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FactCheck: Does cannabis legalisation cause 'exponential' increases in the use of the drug?

The claim was made earlier this week by GP Dr Ray Walley in debate on cannabis legalisation.

A leading member of a group of doctors warning against a liberalisation of Ireland’s approach to recreational cannabis use has claimed that countries and states that have legalised cannabis have experienced exponentially increasing rates of drug use.

He namechecked Portugal, Canada and referred to unnamed US states where the rate of cannabis use was “exponentially” increasing. 

But is it true? Has legalisation of cannabis led to increased usage?

The claim

Appearing on RTÉ Radio One’s Today with Sean O’Rourke to debate with medicinal cannabis campaigner Vera Twomey and TD Gino Kenny, Dr Ray Walley, a former president of the Irish Medical Organisation, said:

The states that have legalised marijuana continue to see a thriving black market and increasing rates of drug use. There is an exponential use of cannabis now happening in Canada, America where the states have legalised it…. In Portugal, where they’ve legalised access, the increased use of cannabis has equally exponentially increased.”

Walley was responding to Kenny’s argument that Ireland should introduce greater access to medicinal cannabis. He said that real-world examples proved that cannabis black markets did not go away following legalisation and usage actually increased exponentially. 

The evidence

Cannabis is one of most commonly used illicit substances globally. However, like all illicit substances, accurately measuring usage is difficult. The question of whether use increases or decreases following decriminalisation or legalisation is difficult because the data used is often contested. 

Simply measuring usage alone is seen as somewhat reductive, because it fails to capture other important details such as the quantity of cannabis used, the frequency of usage and the potency of cannabis being used.

Nonetheless, usage is widely relied upon and is used in various national surveys. When it comes to measuring usage in national populations, two measurements are routinely used: past year or past 30 days. The latter, for obvious reasons, is preferable because it offers a better insight into regular cannabis use. 

Walley, during the interview, did not provide specific statistics but referred to three countries: United States, Canada and Portugal. 

US states

Colorado and Washington became the first states to legalise cannabis for recreational purposes and from 2014 onwards recreational cannabis has been sold legally in Colorado Since then, several other states have followed suit – most recently California in 2018. 

There has been a longstanding academic debate over the impact of medicinal cannabis laws and researchers disagree over whether the introduction of these laws actually did increase usage, especially among adolescents and young people. While some studies have found that medicinal cannabis laws did increase juvenile cannabis use, other researchers have argued the opposite: that medicinal cannabis laws have no impact on the rates of young people using cannabis. 

In October 2018, Colorado issued a comprehensive report on the impact of cannabis legalisation. When contacted Walley to request evidence for his claim, this report was one of the several sent to by a public relations company representing Dr Ray Walley and the group of GPs warning of the risks of cannabis.

Using a sample of the Colorado population, the report estimates that between 2014 and 2017 cannabis use in the past 30 days increased from 13.4% to 15.5%. Among men, past 30-day use increased from 17.3% in 2014 to 19.8% in 2017, while among women the increase was from 10.0% to 11.2%. In terms of daily use among adults, there was an increase from 6.0% in 2014 to 7.6% in 2017.

Among 18-25 year olds, the proportion reporting past-30 day use increased from 27.5% to 29.2% between 2014 and 2017.

The report also cites data from the well-respected National Survey on Drug Use and Health. For young adults aged between 18-25, past 30-day cannabis use increased from 21.2% in 2005/2006 to 32.2% in 2015/16 in Colorado. This put Colorado significantly above the US average of 20.3%.

Among adults, past-30 day cannabis use by adults increased from 5.4% in 2005/06 to 14.0% in 2015/16. Figures have stabilised since 2014/15. 

The report also notes that Colorado has typically always been above the US average when it comes to cannabis use even before legalisation – in 2010 the Colorado figure was 27.3% compared to the average of 18.8%. A point much-repeated in US research is that states cannot be seen as “natural experiments” – states like Colorado that legalised first medical and then recreational use already had exceptionally high rates of cannabis use before any laws were passed. 

