Cannabis and Your Mental Health

Cannabis and Your Mental Health

What is schizophrenia?

Schizophrenia is a type of psychotic disorder. The Royal College of Psychiatrists define it, as ‘a mental disorder which affects thinking, feeling and behaviour’ (RCP, 2008).

In the UK, schizophrenia affects 1 in every 100 people during their lifetime, a staggering statistic which leads one to believe that it deserves greater attention. It is thought that environmental and genetic factors can trigger the onset of schizophrenia. One of these triggers is considered to be cannabis.

Why cannabis?

The link between cannabis and schizophrenia was first noted in the early 1970s (Davidson & Wilson, 1972; Bernhardson & Gunne, 1972). Since then, there has been a considerable amount of interest in this area, not least a political one as this association led to reasoning behind the classification change of cannabis from class C to B (January 2009).

The main psychoactive substance in the Cannabis Sativa plant is delta-9-tetrahydrocannabinol (Δ-9THC) (Rathbone et al., 2008). Whilst it is appreciated that other cannabinoids, particularly cannabidiol (CBD), play a significant and opposing role in mental states to THC, it is THC that will be focused on here. The effects of other cannabinoids will be a discussion for another blog entry.

Although there are many proposed biological causes behind this association, the exact mechanisms are still not known. Some of these mechanisms are thought to be due to an increased level of the endogenous cannabinoid agonist, anandamide which was seen in the cerebrospinal fluid (fluid inside the brain) of people with schizophrenia (Hall & Degenhardt, 2008). It has also been thought that the dopamine (Murray et al., 2007) and acetylcholine (Domino, 1981) systems play a role.

The question here, however, is whether smoking cannabis causes schizophrenia or whether it is one component of many. Is this correlation a cause or an effect?

Five Theories

It has been established that there are 5 hypotheses underlying this relationship between smoking cannabis and the development of schizophrenia (Smit et al., 2004):

  1. ‘self-medication hypothesis’: people who are suffering from schizophrenic symptoms use cannabis in order to cope with the negative symptoms.
  2. ‘effects of other drugs hypothesis’: most people who take cannabis also take other drugs such as amphetamines, opiates and cocaine and these may be provoking the development of schizophrenia.
  3. ‘confounding hypothesis’: the association between cannabis use and schizophrenia is caused by more than one factor so their relationship is spurious.
  4. ‘interaction hypothesis’: only in people who are already at high risk of developing schizophrenia are capable of developing it. This risk may be genetic and triggered by such things as cannabis.
  5. ‘etiological hypothesis’: that cannabis makes its own unique contribution to the risk of becoming schizophrenic.

The Swedish Study

One of the first and largest longitudinal studies (spanning over 15 years) on the effect of cannabis and development of schizophrenia was the Swedish study who found that of the 45,570 cohort (men who were conscripted in the army), very frequent cannabis users (used on more than 50 occasions) had 6 times more of a risk for schizophrenia than non-users.

They also found that those who had only used cannabis at least once had a 2.4 times greater risk than non-users (Andreasson et al., 1987). Although it was seen that conscripts who appeared to have no psychiatric symptoms did have an increased risk of schizophrenia, these findings could not establish whether it was causal or not.

This cohort was re-visited more recently in 2012, where they found a three-fold increased risk for schizophrenia in those using cannabis most frequently. They concluded that this study strengthened findings of association between cannabis use and psychotic disorders. However, it is not clear what the participants’ cannabis use was before the study, whether the participants were under- or over-reporting their cannabis use, and whether there was any association with traumatic experiences either during their time in conscription or otherwise. Their findings are also not applicable to milder forms of schizophrenia, i.e. forms that do not require in-patient care (Manrique-Garcia et al., 2012).

Genetic Link

There have been studies showing the link between effects of cannabis use in people who are genetically at a higher risk of developing schizophrenia. It has been shown that 92% of cannabis users do not develop schizophrenia (Arseneault et al., 2004; Caspi et al., 2005) and this remaining 8% can only be explained by the presence of certain genes.

An interesting study showed that although carriers of a mutated gene (for COMT – an enzyme that is involved in breaking down dopamine and norepinephrine, among others) are more psychosis prone, there is not a ‘main-effect risk’ for carriers and they were not more likely to use cannabis (Caspi et al., 2005). This provides evidence for the interaction hypothesis as people who are genetically at higher risk for schizophrenia have a vulnerability to mental health problems associated with cannabis use (Hollis et al., 2008).

