16 May Does Cannabis Make You Mad?

Reefer Madness

Reefer Madness Or A Cause For Concern?

Are the claims of a link between cannabis and mental illness prohibition inspired hype, or a genuine issue that needs serious consideration? The answer is, perhaps strangely; both.

The term “Reefer Madness” was coined in the 1930’s by a film of that name which claimed to be based on a factual account which supposedly exposed the “truth” about cannabis and mental illness. Reefer Madness wasn’t a true film and the claims it made look somewhat stupid today, they were pure propaganda and a part of the emerging war on drugs. It would be nice to think that media coverage of the whole issue has improved since those days, but it hasn’t.

More recently new claims of a link between cannabis and mental illness were made following the reclassification of cannabis which happened in 2003. On the first anniversary of that downgrading the mental health charity “RETHINK” launched a campaign to highlight the claimed risks of using cannabis.

The RETHINK campaign was, some would say, hijacked by the prohibition movement who saw the whole issue as a reason to oppose any form of cannabis law reform and the media was only too happy to help spread the alarmist stories.

Even supposedly quality newspapers joined in, perhaps the worst example being the Independent on Sunday which ran an “apology” for having previously run a “Decriminalise cannabis” campaign in 1997. The paper made some simply outlandish claims [1] including “Record numbers of teenagers are requiring drug treatment as a result of smoking skunk, the highly potent cannabis strain that is 25 times stronger than resin sold a decade ago“. This sort of rubbish became known as “Reefer madness V2”

There is a correlation between cannabis use and serious mental illness however which several studies have demonstrated; people with mental illness do tend to use a lot of cannabis and people who use a lot of cannabis seem to develop the illness more than those who don’t.

Now the subject of mental illness is one which frightens a lot of people and it’s one of the few remaining taboos – subjects we don’t like talking about. As such the claim that cannabis makes you mad was a very powerful campaigning weapon for the prohibition movement. In truth it’s a complicated issue which has been grossly oversimplified. That said a case can be made for assuming the worst and guarding against a potential causal link by protecting people who might be at risk. But it isn’t the big prohibition supporting argument we have so often been led to believe, indeed the whole issue is perhaps the strongest case for a properly regulated and controlled cannabis trade – which means legalisation.

To summarise the case made by the prohibition lobby:

1: Cannabis strength has increased massively in recent years
2: The increased strength has lead to more cases of serious mental illness
3: The age people use cannabis has dropped and now it’s common for children to be heavy users
4: The younger users start, the bigger the risk of developing serious mental illness.
5: Lax attitudes to cannabis use have increased the number of users, therefore increasing the rates of the illness.
6: Prohibition is the best way to protect people from the dangers of cannabis use.

We will look at these claims in detail here and return to them at the end.

Cannabis The Drug

All drugs used for fun or escapism “do things” in the brain, they are essentially chemicals which change or interact with the brain’s chemistry. All drugs do something along those lines, including alcohol and tobacco. Cannabis is different to most “drugs” in an important way however.

Most drug taking essentially involves the interaction of one active chemical on the brain, but the cannabis experience is the result of two major constituents known as THC and CBD. These two chemicals are very similar in structure, but the small difference means they have very different effects in the brain. Cannabis also contains a range of minor active chemicals which also “tweak” the overall experience, but THC and CBD are the most important for the issue of its effect on mental health.

THC produces psychotropic effects, whereas CBD is totally the opposite and has anti-psychotic properties. “psychotropic ” means perception altering, which is of course the reason people enjoy cannabis. The CBD is thought to moderate the workings of the THC, reducing the potential for undesirable psychotic like effects.

Research by Dr Zerrin Atakan and Prof Philip McGuire [2] concluded: “These studies show that THC and CBD have distinct effects on brain function in humans, and these may underlie their correspondingly different effects on cognition and psychiatric symptoms. Determining how the constituents of cannabis act on the brain is fundamental to understanding the role of cannabis use in the aetiology of psychiatric disorders.”

It is important to understand that the cannabis effect is not the result of just one drug, but the combined effect of the two major drugs and that cannabis can have very different effects depending on the variety or “strain” of the plant because the ratio of active chemicals is different for each strain. Although there are differences between strains, each type of cannabis can be grown to very high tolerances.

