Opposite a bleak government building in suburban Ottawa, Canada, a barebones “cannabis clinic” – with just a cash register, jeweller’s scales and a glass counter – is doing a brisk trade. “Pirate! Muslim! Gangster! Yes! We all smoke!” shouts one teen as he high-fives the owner, Rohmi. He pockets his pungent bag and bounces out, giggling.

On the wall, there’s a menu listing today’s special: moonrocks – buds rolled in cannabis oil then dipped in powdered hash at C$40 (£24) a gram. There are cans of Canna Cola; potent, weed-laced gummy bears; a mound of gooey hashish smelling of dark chocolate, hops and pine resin.

If this is medicine, it’s unclear what the illness is, other than sobriety. It makes an Amsterdam coffeeshop look tame – and it’s this free-for-all, wild-west-of-weed attitude that Canada’s government wants to tame by legalising cannabis during prime minister Justin Trudeau’s first term in office. Laws that will legalise cannabis for recreational use will be announced in the week of 10 April, and will be passed by July 2018, say government sources, making Canada the first G7 country to do so.

Globally, cannabis prohibition is being briskly dismantled, a wave of decriminalisation or legalisation sweeping south through the Americas, with states such as Colorado leading the way. California voted to legalise recreational use in November 2016. The US currently has 29 states offering legal medical marijuana and eight states with legal recreational cannabis markets. Countries including Uruguay, Mexico, Jamaica, Brazil, Colombia and Chile are either creating legal markets for medicinal cannabis or relaxing rules on possession and cultivation. In the EU, Germany is preparing for imminent full medicinal legalisation, and the Republic of Ireland voted in December 2016 to allow medical use. In the UK, medical marijuana is available in the form of a tincture spray, Sativex, but access is severely limited.

What is different about Canada’s plans to legalise is the scope of the law change – it will be legal, nationally, for anyone over the age of 18 to use cannabis for pleasure next year.

Trudeau told journalists early this month that he wanted to seize the profits of the criminalised market and use the income to help those with drug problems. But his main aim in legalising the drug was to make it harder for children to get hold of it, arguing that alcohol laws showed that proper controls and regulations work. “It’s easier for a teenager to buy a joint right now than a bottle of beer, and it’s not right,” he said. “We know by controlling and regulating it, we are going to make it more difficult for young people to access marijuana.”

The evidence is on his side: in Colorado, where cannabis has been legal since 2014, teen use has fallen by about 12%, due to a combination of factors including a smaller black market and better drugs education.

Ironically, however, the upcoming change in Canada has prompted a huge growth in outlaw dispensaries. Rohmi and hundreds of other dealers selling in kerbside clinics are claiming to be operating under rules allowing Canadians to use cannabis to treat complaints as diverse as insomnia, ADHD and chronic pain. In reality, they are cashing in and riding the pre-legalisation green rush.

Police are clamping down, with dozens of raids in recent weeks, but the mood on the streets and in the clinics is one of delighted anarchy, with some outfits simply reopening the day after a raid in a new spot.

Police seize marijuana from a dispensary in Toronto. Photograph: Carlos Osorio/Toronto Star via Getty Images

Rohmi says he takes C$15,000 in cash every day, and claims his business is an act of civil disobedience. “We’re doing this as a Gandhi type of thing. Peaceful protest, activism,” he says. Isn’t he afraid of getting robbed? He laughs and nods at a plank in the corner with the word “Bertha” scrawled on it. “I got Bertha, she’s my security!”

The road to legalisation in Canada has been long and circuitous. Medical cannabis has been legal in Canada since 2000. By 2012, under the Conservative government of Stephen Harper, there were 40,000 Canadians growing cannabis at home, says Chuck Rifici, a cannabis entrepreneur and former chief financial officer for the centre-left Liberal party. “The average Canadian is not particularly concerned with cannabis usage, or its illegality,” he says, with wry understatement. Some of these homegrowers now illegally supply the clinics that are commonplace in all Canadian towns, while there are 38 licensed producers for the medical market, which has 130,000 users.

Trudeau, who won the 2015 general election for the Liberals on a promise to free the weed, has spoken of his own cannabis use in casual terms, cementing his image as a modern progressive. Trudeau sought policy advice from Bill Blair, an old-school cop famed for his zero-tolerance approach to cannabis when he ran the force in Toronto. Trudeau told Blair he wanted to legalise cannabis to stop criminals from preying on kids, and Blair agreed that this was the best way to present the policy to the public.

Cannabis is popular in Canada – one-third of 18 to 24-year-olds use it, as do 3.4 million of all Canadians, 10% of the country’s population. For comparison, in the UK, there are about 2 million users, 3% of the population.

Blair told Canada’s Globe and Mail newspaper in January: “Our intent is to legalise, regulate and restrict. There needs to be reasonable restrictions on making sure that we keep it away from kids, because I think that is very much in the public interest.”

Trudeau set up a taskforce, led by Blair, in June 2016 to write the roadmap towards legalisation. A more moderate and Canadian approach is hard to imagine. Experts included researchers and academics, patients and lawyers, users, chiefs of police and fire departments, and government officials and associations.