In spite of this, there are studies that counter this link by concluding that there is no evidence of genetic-environmental correlation. Cannabis and psychosis may be directional, in that it is not only a causal factor in the development of schizophrenia but also as self-medication for distress or early warning signs of other mental health problems. It may be due to interactions between genetic factors and cannabis rather than the interactions between cannabis use and a predisposition to schizophrenia (Veling et al., 2008).

It must also be taken into account that these genetic studies have predominantly focused on adolescents as their brains are considered to be most vulnerable to changes. These studies do not provide any evidence for adult-onset cannabis use and it is generally considered that cannabis use after adolescence does not increase the risk for the development of schizophrenia, regardless of gene or environment interactions.

Physical Brain Changes

More recent brain imaging and neuroscientific studies have shown that cannabis may alter brain structure. One study in particular found that this was relevant in areas of the brain that are rich in cannabinoid receptors (CB1) in users compared to non-users (Bangalore et al., 2008). In another study, it was seen that chronic cannabis users show alterations in nerve cells within the areas of the brain closely associated with schizophrenia (DeLisi, 2008).

These studies usually cause greatest alarm as it identifies physical and usually irreversible changes to the brain.

However, the association between cannabis users and changes associated to these areas have not been clarified. The first study used a small sample size and the relationship between the estimated brain structure and CB1 receptor changes was vague. It is likely that people who smoke cannabis and develop psychotic episodes have low activity in the gene that codes for dopamine (the dysregulation of which is the key driver in schizophrenia) which potentiates the effects of cannabis and is able to provoke acute psychotic reactions.

Cannabis Psychosis

Recently, there are a growing number of scientists who consider that there is a specific cannabis psychosis. Patients with cannabis psychosis are thought to display more odd bizarre behaviour. They were also seen to have less formal thought disorder and an enhanced insight compared to those with schizophrenia (Hall & Degenhardt, 2004). However, another study found very few differences in the aetiology between schizophrenics who were cannabis users and non-users and opposes the argument that a distinct schizophrenia psychosis caused by cannabis exists (Boydell et al. 2007).

Contradicting Studies on Self-Medication

The Dutch Study

A Dutch study was conducted on 4045 people who had no history of psychotic symptoms (Van Os, 2002). This study demonstrated that the risk for users or non-users of cannabis without a history of psychosis is 2.2% whereas the risk for those with a history is 54.7% (Van Os, 2002). The study implicated that the later onset of schizophrenic symptoms was preceded by prior cannabis use. However, this finding does not confirm that people do not use cannabis to ‘self-medicate’ as the follow-ups were not consistent.

The New Zealand Study

A longitudinal study in New Zealand examined the mental health state of 1037 individuals and concluded that the risk of developing schizophrenia was specific to cannabis and not the use of other drugs (Arseneault et al. 2002). Symptoms of psychosis prior to cannabis use were controlled in this study and it was shown that the use of cannabis was not secondary to symptoms that already existed. This study demonstrates that the self-medication hypothesis is not a valid explanation for the relationship between cannabis use and the development of schizophrenia as it was shown that cannabis use preceded symptoms of psychosis.

On the other hand, it is considered that individuals with the slightest ‘social anxiety or the softest expressions of subtle psychosis-like experiences, may be more likely to start using cannabis so as to ‘self-medicate’ their distress.’ Therefore, it may be that the existence of subtle psychotic experiences rather than prominent clinical symptoms is enough to provoke the development of schizophrenia with the use of triggers such as cannabis smoking (Henquet et al., 2005).

Study on Young People

Another study found that young people who met the DSM-IV criteria for cannabis dependence had twice the rate of psychotic symptoms than those who were not cannabis dependent (Fergusson et al., 2003). This was despite factors such as pre-existing psychotic symptoms, other drug use (etc.).

According to this study, schizophrenic symptoms develop in young people independently to pre-existing symptoms and other social and contextual factors. This occurs only in people who have cannabis dependence.

However, although the study claims that cannabis dependency preceded schizophrenic symptoms; in reality it is uncertain whether the study achieved this. Data was gathered at various points in the development of each subject. Despite the fact that analysis did show a causal relationship, the direction of this causality still remains unclear. This criticism is applicable to many other longitudinal studies using similar methodological analysis.

