Watching Bob Dylan at Slane, I found clinching evidence of the factual basis of the first line in Lenny Bruce’s autobiography, How To Talk Dirty And Influence People: “You wouldn’t believe how many people smoke marijuana.”

In fact, not only would I have believed it, I already knew it. But it was good to have it confirmed when one of the most celebrated Irish entrepreneurs of the age nudged my elbow and asked whether I fancied a toke on his joint. Naturally, I refused, not wanting to seem to be too hugger-mugger with a famous entrepreneur.

A couple of decades on, the legalisation of marijuana has become a relatively respectable cause. Take actuaries – hardly among the cooler professions. Earlier this year, Contingencies, the journal of the American Academy of Actuaries, advised members to reconsider the weight they were giving to marijuana when calculating what premiums to apply to discrete categories of customers. Actuaries should disabuse themselves of the notion that marijuana “conferred the same relative mortality risk as cigarette smoking”.

The available evidence suggested instead that “recreational marijuana users enjoy better physical fitness and get more exercise than non-users” and “have even been shown to have higher IQs”.

Marijuana is now legal in Alaska, Colorado, Oregon and Washington, and has either been legalised for medical purposes or decriminalised for personal possession in another 21 other US states. “The tide is turning – life underwriters would be wise to be at the front end of this curve, and not stubbornly digging in their heels to the detriment of their products.”

In Colorado, tax revenue from marijuana is ring-fenced for the upgrade of schools run down as a result of spending cuts. Last November, the state’s joint budget committee gave almost a million dollars to schools to repair a “critical shortage” of health workers, particularly in poorer districts. The legalisation of marijuana has been one of the best things to happen to Colorado’s young people.

Law Enforcement Against Prohibition has contributed significantly to the shift in US attitudes. The 10,000-strong lobby of retired drugs officers, prosecutors, judges and public officials focuses on the fact that during prohibition in the 1920s and early 1930s, alcohol was associated with bloodshed, gangsterism and vice. Lifting prohibition snapped the connection. The gangsters lost the turf they had been fighting and killing over. The same would happen here under drugs decriminalisation. Yet most politicians in these islands continue to set their faces against an evidence-based approach.

In 2007, Prof David Nutt was sacked by the Blair government from the UK Advisory Committee on the Misuse of Drugs after he published an article in medical journal The Lancet showing that marijuana (and ecstasy and LSD) were less harmful than tobacco or alcohol. The senior chemist on the ACMD, Dr Leslie King, and Marion Walker, the Royal Pharmaceutical Society’s representative, resigned in protest against the sacking.

The government’s chief scientific adviser, John Beddington, commented: “I think the scientific evidence is absolutely clear-cut. I would agree with [Prof Nutt].”

Science minister Lord Drayson said the sacking was a “big mistake” which had left him “pretty appalled.”

None of this made a dent in the doltish certainty of home secretary Alan Johnston. Nutt’s argument was “political”, he told the Commons, mysteriously.

British politicians bristled with anger again last July when Durham Police and crime commissioner Ron Hogg announced that, in future, people found in possession of small amounts of marijuana or growing a few plants would no longer be prosecuted. Contradicting every available scrap of scientific evidence, Tory police minister Mike Penning insisted that marijuana possession should be punished by “significant jail sentences”.

On a brighter note, we may be seeing a shift towards common sense in the Republic. In the Sunday Times last weekend, Conor Brady reported that the Oireachtas committee on justice, defence and equality may recommend that we follow the example of Portugal, which 14 years ago decriminalised all drugs.

On every relevant measure – drugs- related deaths, mental illness, school drop-out rates, addiction levels, crime figures – none of the expressed fears of negative consequences has been borne out, while the predictions of proponents of legalisation have been vindicated.

We shouldn’t be surprised. Marijuana has been used for at least 5,000 years as a medicine and for relaxation, religious rituals and much else. Our ancestors were not stupid, irresponsible or ignorant. It’s just that, in this matter at least, they were more advanced than a dismaying number of 21st-century politicians.

Brady seemed confident that Minister for Justice Frances Fitzgerald might accept the committee’s advice. We shall see.

It was Bob Dylan who turned the Beatles onto marijuana. That appears to have worked out well. On marijuana, Paul McCartney wrote A Day in the Life. Off marijuana, he wrote Mull of Kintyre.