As the quote from the Independent on Sunday above shows, the media has spread a lot of misleading and downright stupid claims about the “strength” of cannabis in recent years. The difference between strains is possibly very important when it comes to the effect cannabis might have on the mental state of a person and therefore a simple measurement of strength is pointless to the degree of being misleading.

The government has always spoken of “potency” in terms of % THC content alone when describing cannabis. This is also misleading as it takes no account of the CBD content, so let’s define the terms properly.

The main componants of cannabis

Strength Vs Potency

Strength is simple, it’s how much of the active chemicals you get per gram of sample. If you have a strong sample, you don’t need as much of it to get the same effect as a large amount of weak stuff, but the effect will be the pretty much same overall providing you don’t slam too much too fast.

Potency is the ratio of THC to CBD. Therefore if a potency figure is going to be quoted it needs two values: %THC and %CBD, or perhaps as a simple ratio of THC:CBD. High potency cannabis would mean a large THC figure combined with a small CBD figure.

Now, it’s easily possible for a sample of cannabis to have lot or a little of the active chemicals, so we can have strong, low potency cannabis and weak, high potency cannabis [3].

Because of prohibition it is impossible to know either the strength or the potency of cannabis being sold.

Has Cannabis Potency And/Or Strength Changed?

There has always been a range of strengths and potencies available and because of prohibition there have never been proper records kept of what people were actually buying. Therefore it’s next to impossible to be sure of how, if at all, they may have changed over time.

Some reports claim certain strains (so called “skunk”) contain a lot of THC but very little CBD, whereas the traditional forms of hash we used to get a lot of before the war on drugs seem to have had close to equal amounts of both chemicals.

It should be stressed again that it’s impossible to be certain about any changes in potency or strength over the past 30 or so years because proper records of strength and potency have simply not been made. CBD content was rarely measured in fact.

A study ordered by the UK government looked (as best it could) at the changes in cannabis and reported in 2008 [4]. The Home Office study claimed to have found a doubling of “strength” over the past few years, a far cry from the claims of 25 fold of the Independent on Sunday story.

However, this study – official as it was – comes close to the definition of “cod science” due to the way they collected the data. Samples analysed came from police seizures and used a very dubious sampling method:

“For operational reasons some forces chose to send in material from only one Borough Command Unit or from one of several forces collection points. Some forces experienced internal logistics problems; others were very enthusiastic and sent in everything received during the trial period”.

A scientific study is only as good as the data it collects and the old adage is always true: garbage in = garbage out. This data was garbage because of the sampling method used and therefore its conclusions should be treated with caution.

Fact is then we don’t really know what’s happened to the strength or potency of cannabis over the period of prohibition, but it might have got stronger and more potent. That said there has always been very strong cannabis – Thai stick, anyone?

There has been a general change however. Cannabis in the UK used to come mainly from North Africa in the form of hashish grown naturally in the sunshine. Because of prohibition these supplies were greatly reduced which lead to the growth of the cannabis farms industry which produces herbal cannabis grown intensively under lights. The strains of plants selected for these intensive grow ops were picked because they gave a high yield of strong cannabis grown under the artificial conditions. It is entirely possible that the CBD content has decreased due to this market change.

It is important to note however that if this change has occurred, it happened because of the workings of cannabis prohibition. Worse, it went unnoticed by the authorities for nearly 10 years until the study in 2008 because the illegal trade is totally unregulated.

What Is Mental Illness?

Mental illness is a wide term meaning illness that affects your perception of the world and your place in it.


Perhaps the worst of the brain problems, it’s often called a “split personality” but that’s wrong. It’s better to think of schizophrenia as meaning “split from reality”; ill people hear voices which aren’t there, suffer hallucinations and, put simply, don’t always experience the real world around them correctly.

Schizophrenia is what is usually understood by the colloquial term “madness” and it was claimed by prohibition supporters, large sections of the media and some scientists that cannabis caused this terrible illness.

Schizophrenia is not a new illness, it’s always existed and usually affects young (mostly male) adults in their late teens and early 20’s, there is also a form which affects old people known as “late onset schizophrenia”, which is usually less serious and doesn’t concern us here.

People can suffer degrees of the illness and no two people’s illness are the same. Most people recover but others, for whatever reason, do not.

The symptoms of schizophrenia are called a “psychosis”, which means ill people experience paranoia attacks; sometimes feeling they are being watched and people are talking about them. They can suffer auditory and visual hallucinations, seeing and hearing things that aren’t there. Hearing voices is very common.