The possibility that the self-medication hypothesis is an explanation between the association of cannabis and the development of schizophrenia is further open to debate.

Furthermore, the exact association between measures of psychotic symptoms and psychosis at clinical levels is still unclear.

Concluding Remarks

Simply put, cannabis users are more likely to show psychotic symptoms than non-users. This suggests the existence of a continuum between the increased use of cannabis in subjects with psychosis proneness and between cannabis use and psychosis proneness in subjects without clinical psychosis.

Cannabis appears to be an independent risk factor for psychotic symptoms, especially in people with a pre-existing vulnerability (Verdoux, 2004). This is supported further by the evidence that cannabis is considered a component cause of schizophrenia and is part of a wider and more complex constellation of other components (Arseneault et al., 2004).

These points support the majority of studies that are available, most of the significant ones have been described here. In spite of this ever-increasing espousal of the observed associations between smoking cannabis and the development of schizophrenia, there are some rather great concerns. In essence, the problem with these studies is that they all examine this relationship differently and many studies also greatly contradict each other.

The difficulty here is that schizophrenia is extremely difficult to diagnose and smoking cannabis does play a part but the implications of this are complex. It can be said for certain that this relationship is not spurious but cannabis is not the only causal factor, it is one of many.

The quote below from the Advisory Council for the Misuse of Drugs, although slightly outdated, is still very relevant:

‘Although there is a consistent (though weak) association, from longitudinal studies, between cannabis use and the development of psychotic illness, this is not reflected in the available evidence on the incidence of psychotic conditions. The most likely (but not the only) explanation is that cannabis – in the population as a whole – plays only a modest role in the development of these conditions.’

- ACMD report, 2007 p. 33



Advisory Council on the Misuse of Drugs. Cannabis: classification and public health. Home Office. 2007.

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Bangalore, S. S., Prasad, K. M. R., Montrose, D. S., Goradia, D. D., Diwadkar, V. A. & Keshavan, M. S. Cannabis use and brain structural alterations in first episode schizophrenia – a region of interest, voxel based morphometric study. Schizophrenia Research 2008 99: 1 – 6

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Hollis, C., Groom, M. J., Das, D., Calton, T., Bates, A. T., Andrews, H. K., Jackson, G. M. & Liddle, P. F. Different psychological effects of cannabis use in adolescents at genetic high risk for schizophrenia and with attention deficit/hyperactivity disorder (ADHD). Schizophrenia Research 2008 105:216-223

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An interesting blog, thanks. I’m not a health expert and I’m not a stakeholder, I’m a 66-year old victim! My daughter is 31 come Sunday and she’s one sad case. Adopted at birth, I don’t know what ‘baggage’ she came with from her birth mother but consider this. She’s tiny, 140cm, she has mental health problems, she’s been in and out of Styal prison (the only place we have ever seen real treatment), and her personality disorder is more than exacerbated by ‘substance’ misuse. She will not see 40 without a miracle.
At 12 or so, the smallest in the class and hating being a girl, she was oft times set up to fail or she would go that extra mile to demonstrate that she wasn’t ‘little’ Emma. And that extra mile was initially cannabis. I know lots of cannabis users and for them it’s a happy drug. For Emma, it took her to dark, dark places. We have never had any constructive support for her mental health issues because Trafford social services were happy to tick the ‘drug user/own fault’ box. My happy, lovely little diamond’s journey to become the little monster she is today began through cannabis.
Were there underlying psychotic problems from birth? Probably, and the ‘exact’ science of hindsight points to a personality disorder – looking back, she ticked all the ADHD boxes at an early age and before I’d ever heard of it! The support at Styal prison was exemplary and they had her as an ‘arrested adolescent’ but sadly, as that work began to help she was released on to the streets of Manchester with no follow up. At school, she had been described as ‘street-wise’ and ultimately expelled.
Cannabis is not for everybody clearly, but for some the result is a disaster and it helps destroy not just the user but those around the user. I’ll never stop loving her, but I don’t like what I see these days and I am helpless. More needs to be done…..and that last quote should end with ‘for most people’.

Thanks for sharing your story. I think this highlights the complexity of the issue, and I'm sorry that you and your family have gone through these difficult times.

Thank you for this excellent review.