Steve Moore, of British thinktank VolteFace, which is campaigning for legalisation in the UK, says Canada’s model is more sensible than that of the US. “The American states that have legalised are too libertarian, with their billboards and TV advertising,” says Moore, who lauds Trudeau’s focus on tackling crime and reducing youth access. The Canadian path to legalisation is at once liberal and conservative, adds Moore, who believes such an approach could work in the UK.

The Canadian taskforce’s paper made more than 80 recommendations, including a minimum age of access – 18 – with restrictions on advertising, and guidance on production, manufacturing and distribution. It laid out measures for testing, packaging and labelling, with a strong emphasis on education around the risks of use, especially driving under the influence. It drew no conclusions on whether legalisation would lead to increased use, and instead took a harm-reduction and public-health approach – acknowledging the risk inherent in cannabis use and proposing ways to mitigate it.

Police will admit privately that the cannabis clinics aren’t a priority. “We have other problems,” says a source, drily. A public emergency has been declared in the state of British Columbia, where opiate users are dying in unprecedented numbers. With a population of just 4.6 million, in 2016, there were more than 900 deaths from fentanyl, a super-strong synthetic opioid. In the UK in 2015, there were just 11 fentanyl deaths, and 1,201 heroin deaths, according to the Office of National Statistics.

The Canadian government is preparing for the public-health problems that an estimated 600,000 new smokers will bring. No one yet has a satisfactory answer as to how driving under the influence will be managed, since a roadside test for the drug is still not sufficiently accurate, and no agreed metric of cannabis impairment has been set.

Another day, another dispensary; this time, Montreal. I decide to register to see how strictly procedures are observed. They have run out of forms, so I am handed a laminated document that I fill in with a whiteboard pen, claiming a bad back and occasional anxiety. I’m registered in minutes, and wander into the striplit room for my medical consultation. The staff know little about the dosage, onset or duration of effects for the drugs they are selling me, and hand me a small pinch-seal bag of capsules with a handwritten label on it: “20mg THC”. A child could open it in a second – and they look like sweets. It is easy to see why a more regulated market solution is being urged.

An altogether more orderly vision of the future can be found a short drive from Ottawa, in the town of Gatineau, western Quebec, where a licensed medicinal cannabis producer, The Hydropothecary, is expanding ahead of legalisation, hoping to supply the new market. We approach the farm and more than 100 security cameras silently track us. We kit up in hazmat suits, shoe covers, hair- and beardnets (the site has to follow safety rules drawn up for opioid factories) before we enter the grow room.

The unmistakable aroma of 1,200, two-foot female cannabis plants is like a gentle slap in the face, but I’m blinded by 1MW of high-pressure sodium light beaming down on the spiky leaves and budsites.

The plants are stretching towards a bespoke arched glass roof that shelters them from the Canadian winter, pumping out THC in a fruitless search for a male. It’s a female-only space; pollen from males would fertilise them, rendering the bud seedy and worthless. This room can produce 350kg of marijuana a month, says Adam Miron, the firm’s co-founder. A similar operation in the UK would currently carry up to a 10-year jail sentence.

A female cannabis plant at the Hydropothecary farm in Gatineau. Photograph: Remi Theriault

The crop will sell for an average of C$10 a gram, and as the company is vertically integrated, selling via next-day mail order, all profits stay in-house. Miron flips fluently between biology and marketing matters, to legal and political issues, and back to branding. He introduces his master grower, Agnes Kwasniewska, a permanently delighted and highly educated Polish-Canadian who takes pride and interest in every plant.

We enter the drying vault through a thick steel door, where millions of dollars of product lie curing in temperature-controlled cases. There’s a litre flask of cannabis oil worth C$80,000, which is passed between the terrified visitors as if it’s plutonium. Every gram is documented from seed to bag. Every gram will be taxed, and children will not be allowed to buy it. Trudeau and Blair would be happy.

Just as the 1,200 plants in Gatineau were being tended by Kwasniewska, a vast, illegal grow was discovered in Wiltshire in a nuclear-bomb shelter, with three trafficked and enslaved Vietnamese teenage boys tending about 4,000 plants, imprisoned behind a five-inch steel door. Industrial-scale cannabis farmers in the UK have often used trafficked children who are imprisoned for months and forced to water and monitor and protect the plants from rival gangs. Upon discovery, the children, already abused and exploited, are jailed.

Canada, meanwhile, is creating a C$20bn-a-year industry that will employ thousands of people, minimise public-health harms, cut teen access to the drug and save the taxpayer millions in enforcement costs. There’s a sense of energised hope and cautious optimism in every conversation you have: Canadians feel that change is coming.

Miron’s desire to grow this business is impassioned and infectious, and seems more than simply financial. What drives him? “Two years ago, my father had terminal lung cancer,” he says. “Very terminal. He was given months to live. He didn’t have much breath capacity, but I will never forget holding a vaporiser to his lips as he sat on the porch with me, and he managed to inhale a few puffs. He slept there, free from pain for the first time in days, resting. He was my first customer.”