Treatment for serious mental illness, unlike for any other forms of illness, is compulsory. If an ill person is sectioned he or she can be given medicine and be detained in hospital against their will. Ill people often suffer social exclusion and many end up in prison because the mental health services in the UK are badly under resourced.

Something like 1 in 100 people in the USA are thought to develop schizophrenia to some extent [5] and rates are probably similar here.

Does Cannabis Cause Schizophrenia?

Some of the effects of cannabis appear similar to some aspects of psychosis – such effects as changed perception – and are the reasons people find cannabis use enjoyable. Less enjoyably, occasionally cannabis can induce attacks of paranoia which again is not unlike the symptoms of schizophrenia. While not pleasant, these bouts of paranoia are mostly short lived and vanish when the drug wears off. Quite a few people who experience this effect decide cannabis is not for them – after all no-one wants to take a drug for its unpleasant effects.

However it is this similarity which has made the claims of a link between cannabis and mental illness seem plausible.

The easy way to see if cannabis is a cause of serious mental illness is to look at the rates of psychosis over the years and to see if they’ve changed along with the massive increase there has been in cannabis use. Because of the concerns raised about the use of cannabis on mental health, the UK government commissioned a study to do just that from researchers at Keele university [6].

The study carried out by researchers from Keele University for the UK government failed to show any increase in rates of psychosis which would have been expected if cannabis did cause the illness. The study concluded their data is “not consistent with the hypothesis that increasing cannabis use in earlier decades is associated with increasing schizophrenia or psychoses from the mid-1990s onwards.” [7]

As rates of serious mental illness have not changed despite cannabis use becoming widespread it is fair to conclude that cannabis is not a significant cause of schizophrenia.

What Does Cause Schizophrenia?

There isn’t a simple answer to that question, although to some extent it seems to be inherited [5] According to the US National Institute for Mental Health “Schizophrenia occurs in one percent of the general population, but is seen in 10 percent of people with a first-degree relative (a parent, brother, or sister) with the disorder. The identical twin of a person with the disorder is most at risk, with a 40 to 65 percent chance of developing the disease”.

No single, clear cut, cause of schizophrenia has ever been identified. The fact it seems to be inherited suggests a genetic link (the COMT gene) and it was proposed that cannabis may impact through this route, although some recent work hasn’t supported this [8]. Some people put birth complications as the main factor, stress and trauma are also risk factors and there are many others.

In all honestly it’s still not known what makes schizophrenia happen. It’s better to think of “risk factors” rather than simple causes – factors which increase the risk of psychosis developing. If more than one risk factor is present, the chances of developing a psychotic condition goes up, or as researchers put it, the risk factors interact.

THC might be one of these risk factors for those at risk of developing the illness, although CBD is likely to reduce the risk, therefore it is claimed high potency cannabis may increase the risk of serious mental illness developing in someone liable to it. This could go some way to explain the apparent correlation between cannabis use and mental illness.

One large study carried out in Denmark in 2008 [9] concluded:

“The results agree with those of other studies that show that cannabis predominantly causes psychotic symptoms in those persons who are predisposed to develop psychosis or show signs of psychosis in the absence of cannabis use”.

Although there is no way to tell if someone is at risk of serious mental illness, a bad reaction to cannabis might be an early warning that a person is likely to develop schizophrenia.

What is undoubtedly true is that people who have schizophrenia are more likely to use cannabis at a higher rate than the population in general. They also smoke tobacco, drink alcohol and use other drugs at higher rates than the general population and this heavy drug use often makes the illness much worse.

People suffering from serious mental illness are clearly a vulnerable minority who need the protection of society. Prohibition does nothing to protect these people and much to put them at far greater risk, not just from uncertain potencies and also from violence due to encountering a criminal supply industry and of course, they may get a criminal record for drug use.

As we’ve seen, the two main constituents of cannabis – THC and CBD – have very different, almost totally opposite effects on schizophrenia. CBD is actually anti-psychotic and may even be beneficial to ill people, whereas high THC cannabis may make things worse. If the theory that high THC content cannabis is detrimental to people suffering from mental illness is true then it is important to properly regulate the trade to either allow them access to low potency supplies or restrict their access to the trade entirely, as we do with problem drinkers who can be banned from off licences or pubs. No-one would suggest that gifting the alcohol trade to the mob and supplying the trade with moonshine or bathtub gin would help problem drinkers, so why use that logic for cannabis?

