On Wednesday October 17, 2018, Canada became the second country in the world to legalize marijuana, joining Uruguay. While a few states in the United States have moved to legalize marijuana in recent years, this highly politicized issue has been the subject of debate and divisive science for decades.

Those against the legalization of marijuana suggest the drug is dangerous, increasing a person's risk of mental illness and damaging to long-term memory. Making it legally accessible will mean more people will use it, more children will experiment with it and the overall perception of the drug will be that it is safe because it is legal.

Those for marijuana legalization suggest the positives far outweigh the negatives. The argument is that legalization will reduce crime, eliminate the drug's black market origins, and deliver the government hundreds of millions of new tax dollars. Pro-legalization advocates also suggest there is no evidence marijuana use will increase following legalization.

This gigantic social experiment beginning in Canada will offer the biggest insight into the pros and cons of legal recreational marijuana the world has ever seen. Over the next few years academics, law-makers and governments around the globe are sure to be watching the country closely. The legal roll-out of the drug on such a massive scale will undoubtedly offer some clearer answers to these questions that have been so vociferously debated for years.

But what are the big questions we hope the Canada experiment will answer?

A marijuana dispensary in Oregon Rick Obst / CC BY 2.0

Will use increase?

A big concern often raised over legalizing marijuana is that by removing the taboo of its illicit status more people will experiment with it, including more children and adolescents. The data up until now has been decidedly mixed on this question. In the United States, Colorado offers the best indication of what effect legalization has on general use rates. The state was one of the first to institute full recreational legalization in 2013 and the results so far have been divisive, to say the least.

One study extravagantly concluded that past-month adult use of marijuana in Colorado increased 63 percent in a two-year average post legalization compared to the two year average prior to legalization. And, in the same time period, youth marijuana use increased 20 percent.

Another more recent survey suggested that while adult use of marijuana has risen slightly since legalization, youth use has remained unchanged. The general overview from most research into Colorado use post-legalization has generally followed this pattern, a mild uptick in adult use and an interesting stability in teen and youth use.

Globally, we have seen reasonably clear evidence suggesting there is no connection between relaxed drug laws and increased use. Portugal is the most compelling case study the world has for the effects of mass drug decriminalization.

In 2001 Portugal made the huge decision to essentially decriminalize all drugs. The law moved personal use and possession of recreational drugs from a criminal offense to a health care issue. Drugs were still illegal but if you were caught with small quantities you would not go to court, but instead attend a "Commission for the Dissuasion of Drug Addiction." These panels, made up of a social worker, a psychiatrist and an attorney, were designed to divert drug users out of a criminal system and into either rehabilitation or social justice scenarios.

The long-term data from the Portugal experiment, at the very least in terms of overall drug use, was incredibly positive. There was no spike in use after criminal penalties were removed, in fact as of 2017, more young people used illicit drugs in the United Kingdom than Portugal.

However, data can be spun into a variety of conclusions, and the Portugal experiment has been rife with alternating impressions of whether country-wide decriminalization has been beneficial. One journalist suggested the data has become a "Rorschach test," where anyone "can look at these numbers and make almost whatever argument they'd like to make."

An example often raised is that personal drug use over the course of an individual's lifetime has increased nearly 50 percent since 2001, however "people reporting drug use over the last 12 months of their lives has actually gone up only slightly."

Will Canada help offer some clarity into the "use versus legalization" issue? Only time will tell. But any issue with increased use will be inevitably be linked to the constantly debated health effects of marijuana.

What are the long term public health consequences of marijuana legalization?

Is it safe?

A couple of days before Canada pressed play on marijuana legalization a strongly worded editorial was published in the Canadian Medical Association Journal suggesting this was "a national, uncontrolled experiment in which the profits of cannabis producers and tax revenues are squarely pitched against the health of Canadians."

The Canadian Psychiatric Association (CPA) also joined the chorus of medical professionals concerned with the looming legalization, releasing a statement suggesting that due to the effects of the drug on developing brains it must be more restricted to those under the age of 25.

"There is a strong evidence-base showing that early and regular cannabis use can affect cognition, such as memory, attention, intelligence and the ability to process thoughts and experiences," says CPA president, Dr. Wei-Yi Song. "It can also increase the risk of developing a primary psychotic disorder as well as other mental health issues such as depression in those who are already vulnerable to these disorders."

It is reasonably clear that for those prone to mental health problems, marijuana can amplify those issues. However, what effect legalization (or decriminalization) has on these factors from an overall societal perspective is not clear. After just four or five years of legalization in parts of the United States we do not know whether rates of mental health problems are rising. It is probably fair to hypothesize that if use rates do not markedly rise then neither should rates of mental health problems.

One focused study looked at the rates of suicide and admissions into drug treatment centers across Colorado and Washington over the first two years of marijuana legalization. Suicide rates were found to trend ever so slightly up, but the report concluded that it was too marginal to correlate with marijuana legalization.

On the other hand, admissions into drug treatment centers for problematic marijuana use trended downwards after legalization. Instead of making an assumption that legalization was a cause of these trends, it is safe to say this offers an interesting metric indicating legalization does not increase abuse of the drug.

Does legal marijuana make the roads less safe? Cannabis Culture / CC BY 2.0

But what about all the stoned drivers?

Connections between marijuana use and traffic accidents is another one of those areas jam-packed with contradictory data. This discordancy came to a head when two studies were released in the same week in 2017. Both studies were examining the impact of marijuana legalization on traffic accidents and both studies came to opposite conclusions.

