DEBATE: Should we delay legalisation until we understand the link between cannabis and psychosis?

by Ella Walsh

In a recent Sunday Times Magazine article Megan Agnew sheds light on the issue of cannabis and psychosis. The article features some compelling personal stories from people who have struggled with cannabis use, and subsequently developed psychosis.

As stated in the article, cannabis is the most commonly used drug in the UK. While cannabis use among the general population has decreased over the years, the number of people being hospitalised for cannabis-related mental health problems has continued to rise. The increasing number of people experiencing psychosis is thought to be a consequence of high potency cannabis dominating the illicit market. 

What is ‘Skunk’?

High potency cannabis, known as ‘skunk’, is where cannabis contains much higher levels of THC than CBD. The article cites a study which found that the average ratio of THC to CBD in 2008 was 1:1, then by 2016 the average ratio was 3:1. 

THC, CBD, and all the other hundreds of active compounds found in the cannabis plant, have a wide variety of effects. Put simply, THC is a psychoactive compound found in cannabis which causes the ‘high’ effects, and can lead to anxiogenic effects. CBD, while still technically psychoactive, produces almost counteractive calming effects, which we understand is beneficial for reducing the symptoms of anxiety. When the ratio of these two compounds are out of balance, i.e there are higher levels of THC than CBD, the effects of the plant dramatically alter.

Why has cannabis become more potent? 

The prevalence of high potency cannabis can be attributed to innovative indoor growing methods which have developed over the past 30-50 years.

With the introduction of indoor growing systems, and increasingly precise growing techniques, growers now have full control over the conditions under which the plant is grown, as well as the seeds they are using. This has led to the development of ‘strains’ which have higher levels of THC than would normally be found in cannabis grown outside.

Does cannabis cause psychosis? 

This article has opened up a debate around whether we should hold off from legalisation until we fully understand the relationship between cannabis and psychosis. Or, whether we should  regulate cannabis to address the existing harms of high potency cannabis. 

Giving his opinion in the Sunday Times Magazine, Philip McGuire said that it was “really important that big decisions like that are deferred until we really know how linked cannabis addiction is with psychosis”. 

The link between cannabis use and psychosis is a thoroughly explored topic, and often used as an argument against the legalisation of cannabis. The research is clear in establishing that there is a link between cannabis use and psychosis. With this, many fear that if cannabis were to be legalised, people would assume that it is safe, consume more, and rates of psychosis would rise even more rapidly.

Those in favour of reform argue that potency of cannabis and associated harms have only ever increased during the century that cannabis has been controlled, and half a century that it has been criminalised. On this side of the debate, existing prohibitionist drug policy is seen as perpetuating the harms of cannabis, and a regulated cannabis market is seen as the best way to regain some control over the problem. 

THE CASE FOR: Refraining from legalisation until the link between cannabis and psychosis is better understood.

The link is clear.

There is a clear trend among the stories and experiences shared in the article, where those who have experienced psychosis strongly identified cannabis use as a contributing or driving factor. While the evidence base may be inconclusive, this very fact is cause for concern. Scholars are still uncertain of the extent to which the development of psychosis can be attributed to cannabis use.

One multicentre case-control study spanning over 5 years analysed data from 11 sites across Europe to further explore the relationship between cannabis use and psychotic disorders. The study found that those who used cannabis daily, compared to those who had never used cannabis, were more likely to develop a psychotic disorder. They also found that if high potency cannabis was used daily, people were 5 times more likely to develop a psychotic disorder. 

The results of this study show that there is a clear link between frequency of cannabis use, as well as the potency of cannabis used, and the likelihood of developing a psychiatric disorder. This becomes extremely worrying when we begin to discuss a legalised market, which could potentially mean that more people will be exposed to cannabis than while it remains illegal. 

Bad precedent abroad. 

In theory, it may be possible to legalise cannabis without increasing the prevalence of use, or availability of high potency cannabis. However, reforms which have taken place across North America shows that legalisation doesn’t necessarily translate to harm reduction. 

One study exploring the impact of legalisation in the US found in states where cannabis is legal, the price of cannabis had fallen, and cannabis use as well as dependence had increased. 

Ultimately, there has been a precedent set by the regulatory models from the United States and Canada that most worryingly that doesn’t include THC limits. The commercialisation of cannabis in places such as California has instead allowed for high potency cannabis to be legally sold without much regulatory oversight into the potential harms of high potency cannabis, or measures to mitigate that risk.

What we don’t know.

The reality remains that legal regulation has not yet been done in a way that sufficiently reduces harms. While a legally regulated market would, after some years, bring us a deeper understanding of these unknowns, is it morally permissible to even establish this?