Would preventing adult cannabis use reduce the incidence of serious mental illness? The answer to that is “probably not”. A study by Stan Zammit and others [10] concluded that if cannabis did cause schizophrenia then to prevent just one case in adults over 20 or so, we would have to prevent around 3,000 heavy cannabis users, or 150,000 light users.

Therefore if the aim of prohibition is to reduce the number of adult cannabis users in an effort to reduce the rates of serious mental illness, it isn’t going to work. Indeed, by making adult cannabis use illegal, all that happened was a massive uncontrolled illegal trade developed and that has spilled over to children.

Young People – Children And Teenagers

Prohibition supporters have drawn attention to the fact that the age of first use of cannabis has decreased over recent decades, and in this they are correct. When prohibition was introduced, cannabis was seen as an adult pastime. Now it’s not uncommon for children to be regular smokers in some areas. If prohibition is supposed to prevent young people getting involved in the drug trade, it has obviously failed badly. Quite how this is used to justify continued prohibition isn’t clear but it is typical of the twisted logic of prohibition supporters.

Cannabis and drug use in general presents a far bigger risk to children than to adults. A young brain develops as it learns, a process called “plasticity” [11]. If the learning process is disrupted by drug use then the brain’s ability to grow and develop properly will also be disrupted. This is true for all drugs, not just cannabis – and maybe some things that aren’t drugs such as too much TV watching or internet use.

There is some research which seems to indicate that children under 15 or so run a far higher risk of developing serious mental illness if they use cannabis and several studies seem to support this. For example one carried out by Jouko Miettunen and others and published in 2008 looked at early symptoms indicating the onset of an attack (prodromal symptoms) of psychosis in adolescence amongst young cannabis users [12]. They concluded that “cannabis use is associated with prodromal symptoms of psychosis in adolescence.”

However it should be noted that some other research [13] into children’s cannabis use carried out in Holland, where cannabis use is more open and can be properly assessed seems to indicate a far less clear association:

“Cannabis use, in individuals who did not have psychotic symptoms before they began using cannabis, predicted future psychotic symptoms. However, psychotic symptoms in those who had never used cannabis before the onset of psychotic symptoms also predicted future cannabis use”.

One of the features of schizophrenia is what psychiatrists call “premorbid indications”. A pre morbid indication is a bit like a consequence of an illness that hasn’t happened yet. It may be that heavy cannabis use in a young person could be one of these premorbid indicators [14] and so should be a cause for concern. All the cases of young people highlighted by the media have shown this unusual pattern of heavy use before they developed their illness.

Regardless of whether or not drug use leads to schizophrenia, it makes sense actively to protect all children under the age of 15 or so from drug use of all kinds including cannabis. That should be the primary aim of any sensible drugs policy and, indeed it’s what prohibition claims to do and is the main reason used by the government to justify its drugs policy. However, as prohibition supporters have pointed out, children have easy access to prohibited drugs under the current regime and it clearly doesn’t work.

An age limit of 18 for sales – similar to that for alcohol and tobacco – would be sensible and is urgently needed. Under prohibition the age limit for cannabis is £10, there is no protection for children.


In the UK, cannabis is still usually smoked mixed with tobacco. It is known that tobacco is intensely addictive and that people with psychosis tend to be heavy tobacco smokers. If they are used to smoking cannabis with tobacco their tobacco habit could lead to them smoking far more cannabis than otherwise.

It is interesting to note that whereas many of the studies into cannabis and mental illness claim to control for other drug use, very few of them take any account of tobacco use.

A safer use campaign aimed at separating the use of cannabis from tobacco would be useful in reducing the risk to people at risk of psychosis. It would also give wider health benefits to all cannabis users. [15]

Conclusion: Cannabis And Mental Health, The Case For Reform

Concerns of a link between cannabis and mental illness areperhaps the strongest argument for cannabis law reform.

Prohibition claim 1: Cannabis strength has increased massively in recent years

It may have done, but we don’t know for sure because prohibition makes proper monitoring of the trade impossible. If it has increased, the change was brought about by the workings of prohibition. Certainly a market shift did occur and went unnoticed for nearly 10 years because of the lack of proper control. If the fears that high THC levels impact on mental health are true, prohibition has made things worse. Under prohibition, cannabis is not a controlled drug.

