What are the lessons from America’s great cannabis liberalisation experiment?

The US has come quite some way in legalising cannabis use, with nearly half of states having laws permitting recreational cannabis use. Yet the legalisation of cannabis remains an ever-present, heated and polarising debate . One that is often exploited by politicians to win votes, either by stoking fear of the harms that will come from legalising cannabis, or by the promise of decreased violent crime and increased tax revenue from cannabis sales.

The science around the harms of cannabis is relatively well-established. Frequent use of high-potency cannabis and starting use at a young age is a risk factor for various mental health conditions – particularly schizophrenia. It can also lead to cognitive impairment and poor educational attainment, as well as increasing the risk of developing cannabis-use disorder (addiction).



Using cannabis before driving – within about eight hours of consumption – increases the risk of road traffic accidents . And if it is used during pregnancy it can cause birth complications. The important question is: how are these harms affected by legalisation? Cannabis potency is a main driver of cannabis harm, and US states with legal cannabis have seen a dominance of high-potency products .

The increase in potency began, however, during prohibition in both the US and Europe . There is also some evidence that cannabis potency levels in legal retail stores are exaggerated to make the products more appealing to customers. Cannabis use rates are higher in states where cannabis is legal .

However, this does not mean that legalisation necessarily drives people to use cannabis. Overall, the evidence is somewhat mixed. On the whole, though, the data suggests that cannabis legalisation is associated with an increase in adult cannabis use.

But most studies suggest no change in adolescent use. This topic is complex to research because background trends are that cannabis use rates in both the US and Europe have been rising since the early 2000s, long before legalisation was introduced. Rates of substance use, such as cannabis, are normally gathered from surveys, which have historically been prone to under-reporting due to “social desirability biases” – when respondents to surveys conceal their true opinion on a subject to make themselves look good.

One good example of this is from research conducted in Sweden, which, despite its well-known progressive social values, has a prohibitionist attitude to drugs compared with other European countries. For example, police can forcibly take blood samples if they suspect someone has taken drugs. Ordering drugs from a drug dealer is also a specific crime (“attempted drug offence”).

In an innovative study approach, where researchers indirectly asked people about their cannabis use (known as the “crosswise model” ), Swedes reported almost two to three times as much cannabis use as in standard surveys. This is in line with other research which found that people are more likely to admit to doing risky things depending on how they are asked about it . One cause for concern is the observation that legalisation has increased visits to emergency departments due to cannabis-induced psychosis , as well as accidental poisoning in children .

This is especially pronounced in places with high densities of cannabis shops. A contributor to this has been the increase of high-potency edible products , often in the form of sweets. The province of Quebec in Canada has counteracted this by putting restrictions on edibles’ potency and restricting products that could appeal to children.

This has resulted in shops selling cannabis-infused vegetables instead of sweets. Early results suggest Quebec has fewer children needing emergency care and has not seen a rise in acute psychoses . Depends on what measures you put in place The legalisation of cannabis is not a binary choice.

Drug policy decisions within a legal framework can have a substantial effect on the number of people who use a drug and the level of harm the society experiences. For instance, we know that marketing , number of stores , opening hours , taxes and price , labelling , minimum age limits , minimum unit pricing influence our most commonly consumed intoxicant, alcohol, and its harms. Uruguay legalised cannabis in 2012 and adopted several of these measures.

It has not seen an increase in cannabis use , perception of harm , or rates of cannabis addiction compared with neighbouring countries. Lastly, a serious debate on legalisation should also consider the effectiveness and potential harms of prohibition. Prohibition leaves the vast profits of cannabis sales (and other drugs) to criminals, which helps them grow and expand their organisation and activities.

And studies have found links between increased drug law enforcement and increased violence . Many more harms of prohibition have been discussed by experts , and the evidence that punitive laws prevent cannabis use is very weak . Overall, there is no perfect drug policy, and drugs such as cannabis will always exist in human societies.

Future cannabis policy considerations should take a nuanced and evidence-based approach, learning from the US and other parts of the world where cannabis laws have been relaxed..