Canada has embarked on one of its most ambitious public and economic experiments in decades, with adults now able to buy marijuana legally.

Canadians in Newfoundland in the east were the first to have legal access to the drug on Wednesday, when retailers began selling at midnight. The Cannabis Act makes the Great White North the world’s second country, and largest economy, to legalise the substance.

Prime minister Justin Trudeau, who promised on the campaign trail to legalise marijuana, tweeted: “Profits out of the hands of criminals. Protection for our kids. Today cannabis is legalised and regulated across Canada.”

The new law allows Canadians who are 18 and older of age to carry and share up to 30 grams of marijuana in public. They will have access to items including pre-rolled joints, cannabis oil, and dried flowers in shops and online.

The government is expected to go further, providing pardons for individuals who have been convicted for possession of small amounts of marijuana.

As many as 500,000 Canadians have marijuana convictions on their criminal records, and more than 15,000 have been charged with marijuana-related offences since Mr Trudeau became prime minister in 2015.

In pictures: 4/20 Marijuana world rallies Show all 26 1 /26 In pictures: 4/20 Marijuana world rallies In pictures: 4/20 Marijuana world rallies A man wears a marijuana leaf mask during the annual 4/20 cannabis culture celebration at Sunset Beach in Vancouver, British Columbia AP In pictures: 4/20 Marijuana world rallies A lady smokes marijuana on Parliament Hill on 4/20 in Ottawa, Ontario Getty Images In pictures: 4/20 Marijuana world rallies James Reed smokes a joint during the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies People sign a 4/20 sign on Parliament Hill on in Ottawa, Ontario Getty Images In pictures: 4/20 Marijuana world rallies A woman smokes marijuana on Parliament Hill on 4/20 in Ottawa, Ontario. Polling released showed strong support in Canada for a government drive to legalise recreational use of marijuana, but many would like the proposed minimum age for consumption to be raised. Sixty-three percent of respondents told the Angus Reid Institute they support legalisation Getty Images In pictures: 4/20 Marijuana world rallies A man smokes marijuana during the annual 4/20 marijuana rally on Parliament Hill in Ottawa, Ontario, Canada Reuters In pictures: 4/20 Marijuana world rallies Demonstrators smoke marijuana during the '4/20 Santiago' rally in favour of legalisation in front of the La Moneda presidential palace in Santiago, as part of the Global Marijuana March which is being held in hundreds of cities worldwide Getty Images In pictures: 4/20 Marijuana world rallies People play with a mock marijuana joint during a 4/20 party to demand legalisation and to celebrate marijuana culture outside the Senate building in Mexico City, Mexico Reuters In pictures: 4/20 Marijuana world rallies Adam Eidinger, co-founder of DCMJ, hands out free marijuana joints to DC residents who worked on Capitol Hill as part of the 1st Annual Joint Session to mark '4/20' day and promote legalising marijuana on Capitol Hill in Washington, DC Getty Images In pictures: 4/20 Marijuana world rallies In pictures: 4/20 Marijuana world rallies Police arrest Rachel Ramone Donlan after she handed out free marijuana joints to DC residents who worked on Capitol Hill as part of the 1st Annual Joint Session to mark '4/20' day and promote legalising marijuana on Capitol Hill in Washington, DC AFP/Getty Images In pictures: 4/20 Marijuana world rallies Police arrest Rachel Ramone Donlan after she handed out free marijuana joints to DC residents who worked on Capitol Hill as part of the 1st Annual Joint Session to mark '4/20' day and promote legalising marijuana on Capitol Hill in Washington, DC Getty Images In pictures: 4/20 Marijuana world rallies Thousands of people gather to smoke marijuana during the '420 Santiago'rally in front of the La Moneda presidential palace in Santiago Getty Images In pictures: 4/20 Marijuana world rallies People attend the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies Kevin Barron and Lasean Moore of Raleigh, North Carolina, share a joint during the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies Nic Ruhl takes a pull on a giant hand rolled joint at precisely 4:20pm MDT during the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies Various cannabis paraphernalia on display at a vendor's stall during the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies Mo Banez, of Austin, Texas, lights a joint during the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies A man displays a large container of cannabis during the Denver 420 Rally at Civic Center Park in Denver, Colorado. The rally, held annually, is a celebration of both the legalisation of cannabis and cannabis culture. Colorado is one of twenty-six U.S. states along with the District of Columbia that has legalised the use of cannabis either recreationally or medically AFP/Getty Images In pictures: 4/20 Marijuana world rallies Sitting in small groups on mats shaded by trees in the Rose Garden just across from the Knesset, participants lit up as the clock struck 4:20 for the local version of the traditional worldwide April 20 pro-marijuana events, known as '420' rallies Getty Images In pictures: 4/20 Marijuana world rallies An Israeli smokes a marijuana joint in Jerusalem during a rally at the Rose garden Getty Images In pictures: 4/20 Marijuana world rallies Sitting in small groups on mats shaded by trees in the Rose Garden just across from the Knesset, participants lit up as the clock struck 4:20 for the local version of the traditional worldwide April 20 pro-marijuana events, known as '420' rallies Getty Images In pictures: 4/20 Marijuana world rallies An Israeli girl poses with a mock marijuana joint in Jerusalem during a rally at the Rose garden, to celebrate 420 and to express their defiance of current laws Getty Images In pictures: 4/20 Marijuana world rallies aelis pass around a marijuana joint in Jerusalem during a rally at the Rose garden, to celebrate 420 and to express their defiance of current laws Getty Images In pictures: 4/20 Marijuana world rallies An Israeli smokes a marijuana joint in Jerusalem during a rally at the Rose garden, to celebrate 420 and to express their defiance of current law Getty Images In pictures: 4/20 Marijuana world rallies An Israeli smokes a marijuana joint in Jerusalem on April 20, 2017 during a rally opposite the Knesset to celebrate 420 and express defiance of current laws AFP/Getty Images