Hospitalisation for cannabis induced psychosis is rising, and as mentioned in the article, there is already a lack of suitable treatment pathways for these people. Therefore, developing a better understanding of the relationship between psychosis and cannabis will be fundamental for informing and constructing regulatory frameworks which can effectively address the harms associated with cannabis use. 

Alongside developments within the UK’s own research, holding back from legalisation will allow us time to receive and interpret data from the US and Canada’s markets to best inform policy. 

The need to wait.

There is undoubtedly an urgency to reform cannabis policy.  BAME people in the UK are more likely to be stopped and searched, arrested, and prosecuted for crimes related to cannabis possession. The over-criminalisation of young people, specifically those BAME young people, has deleterious effects on their future education and employment prospects.

But perhaps it should be considered whether these harms could be alleviated by decriminalisation?

THE CASE AGAINST: Progressing with legalisation because of the harms caused by cannabis use in an unregulated illicit market.

Prohibition isn’t working. 

Agnew’s article states that there are increasing levels of hospitalisations related to cannabis use and psychosis in the UK. However, it could be argued that these revelations only further highlight how the harms of cannabis are amplified in a non-regulated illicit market. 

One of the conclusions from the multi-centered case control study found that if high potency cannabis were no longer available, 30% of cases presenting with psychosis for the first time could be prevented in London. Moreover, another study found that in order to prevent one case of psychosis, you would have to prevent 20,000 from using cannabis.

The fact of the matter remains that prohibition isn’t preventing anyone from using cannabis, or suffering from psychosis. Non-regulation of cannabis has instead allowed for high potency cannabis to dominate the illicit market. Meanwhile those who use cannabis are stigmatised, criminalised and ostracised.

Causes of psychosis.

There may be an ever growing evidence basis exploring the relationship between cannabis and psychosis, but it is important to consider by nature of the scientific method that research findings and conclusions have inherent limitations. Often when links are established in research it is difficult to determine whether the relationship is one of correlation or causation. Where cannabis and psychosis is concerned, most studies contradict each other and we are yet to form a cohesive view.

It is the classic chicken and egg dilemma. 

What we do know, however, is that cannabis use isn’t the only risk factor for developing psychosis. Therefore, it’s important to question whether instead of trying to reaffirm the link between cannabis and psychosis, we should place more focus on research which seeks to comparatively explore cannabis alongside other risk factors. 

Nature, Nurture or Cannabis?

Within all that is unknown about the nature of the link between cannabis and psychosis, one important area of research is the role that both genes and environment play in this relationship.

A longitudinal co-twin study recently found that the association between adolescent cannabis use and risk of developing a psychotic disorder is more dependant on ‘familial confounding factors’, such as genes, environment, shared childhood experiences, than cannabis use alone.

The conclusions of this research have the potential to profoundly change our understanding how genes and environment influence the relationship between cannabis and psychosis.

Cannabis vs Alcohol.

One study examined the impact of alcohol use on the relationship between cannabis use and developing psychosis in ‘clinically high risk’ people. The study found that the association between cannabis use and psychosis was confounded by alcohol use, meaning that the association between cannabis and psychosis becomes weaker when you also consider the role of alcohol. Moreover, the research also concluded that there was no relationship between impairing cannabis use and psychosis.

Consuming a bottle of vodka every day is completely legal, however, the majority of people make a choice not to. 


Alcohol regulation is complemented by public health policy which prioritises education and harm reduction. People who choose to consume alcohol are provided with evidence-based guidance for how they can do so recreationally, whilst reducing the harms associated. In this regulated market, alcohol consumers are offered a variety of products, with clear information on strength which enables them to make informed choices. 

One review of alcohol policy reforms which took place in Russia shows how regulation can play a role in addressing the rising levels of psychosis. Prior to policy reforms, Russia had high rates of psychosis associated with widespread consumption of vodka. The review found that once regulation had been introduced, the prevalence of alcohol-induced psychosis dramatically decreased because consumers we drinking lower percentage alcohol such as beer. Moreover, the findings also concluded that if vodka sales increased by just 1 litre, the rate of alcohol induced psychotic incidents would increase by 27.4%.

There is already widespread availability of high potency cannabis in the UK, therefore regulation is an essential step towards changing the nature of the market. Preventing cannabis users from having a legitimate choice, and making informed decisions with regards to the cannabis they consume is compounding the harms and risks associated. 

Both drugs carry a risk of psychosis, why should cannabis be treated differently to alcohol?


Should we hold back from legalising cannabis until we better understand the relationship with psychosis? 


Should we instead legalise and regulate cannabis to reduce existing harms created by prohibition? 

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