The more negative study suggested a 3 percent average rise in traffic collisions could be attributed to marijuana legalization across Colorado, Washington and Oregon. The other study, examining crash fatality rates, found that no rise whatsoever in Colorado and Washington compared to the rest of the country. Perhaps it is unsurprising that mild collisions increased yet fatalities did not, however this again proved to be a litmus test of political bias with each study allowing individuals to confirm whatever prior prejudice they had on the subject.

A broader crash and fatality rate study shows almost no statistical shift in accidents or deaths on the road prior to, and post, marijuana legalization in Washington and Colorado. Yet the chief research officer of the Insurance Institute for Highway Safety, David Zuby confidently concludes: "Worry that legalized marijuana is increasing crash rates isn't misplaced. [The] findings on the early experience of Colorado, Oregon and Washington should give other states eyeing legalization pause."

Again we turn to Canada. The mass legalization across such a large country of nearly 40 million people will undoubtedly deliver some interesting data on this matter over the coming years. Despite legalization ticking into reality, Canadian police are reportedly not ready to deal with a raft of new impairment laws brought in to coincide with the legalization. It's unclear exactly how the country, on a broad scale, is planning on dealing with the issue, and everything from impairment tests to blood, saliva and urine testing are apparently about to be implemented.

A federal marijuana warning from 1935

Legalization reduces crime though, doesn't it?

One of the big arguments often made by advocates for marijuana legalization is that it reduces crime. By eliminating the organized crime networks that distribute and sell the drug, all the negative associations that come with the presence of those networks will inevitably disappear, thus reducing overall crime rates in a community. Again, the data we have from the last few years is decidedly mixed, with politics on both sides infiltrating conclusions made.

Several studies have suggested that overall, legalization of marijuana in the US has not dramatically altered crime rates. It's incredibly difficult to claim a causal connection between legalization and violent crime rates, but both Washington and Colorado have seen decreases in murder rates and violent crime over the past few years. Some suggest this cannot be attributed to marijuana legalization as they are simply continuations of longer, more complex trends.

An interesting report examining some specific areas in California following marijuana legalization has suggested that "home invasions, violent crimes and robberies" have actually increased over the past year. This has been attributed to criminal organizations descending on outlying areas in the state, stealing cash and marijuana to move it across into other states, where the drug is still illegal and the black market thrives.

This, of course, becomes a great datapoint for anti-legalization advocates to jump on, and in 2017, rabid anti-marijuana advocate Attorney General Jeff Sessions said to the media, "We're seeing real violence around that. Experts are telling me there's more violence around marijuana than one would think and there's big money involved."

And technically, Sessions is not necessarily incorrect, although it would be fair to argue that the violence that may be accompanying the legalization of marijuana in some counties is more associated with the states that maintain prohibition that the ones that have legalized it.

One fascinating knock-on effect from legalization could be an associated decrease in violent crime related to Mexican drug cartels. A 2017 study entitled "Is Legal Pot Crippling Mexican Drug Trafficking Organisations? The Effect of Medical Marijuana Laws on US Crime" examined the correlation between the introduction of broad medical marijuana laws in the United States and a decrease in violent crime across states that border Mexico.

The hypothesis is that marijuana, the largest drug market in the United States, results in competing drug cartels battling each other for supremacy along smuggling routes. This is associated with violent crime, murder and robbery. The study found that across California and Arizona border areas, drug cartel associated crime dropped between 1994 and 2012.

Critics of the study could easily suggest that those crime rate drops do not have anything to do with the spread of medical marijuana legislation across the United States. Some have also suggested that drug cartels have simply moved from the marijuana business into human trafficking, kidnapping and cultivation of other drugs such as heroin.

The unexpected effects

Legalization of marijuana is such a significant change to society that it can also result in a number of unexpected effects, both positive and negative. One study examined the efficacy of law enforcement in states post-marijuana legalization, assuming that once police were lifted of the burden of enforcing marijuana offenses, they suddenly had more time to better solve other crimes.

"Our models show no negative effects of legalization and, instead, indicate that crime clearance rates for at least some types of crime are increasing faster in states that legalized than in those that did not," write the authors of the study in Police Quarterly.

There also have been some claims that states with either medical or recreational marijuana have seen dramatic drops in opioid prescriptions and usage. Again, this is correlation and not causation, but it raises interesting questions over the role marijuana legalization could play, especially in a country such as the United States, which is suffering from a major opioid use epidemic.

Another interesting side effect of marijuana legalization could very well be decreases in general alcohol use. One study from Georgia State University concluded that alcohol sales notably dropped in states that instituted medical marijuana laws. The research suggests a drop of about 15 percent in alcohol sales could be seen over a 10 year period in states with medical marijuana legislation.

Canada, the world is watching

Ultimately, we just don't know what the effects of mass marijuana legalization are on a large society, especially over the long-term. So far the localized recreational legislations in the United States have delivered mixed results, with data that has obviously been able to support both pro and con arguments.

What we can be sure of is that the day after legalization of marijuana passes, the sky will not cave in, and society will not collapse. The Canada experiment will undeniably take several years before, at the very least, we can understand if use rates rise or crime rates are effected. And it will take many years more to understand what the mental health effects may be. But the world is watching, and Canada's success or failure with this grand experiment will undoubtedly determine the drug policies of many countries around the globe for decades to come.