The age of first use of cannabis in Colorado remained relatively stable between 2008 and 2017 at between 14.6% and 14.8%. National Survey on Drug Use and Health figures, from the report, show that past 30-day use of cannabis among 12-17 year olds increased to a high of 12.6% in 2013/14 before declining to 9.1% in 2015/16.

Overall treatment admission rates for those reporting cannabis as the primary drug also decreased, from 222 in 2012 to 176 in 2017. 

The report also includes a survey of 53,850 students from Colorado schools. The proportion of high school students, it finds, reporting cannabis use ever in their lifetime remained largely unchanged between 2005 and 2017. This trend was echoed in past 30-day use.

The report also offers a health warning for the data. The authors warn that legalisation may result in people being more comfortable with reporting use, rather than actually capturing changes in usage. It also states: “Complex and sometimes conflicting laws have caused law enforcement officials and prosecuting attorneys to modify policies and practices that cannot be disentangled from available data. For these reasons, it is critical to avoid ascribing changes in many social indicators solely to marijuana legalization.” was provided with a number of other reports and articles on US cannabis usage by the GP group. No report contained evidence of an exponential increase in cannabis use in the US in states following legalisation of medicinal cannabis. One report also states: “No information is yet available on the relationship of recreational marijuana laws to changes in the prevalence of adult cannabis use and consequences.”

Another article sent by the GPs came from the Canadian Medical Association Journal, dated October 2018. said: “For better or worse, it is unclear how much has changed in the five years since Colorado legalized cannabis… It’s too soon to know the long-term health impacts of the policy.” The article also states: 

Legalization didn’t change prior trends showing that “Coloradans like marijuana and they like it more over time.”

An article from 2017, published in an American Medical Association journal, looked at cannabis use across the US. The authors did find that adult usage increased to a greater degree in states that had legalised medicinal cannabis. However, they also noted that studies have not shown an increases in adolescent cannabis use follow legalisation of medicinal cannabis. It also notes that research is still needed on the effect of recreational cannabis laws.

There is a general consensus that cannabis use has increased over time in the US. However, no report or article suggests that states have experienced an exponential increase in cannabis use following legalisation – whether for medicinal or recreational purposes. While many studies suggest that adult use of cannabis does increase somewhat following legalisation for medical use, there is no consensus on whether it has increased in use among adolescents.

As one academic review also states: “The legalization of marijuana is not a single event but rather a series of unfolding changes—the vote itself, the gradual establishment of a regulatory system, the emergence of retail sales outlets, and the development of an adequate legal supply of the product.” That particular study, sent to by the GP group, also states:

Methodological issues aside, those pushing for marijuana legalization can now refer to several peer-reviewed studies from some of the best journals in the world to argue that increasing access to medical marijuana does not increase marijuana use among kids and may yield significant public health benefits.”
It is hard to come to definitive conclusions about the impact of legalistion and many academic studies stop short of drawing clear links between legalisation for medicinal or recreational use and increasing use among adults. 


Cannabis is not legal in Portugal. Since 2001, the consumption, acquisition and possession for personal consumption of all drugs has been decriminalised. 

This means that a person caught using or possessing a small quantity of drugs for personal use, where there is no suspicion that the person is involved in drug trafficking, will be given a warning, a small fine or told to appear before a three-person commission consisting of a lawyer, a doctor and a social worker.  

The available data is somewhat limited in Portugal. Data from the European Monitoring Centre for Drugs and Drug Addiction (ESPAD) for Portugal in 2016 found that last-year cannabis use among young adults aged between 15-34 years was 8%. 

Usage has fluctuated over the last decade. In 2007, among 15-34 year olds, last-year drug use was 6.7%. It dropped to 5.1% in 2012 and then rose steadily between 2012-2016. EU figures suggest this is well-below the European average, going by last-year use. Among young adults in France, for instance, last-year use was 21.5% and in Ireland it was 13.8%. 

