an attitude of “Do as I say, not as I do,” comes to light.

There are members of this government who have dabbled with drugs – one of the few experiences they share with millions of non-privileged people in this country – even as they criminalise others who do so. Cameron himself has refused to answer questions over his own past consumption, glibly replying that everyone was allowed to “err and stray” in their lives.

Except, of course, they are not. If you are a black Londoner, you are more than six times more likely to be stopped and searched for drugs, and five times more likely to be charged if possessing cannabis. To pre-empt the inevitably prejudiced arched eyebrows, the research shows that black people are significantly less likely to use drugs.

What a country to live in. Ministers with pampered backgrounds can dismiss past flirtations with drugs as youthful japes while promoting legislation that criminalises working-class black men, damaging their lives and future prospects.

Where is the courage displayed by David Cameron Mark I? The Lib Dem leadership candidate Norman Lamb has done us all a service by calling for the legalisation and regulation of cannabis.

It is an eruption of common sense that must not go to waste. As a London School of Economics report highlighted last year, the 44-year-old so-called war on drugs has achieved nothing, except “mass incarceration” in the US, repression in Asia, “vast corruption and political destabilisation” in Afghanistan and west Africa and the spread of HIV in Russia. “The strategy has failed based on its own terms,” the report declared. “Evidence shows that drug prices have been declining while purity has been increasing.”

We have a choice. Do we leave drugs in the control of murderous gangs or regulate them and stop locking up youngsters?

We have a clear choice. Do we leave drugs in the control of murderous drug gangs who destabilise entire nations – or do we regulate them, bring in tax revenues, and stop locking up harmless youngsters?

Look at Mexico: a country partly destroyed by the “war on drugs”. Since an all-out military offensive was unleashed in 2006, around 100,000 Mexicans have perished and perhaps 20,000 are missing.

US policy has played an important role: the country has spent billions on helping to turn Mexico into a failed state. Much of this money has been splashed out on a state apparatus heavily infiltrated by the cartels. The signing of the North American free trade agreement in 1994 was good for US big business, but bad news for Mexican farmers who were impoverished when the country was flooded with US agricultural goods, leading some to turn to cultivating drugs instead.

Isis is rightly a source of horror, but one wonders if it looks to the cartels for inspiration. These cartels, too, behead hundreds of people, sometimes posting their sickening murders on social media; they publicly display their horribly mutilated victims to terrorise others; they recruit child soldiers and traffic women. It is a tale of horror replicated across the globe. And in Afghanistan, more land is being used to cultivate opium poppy than when the Taliban were overthrown, despite $7bn (£4.6bn) of US gold wasted on counter-narcotic operations. There are warnings that the country is on the brink of becoming a “narco-criminal state”, with opium-related trade worth around 14% of Afghanistan’s GDP.

In west Africa, warns the UN’s former secretary general Kofi Annan, “drugs may have destroyed many people, but wrong governmental policies have destroyed many more”.

Over in the US, about half of the prisoners caged for drug offences are black, their employability and lives often trashed as a consequence.

This is madness. Let us look – as Normal Lamb suggests – to the state of Colorado, where the possession of small amounts of marijuana for recreational use has been legalised since 2012. Let’s look at the example of Uruguay, where the government has seized control of cannabis from the drugs gangs and now runs it as a state monopoly.

Yes, politicians who abandon the failed mantra of the drug war risk the incandescent rage of the Daily Mail. But how many lives have to be lost – or simply ruined – before reality and common sense finally prevail? Rather than expanding the efforts of a disastrous policy, the old failed approach must finally be abandoned. An earlier David Cameron would have agreed. It is a tragedy the current incarnation does not.