Supporters of legalisation have argued that the country will benefit from tax on sales to residents and international tourists.

“The fact that we are moving away from a prohibition model is a victory for human rights and social justice, an economic windfall for the Canadian economy and a sign of social progress,” Adam Greenblatt, of producer Canopy Growth, told The New York Times.

The new law has investors buzzing about the potential for profits from marijuana sales, adding to an already robust year for weed stocks as interest in the market has soared. While shares in marijuana companies dipped this week, the speculation over the year has seen growth for companies such as Mr Greenblatt’s – up 130 per cent this year – and Aurora Cannabis, up 510 per cent on last year.

But not everyone is convinced that legalising marijuana is a good idea. In a testimony to the Canadian legislature, members of the country’s medical community noted evidence that early and frequent use of the drug can impact mental health. And Ontario’s premier, Doug Ford, sent a letter to Mr Trudeau on Tuesday suggesting that police forces don’t have adequate capacity to handle a surge in drug-driving offences.

Bernard Le Foll, an addiction specialist at the Centre for Addiction and Mental Health in Toronto, told The Independent that while his organisation supports the legalisation of marijuana, not enough has been done to educate the public about the dangers of the drug, which include addiction, impaired driving, and mental health issues.

“There is no doubt now in the medical community that cannabis is an addictive substance,” Mr Le Foll said.

“It has been estimated that around 9 per cent of users will develop what is called cannabis dependence.

“I think the problem is ... there is a perception that it is a benign substance that does not affect them badly,” he continued, before noting that a large portion of marijuana users report driving under the influence.

“They feel that it is safe to do but clearly it is not. The issue is an issue of education. I think there has been for so long such little information going to the public that people have made their own judgement of cannabis, and the feeling is probably more favourable than what it (truly should be).”

Statistics Canada, the national data agency, has projected that marijuana spending could reach $6.3billion Canadian dollars (£3.6bn) annually. Around 20 per cent of the population is expected to use marijuana, with users soaring past seven million.

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Starting on Wednesday, customers in most parts of the country were able to buy cannabis from either government or private stores – with some provinces taking more time to acquire licences.

Bill Blair, the minister in charge of legalisation, said his government expects there to be sufficient supply to meet demand but added: “It may take some time before illegal marijuana dealers are pushed out of the market.”