Cannabis does not just simply "cause" schizophrenia. It may induce it, but does not "cause" it. The causes are deep rooted within the persons psyche, ultimately their sense of self. For example, if someone has a feeling their whole lives that they are "different" or struggle to interpret the world around them, smoking cannabis will deeply show this truth to them and they will realise that they cannot relate to others within the world and that they are truly different. Not being able to fit in and have a true sense of self will spiral down into what some people may call "schizophrenia". This will happen to these people at some point in their lives, but cannabis may speed up the process revealing truths to these people.

From personal experience, I have Asperger's but I didn't know this until I was nearly 19. My parents and others had hidden this fact from me my entire life. I had been smoking cannabis since around 15-16, no problems at all. But when your life and your ego is developing, and should be reaching pivotal points in development and you cannot relate nor understand- this leads to a very poor sense of self. The ego is my, your and everyones everything. Instead of pushing the blame onto cannabis, the best way to deal with issues like this is to deal with them directly through psychological treatment. Cannabis is not the problem, it is the person's sense of self.

Thank you for your comment. The point was to make readers aware of the range of evidence on this subject. Of course, cannabis does not simply cause schizophrenia, the point here, in my opinion, is that cannabis can be considered one cause of many.

Destruction of one's ego leads to inner peace, freedom from outside and internal influence and allows for clear, objective thinking. Once in this state, mental disease becomes much less likely due to having developed a strong executive function, that is, the real and actual "you".

the most informative piece of writing i have yet seen on this topic. thank you

There is an elephant in the room: Tobacco. Most cannabis users mix cannabis with tobacco, a substance known to have a complex effect on mental illness.

At the very least tobacco use is an important confounding issue, yet is never acknowldged as such.

I would also draw attention to the point made at the start of this blog regarding other cannabinoids present in cannabis. Although acknowldged at the start the term "cannabis" is used throughout as as if we are talking about a single, consistent, product here and we are not. The term "cannabis" can mean varieties with virtually no CBD to almost equal proprtions of THC and CBD. I would argue that without knowing the variety of cannabis consumed making a definitive statement about the relationship between cannabis and menatl health is never going to be easy. Of course, to accept this is to draw attention to the problems caused by prohibition, so the cynic in me concludes it's unlikely to be acknowledged...

I take on board that point. Of course 'cannabis' as a whole refers to all cannabinoids, but this blog's aim was to highlight the studies on this issue; almost all the longitudinal studies used the term 'cannabis', and very few examined the variety of cannabis used, unfortunately. We do believe that CBD has not received as much attention as it deserves, and that's the reason we intend on discussing cannabinoids (other than THC) and mental health at a later date.

I understand that, it was just for anyone else who may come across this article. I agree with you also, the CBD content is everything. THC is one chemical it will not produce nearly as wide a range of subjective effects.

Cannabis does not cause anything but happy, hungry & sleepy people!
The problems come from having to associate with criminals to get it, being affraid of getting caught & all the other things that go with keeping secrets and avoiding the law!
I have met people who will flip out on me for using cannabis! The same people usually have a beer in one hand & a cigarette in the other!
Lucky for me I'm a happy head strong individual with an education & a good job.
If I was a kid from a broken home surrounded by future criminals then it would possibly be a different story! Not that under 18's should be using it!
I'm tired of cannabis getting blamed for societies ills. The herb has been used since the beginning of human civilization before alcohol, before tobacco, before tea & most definitely before coffee-eurgh! This government has systematically destroyed the average British family as well the economy! Alcohol & tobacco cause more problems even coffee or lack of will make a person crazy, but lets keep blaming cannabis because you cant patent cannabis.....unless your Monsanto & GW pharmaceuticals!

PS.......People do realise that CBD turns into THC as part of its breakdown!

It is known that we can be possessed by angels.. You have to use sense , prays , meditation and love from your mind heart body and soul to get rid of the bad angels(gjins) that lead to missbelief weekness and bad at the end. Egjins the good angels is who you want to be lead of. There are people who are so overtaken by these bad angels that you can notice by the way the act talk ect... You can specially notice if a person is possessed and se the difference and shift more clearly when the person is intoxicated.. So why do we who are strong hace to suffer for those people? I need the weed and only the WEED cause of painreliefe.. And these Shrinks.. Doktors ect just lie and give people different diognessis and play along with the moneymaking and counter religion.

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