Prohibition claim 2: The increased strength has lead to more cases of serious mental illness

There has been no increase in the rates of mental illness. But if increased potency (not strength) has taken place, it might make existing illness worse and be bad for those at risk. The need for a better understanding of the issues of potency and strength is obvious and proper regulation of the trade would address this.

Prohibition claim 3: The age people use cannabis has dropped and now it’s common for children to be heavy users

This is true, but it’s hardly an issue to support the workings of prohibition. That children have become ensnared in the unregulated and uncontrolled cannabis trade is entirely caused by prohibition.

Prohibition claim 4: The younger users start, the bigger the risk of developing serious mental illness

We might as well assume this is true, whether true or not. Children deserve the protection of the law which only a regulated trade could provide, prohibition treats them as criminals and drives all use – including problematic use – underground.

Prohibition claim 5: Lax attitudes to cannabis use have increased the number of users, therefore increasing the rates of the illness

As rates of serious mental illness have not increased in line with the increase in cannabis use, there is no evidence to support this, but the “lax attitudes” have come about through the failure of prohibition. Proper legal control and regulation would mean an end to “lax attitudes” and the introduction of proper, workable laws.

Prohibition claim 6: Prohibition is the best way to protect people from the dangers of cannabis use.

Prohibition means we don’t know what is sold as cannabis, how strong it is or how potent it is. We don’t know if it’s contaminated either after harvest or in the growing process through overuse of pesticides for example. We don’t know who sells it, where from or in what amounts. The only qualification to be a dealer is unaccountability. Cannabis users have no recourse to the law when things go wrong and sometimes violence is used. When this happens it will be those least able to defend themselves – such as ill people – who suffer most.

Prohibition is a con, it is not drug control. The only way to achieve drug control is to control the commercial supply industry, which means proper legalisation, licensing of dealers and premises, age restrictions and strength/potency information.

Treating those you claim to want to help as criminals is, well, simply mad.



[1] Independent on Sunday – “Cannabis Apology”

[2] Institute of Psychiatry News – New research reveals how cannabis alters brain function

[3] THC, CBD and the misleading concept of “Potency”.

[4] Home Office Cannabis potency study

UKCIA Comment

[5] Mental Illness – Source: USA National Institute of Mental Health. Facts about Schizophrenia

[6] Keele Study – Assessing the impact of cannabis use on trends in diagnosed schizophrenia in the United Kingdom from 1996 to 2005 Martin Frisher, Ilana Crome, Orsolina Martino, Peter Croft

[7] Further reading: Cannabis and mental illness – the Keele Study

[8] – Cannabis and smoking gene links to schizophrenia ‘unfounded’

[9] Familial Predisposition for Psychiatric Disorder – Comparison of Subjects Treated for Cannabis-Induced Psychosis and Schizophrenia, Mikkel Arendt, MScPsych, PhD; Preben B. Mortensen, DrMedSc; Raben Rosenberg, DrMedSc; Carsten B. Pedersen, MSc; Berit L. Waltoft, MSc


[10] If cannabis caused schizophrenia—how many cannabis users may need to be prevented in order to prevent one case of schizophrenia? England and Wales calculations – Matt Hickman, Peter Vickerman, John Macleod, Glyn Lewis, Stan Zammit, James Kirkbride, Peter Jones

[11] Plasticity

[12] Association of cannabis use with prodromal symptoms of psychosis in adolescence – Jouko Miettunen, Sari Tormanen, Graham K. Murray, Peter B. Jones, Pirjo, Maki, Hanna Ebeling, Irma Moilanen, Anja Taanila, Markus Heinimaa, Matti, Joukamaa and Juha Veijola

[13] Cannabis use predicts future psychotic symptoms, and vice versa – Robert F. Ferdinand, Frouke Sondeijker, Jan van der Ende, Jean-Paul Selten, Anja Huizink & Frank C. Verhulst

[14] Pre-morbid Conduct Disorder symptoms are associated with cannabis use among individuals with a first episode of psychosis. – Malcolm CP, Picchioni MM, DiForti M, Sugranyes G, Cooke E, Joseph C, McQueen G, Paparelli A, Stilo S, O’Connor J, Morgan C, Murray RM, Hodgins S.

[15] Toke pure – CLEAR anti tobacco campaign

Author Derek Williams