For half a century it has been easy for politicians largely to ignore the existence of a vast black market supplying cannabis in the UK and its attendant consequences. The lack of saliency of drugs, as a political priority, is perhaps a consequence of the public’s ambivalence regarding this most popular of illegal narcotics. A Populus survey late last year revealed that over 60% of British adults said they had never consumed any form of cannabis, nor did they know anyone in their family or social circle who had ever taken as much as a puff of a joint. But the same survey suggested that some 2.4 million Britons consumed cannabis in the past year. While the estimated £2.5 billion people spent on it generated no tax revenues, it proffered a handsome dividend for perpetrators of industrial level criminality. As the social historian James Mills recounts in his definitive history of cannabis in the UK, successive governments have generally avoided going anywhere near this issue, leaving it to the discretion of the police to enforce.

Is any other criminal offence so routinely committed and yet enforced so sporadically?

Perhaps only the illegal downloading of music and movies comes close, but in the past few years the emergence of new media streaming services such as Spotify and Netflix have all but consigned illegal digital piracy to history.

Can this ‘fudge on drugs’ endure? If so, at what cost?

It does seem things may be about to change.

In January this year Rupert Green, a 21 year old student committed suicide in his childhood garden. His father Lord (Nicholas) Monson claims his son had suffered from psychosis as a result of an addiction to skunk — a virulently strong cannabis. Lord Monson raised a storm in the media bringing public attention to the addictiveness and deleterious impact on mental health of skunk. Today the Mail reports that he has engaged in a dialogue with the Prime Minister on the issue.

Skunk’s origins date back to the 1970s, rising to prominence in the early 2000s. The term is now used to refer more broadly to potent high THC, low CBD cannabis. THC helps produce the ‘high’ associated with cannabis use. It has also been correlated, particularly when consumed heavily over long periods and in high concentrations, with greater incidence of psychosis and increased development of dependence. CBD, while not psychoactive itself, modifies the effects of THC, including reducing its anxiety and paranoia inducing effects. There is evidence that the incidence of psychosis is not seen to increase when CBD is consumed alongside THC. CBD containing cannabis also seems to be less dependence forming than that without it. Whereas other forms of weed often contain the two substances in more equal ratios, skunk tends to contain high amounts of THC and hardly any CBD.

Estimates suggest that somewhere between 80–90% of street cannabis sold today in the UK is skunk.

From 2015–16 there were 81,904 admissions into hospital psychiatric units for drug-related mental health or behavioural disorders, compared to 38,005 in 2005–06.

But as evidence becomes clearer that dealers are profiting from growing and selling a form of cannabis that endangers public health, the number of actual arrests and prosecutions for cannabis possession is at an all time low. In England and Wales police recorded 19,115 arrests for cannabis possession in 2015, compared with a high of 35,367 in 2010 — a fall of 46 per cent. Indeed some police forces — such as Durham — have more or less decriminalised cannabis use within their jurisdiction.

In the course of this year the Prime Minister has made just one major social policy speech. It was on mental health with a particular focus on young people. She vowed that only by “challenging the established way of doing things can we improve mental illness”.

Allowing criminals to sell skunk to kids as young as 14 with near impunity, is a curious way of dealing with what May calls “the burning injustice of mental health”.

But drawing attention to cannabis’s dubious legal status and how lightly it is enforced is hardly new. For decades it was easy to dismiss calls for review in the absence of any policy alternatives across the world. That is no longer the case. The most interesting new approach is Canada’s and it is one that is now catching the eye of leading British social conservatives. Today in Ottawa, Parliament will almost certainly vote to legalise cannabis.

Bill Blair, the most revered former law enforcement officer in the history of Canada may be a Liberal MP, but he is from staunchly conservative stock. Last month when I met the former Chief of Toronto Police in his home city it was clear he had lost none of his crime fighting zeal. Overseeing Prime Minister Trudeau’s plans to create a legal cannabis market Blair’s focus is on removing the black market that funds criminal gangs, limiting the supply of high potency cannabis and denying children any access. That this is being implemented with broad public support, barely a murmur of dissent from the Conservative opposition or the public does suggest that this is an issue that can be resolved with equanimity, if handled openly and responsibly.

Trudeau’s argument for legalising cannabis is almost certainly the best one ever made. Should it succeed in reducing teenagers’ consumption and removing the crime associated with the illegal trade, it is a policy that is bound to be replicated globally. It also has the potential to hugely positively impact the mental health of those who use cannabis through controlling the cannabis on offer and changing consumption habits to direct people towards less harmful forms of cannabis and improved methods of consumption.

If the Prime Minister is as committed to tackling the UK’s mental health problem, as she seems to be, the calls for an urgent review of the impact of the UK’s multi-billion pound illegal cannabis market on mental health and family life will only grow. She may not be able to ignore the clamour for reform.

Nor should she.