Steven Hoffman is director of the Global Strategy Lab at the University of Ottawa's Centre for Health Law, Policy & Ethics

Last Thursday, the federal government unveiled its long-anticipated legislation to legalize cannabis. We heard that cannabis will be strictly regulated and kept away from children and criminals. But we didn't hear how the government will address Canada's international legal obligations under the UN drug control treaties that specifically require cannabis's prohibition.

For decades, the world has collectively criminalized and controlled access to drugs through three international treaties: the 1961 Single Convention on Narcotic Drugs; the 1971 Convention on Psychotropic Substances; and the 1988 Convention against Illicit Traffic in Narcotic Drugs and Psychotropic Substances.

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Canada's treaty obligations are clear. Article 4(c) of the Single Convention limits drugs' use "exclusively to medical and scientific purposes", and Article 36(1)(a) requires state punishment for their possession, production, sale and delivery. Article 3(2) of the Trafficking Convention specifically criminalizes drugs' possession, even if just for personal consumption.

Of course, countries do have some flexibility in implementing these treaties. Portugal prohibits drug use but diverts offenders toward mandatory education classes, treatment sessions and fines. The Netherlands criminalizes cannabis but doesn't enforce this prohibition against people possessing small quantities. Countries also don't need to implement treaty provisions that violate their constitutions. Bolivia created a constitutional right to consume coca leaf in 2009 to take advantage of this loophole.

Yet the treaties' flexibilities are rather limited. Unless we change our constitution, Canada cannot legally legalize cannabis without either renegotiating the UN treaties, obtaining special exceptions, finding creative workarounds, or withdrawing from them.

Negotiations, exceptions and creative lawyering will only get Canada so far. In a report published last week, my University of Ottawa law students explained why Canada cannot easily take advantage of them. Convincing the 32 countries with death penalties for drug smuggling to renegotiate the treaties or grant Canada a special exception seems as politically impossible as adding a constitutional right to consume cannabis into the Canadian Charter of Rights and Freedoms.

As the University of Ottawa report details, there is only one promising legal workaround – one that utilizes the treaties' "scientific purposes" exemption. This would involve claiming that cannabis legalization was necessary to conduct a big natural experiment on the inter-generational effects of legal cannabis and likely require enrolling every cannabis purchaser into a long-term cohort study. The Canadian Institutes of Health Research already announced small research grants to evaluate the forthcoming legislation's effects; the government agency could be funded to organize additional research on a scale that justifies national cannabis legalization for scientific purposes.

But if the government deems this workaround infeasible, then withdrawing from the treaties is the only other option. Yet this requires giving at least one year's notice. If Canada wants to legalize cannabis on July 1, 2018, it needs to trigger its withdrawal by this July 1. That leaves only a couple of months for diplomats to try negotiating changes or special exceptions before just unilaterally withdrawing from the treaties.

Either way, the government should immediately announce its intentions to ensure the world is not left to wonder whether Canada even cares about international law.

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The consequences of undermining international law are severe. It's not just our reputation or possible UN Security Council seat that are at stake. This week, we saw Syria again use chemical weapons and there are rumours of a forthcoming North Korean nuclear test. How can Canada condemn these violations of international laws while planning for its own violations? This past week's events show how desperately we need a rules-based world order – even if international rules and their enforcement mechanisms are far from perfect.

In other words, Canadians may not care about international law when it's just about drugs, but they probably do care when these laws help fight chemical weapons, nuclear proliferation, and human rights abuses. Canada cannot pick-and-choose which international treaties to follow without encouraging other countries to do the same.

While there may be significant health and social benefits from legalizing cannabis, we must not allow international law to become collateral damage in the pursuit of other objectives. Taking advantage of the "scientific purposes" exemption would allow us to have our cannabis cake and eat it, too. Otherwise, Canada should immediately start negotiating with other countries and preparing to withdraw from the drug control treaties after negotiations inevitably fail.

The alternative – undermining international law – is unacceptable, unfair and unjust to all.

Steven Hoffman is associate professor of Law, Medicine and Public & International Affairs and adjunct associate professor of Global Health at Harvard University.