The ESPAD survey also found that between 1995 and 2015, use of marijuana or hashish during the last 30 days had risen from 4% to a high of 9% in 2011, before dropping to 8% in 2015. 

In 2012, a national population survey on drug usage in Portugal was held. The survey found that in the general population, aged 15-64 years, lifetime prevalence of cannabis use was 7.6% in 2001, 11.7% in 2007 and 9.4% in 2012.

In terms of the last 12 months use, cannabis use was 3.3% in 2001, 3.6% in 2007 and 2.7% in 2012. 

While cannabis use has not increased exponentially, it has risen since decriminalisation


Since October 2018, cannabis has been legal for recreational use in Canada. Adults who are 18 years of age or older are able to possess cannabis for recreational purposes, with restrictions on the amount you can possess and the amount you can share with other adults.

The transition to legal production has been challenging. In April, The Guardian reported that due to the inability of legal producers to meet surging demand, many consumers found themselves using illegal markets. The Guardian also quotes government figures from January 2019 which suggest that cannabis sales on the illegal market are $5 billion, compared to $2 billion in legal sales. 

The Canadian experience of legalisation is very recent and very limited data exists. But Canada’s statistics office has been conducting a national cannabis survey to monitor changes following legalisation and while the data is self-reported, making it more limited, it does offer quarter-by-quarter figures for cannabis use from January 2018.  

In the first quarter of 2018, reported usage was 14%. One year later, the figure has risen to 17.5%. The number of first-time users was nearly double the 327,000 estimated first-time users a year earlier. Statistics Canada did find that “first-time users in the post-legalization period are older. Half of new users were aged 45 or older, while in the same period in 2018, this age group represented about one-third of new users.”

They also report that, following legalisation, 6% of Canadians aged 15 or older reported using cannabis on a daily or almost daily basis. Compared to the first quarter of 2018 before legalisation, the report finds, daily use has remained stable. Weekly use did increase, however, from 2% to 4%.

Data problems

One problem with international comparisons is that countries are all incredibly different and even the experience of US states following legalisation varies significantly. As one 2017 study notes, states that have legalised medicinal cannabis use are not “natural experiments” and it is important not to confuse causation and correlation when attributing straightforward relationships between legalisation of medicinal or recreational cannabis and changes in usage among the general population. This is due to the wide range of other factors that could impact usage, such as attitudes to cannabis pre-legalisation and the specific regulations put in place.  

One other factor is that in the US and Canada, legalisation of recreational cannabis use only occurred within the last few years. This makes assessing changes in use and coming to definitive conclusions about the cause of any change particularly difficult. 


“Exponential” has a specific definition and means that an increase has become more and more rapid over time. However, it is often used colloquially, or non-scientifically, to describe a significant increase. 

The difficulty in proving and disproving claims over cannabis usage following legalisation or decriminalisation is that various studies of various states disagree on how rates of usage should be measured and emphasise that causation and correlation cannot be confused. 

Some places have experienced an increase in cannabis use following legalisation of medical cannabis. However, many US states experienced higher than average cannabis usage before any laws were introduced. 

The claim that legalisation of cannabis for recreational use has caused exponential spikes in usage is also difficult to prove, simply because the introduction of these laws in Canada and the US is so recent. 

Measuring numbers alone can prove difficult because it also doesn’t reveal the potency of cannabis people are using, for instance, or the various other motivations for reform of cannabis laws.

So while it is difficult to find settled evidence that use has increased significantly following legalisation of cannabis, it is also hard to come to definitive conclusions considering the vast array of studies and reports out there. 

As a result, we rate the claim legalisation of cannabis in Ireland could lead to an “exponential” increase in use: UNPROVEN

As per our verdict guide, this means: The evidence available is insufficient to support or refute the claim, but it is logically possible.’s FactCheck is a signatory to the International Fact-Checking Network’s Code of Principles. You can read it here. For information on how FactCheck works, what the verdicts mean, and how you can take part, check out our Reader’s Guide here. You can read about the team of editors and reporters who work on the factchecks here.

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