Many people who have already heard of Johann Hari have done so for what is now the wrong reason: the Swiss-Scottish reporter caught in a plagiarism scandal, obliged to return his 2008 Orwell prize and leave the Independent. But Hari, shamed, ploughed his furrow as a campaigning writer on the manifestly failed “war on drugs”; here is the prodigal fruit of that work, and with it redemption, if that was needed.

This long-awaited history of the “war” – which imprisons millions and persecutes more – begins with an enthralling account of the obsession of an obscure American public official called Harry Anslinger with the heroin addiction of Billie Holiday, and how his fear of jazz and all things black spurred him to launch the prohibitionist crusade which now fills the air above Latin America with helicopter gunships and jails worldwide with wretched victims.

Hari’s journey is personal: that of a former Stonewall journalist of the year who has lost friends to addiction, but his rage is reported to a fault. He mines archives to sketch the life of the first real narco baron, Arnold Rothstein, and later enters a US jail to interview a former executioner for Rothstein’s most savage descendants, the Mexican Zetas cartel, who crossed the border to give himself up when he couldn’t take it any more.

Hari savages American and British policy, which criminalises and jails drug addicts, for both its odious morality and cruelty. He describes the hellish world of a female chain-gang, incarcerated and supposedly “penitent” – forced to wear slogans like “I’m Breaking the Need for Weed”. They work in the Arizona desert where one of them was “cooked” to death after collapsing of heat and exhaustion

He aptly summarises the international application of this mentality: “The US government has approached Mexico with the same threat as the cartels – plata o plombo. Silver or lead. We can give you economic “aid” to fight this war, or we can wreck your economy if you don’t.

We meet heroes like Bruce Alexander who treats and cures addicts – and even increased average life expectancy in Downtown Eastside Vancouver – on the incontestable principle that “the core of addiction doesn’t lie in what you swallow or inject, it’s in the pain you feel in your head”. Hari finds a similar project in Birkenhead which, by prescribing “heroin reefers”, saved and nurtured lives on Merseyside.

Hari demonstrates beyond rebuke that “if you arrest a large number of drug dealers, drug-dealing doesn’t go down”. But it did on the streets around the projects in Vancouver and Birkenhead.

Hari’s searing argument should not need celebrity blurbs from Elton John, Stephen Fry and Noam Chomsky to promote it – it is self-evident. But although it is absurd to expect any book to cover the entire ground, one finishes this excellent volume with two crucial questions unaddressed. The first concerns how the “war on drugs” is actually one against addicts, and in no way against the profits of drugs. The Wachovia and HSBC banks were caught, and admitted – with impunity apart from a ticking-off and token fine– laundering hundreds of millions of dollars in drug profits. Drug and blood money still swills into the coffers of other banks, but no one from HSBC went to jail, and nor will their executives – the punishment meted out to Lord Stephen Green, chief executive of HSBC during the laundering, was promotion to the British cabinet.

The second omission concerns how legalisation of hard drugs would actually work – a general failing of the movement which seeks solutions in legal and social policy rather than political economy. Legalisation would no doubt suit places such as Vancouver, New York or Liverpool. But how would it work in wretched barrios around the cities of central and South America, townships of Africa and eventually dormitory towns of China and Bangladesh? Hari insists that “responsible drug use is the norm, not the exception”. He reports a UN statistic that “only 10& of drug users have a problem with their substance. Some 90% of people who use a drug – the overwhelming majority – are not harmed by it.” But this is not the whole story in the desperately poor, wider world that services the countries where Hari’s book is set. On the outskirts of Tegucigalpa, Rio or Cape Town, people take drugs not like they do at the hedonistic “drug party on a vast scale” Hari attends around a Greek temple, but to self-obliterate because life is intolerable. In these accursed places, the Hari/UN statistics – which may well be true of Camden or Downtown Eastside in Vancouver – don’t explain the ubiquitous, irredeemable misery wreaked by the savage entwinement of abject poverty and drug use.

What would all those women’s groups and priests who work in overwhelmed detox clinics think if leading legalisers such as Richard Branson and Geoffrey Robertson QC arrived by helicopter in the crack- and meth-addicted colonias of Ciudad Juarez and other smuggling entrepots along the US-Mexico border – where people drink from the deadly river that flows through – to assure them: “Everything’s fine… it’s all available for sale at Superdrug!”? They’d be incredulous and despairing.

Because if hard drugs are legal, who is going to make them? Presumably the experts who already do, working not for narco syndicates but Big Pharma, another kind of cartel. And do we really trust Big Pharma to manufacture methamphetamine and process crack or heroin in order to sell as little as possible in the developing world? That’s not how Big Pharma works; that’s not how capitalism works.

None of this detracts from the book’s argument, or the righteous movement of which Hari is an estimable spokesman. But the movement hasn’t thought it through and nor does the book.

With it, though, Hari affirms the role he has already established for himself: a crucial voice in, as well as commentator on, the urgent cause of not merely “reforming” the way society deals with the drugs crisis but tearing it up completely – and either starting again along an entirely different track, or else becoming overwhelmed by the eventually inevitable mass addiction of the new wage-slaves, the global assembly plant and lumpen proletariats, to hard drugs.

Ed Vulliamy is author of Amexica: War Along the Borderline, winner of the Ryszard Kapuściński award for literary reportage.

Chasing the Scream by Johann Hari is published by Bloomsbury Circus, £18.99. Click here to buy it for £14.99