Royal College Of Psychiatrists “Cannabis and mental health” leaflet – a critique revisited

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RCPsych logo

Back in 2009 this blog critiqued a terrible leaflet from the Royal College of Psychiatrists entitled “Cannabis and mental health” (read it here). The link at the top of that blog now links to the latest version, which you can see here.

The leaflet was reviewed in June 2014 and was due for review in June 2016, but it doesn’t seem to have happened.  It’s interesting to see how the information on the leaflet has changed in places although things haven’t improved greatly, there is still much of concern and the evidence is heavily slanted, using cherry picked data and emotive language. Some of the writing is simply terrible, with parts seemingly written by children.

Considering this leaflet is published by the Royal College Of Psychiatrists it might be expected to be a truthful and factual summary of the effects of cannabis on mental health, it is anything but.

About this leaflet


This leaflet looks at the research on the effects of cannabis use and mental health and is for anyone who is concerned about the issue. We hope that this will help people to make informed choices about using – or not using – cannabis.

Given that description you might expect it to be full of impartial, factual information about cannabis and its effect on mental health. You would be wrong. A large proportion is in fact given over to what can only be described as the government’s cannabis prohibition agenda. It goes into some detail about the legal status of cannabis, it affect on driving and repeats some very dubious claims as fact. It also uses the term “skunk” as a catch-all term but without the context of being a dodgy street drug with a range of uncertain properties created by the prohibition policy.

Indeed, the leaflet makes no mention of the present drugs policy impact on any of the problems they allude to.

The leaflet also misuses the term “strength”, confusing it with “potency” and fails to explain the most important role of CBD in moderating the psychotic effects of THC.


Things don’t get off to a good start:

Cannabis is the most widely used substance in the UK. Even though there has been a steady reduction of use since 1996, about 2.3 million 16-59 year-olds have reported using cannabis in the past year. Frequent use of cannabis is about twice as likely amongst young people, and nearly 5.3 million 16-24 year-olds have used it in the last year.

So more people aged 16-24 have used cannabis in the past year than those aged 16-59? That’s clearly rubbish. This is sloppy writing that no-one seems to have checked.

It then mentions that “despite government and media warnings”, people still view cannabis as

a harmless substance that helps you to relax and ‘chill’ – a drug that, unlike alcohol and cigarettes, might even be good for your physical and mental health.

It doesn’t speculate why of course, but in any case that’s wrong. Most people see cannabis – correctly – as less harmful than alcohol and tobacco, which is reasonable because it’s true.  Less harmful of course doesn’t mean harmless, this seems to be a concept not only governments have a problem with, but also Royal Societies. Good for health? Well in some  ways it clearly is and has well documented medicinal uses.

Then of course we have the doom laden bit, typical of prohibition writing

On the other hand, research over the last 10 years has suggested that it can have serious consequences for people, such as the development of an enduring psychotic illness, particularly in those who are genetically vulnerable.

Interestingly the earlier version said

On the other hand, recent research has suggested that it can be a major cause of psychotic illnesses in those who are genetically vulnerable.

So it’s no longer a “major cause of psychotic illnesses”, but now can have “serious consequences”. I bet they worked really hard on that phrasing. Now though, they imply everyone is at risk, not just those who are genetically vulnerable which is highly questionable.


What is cannabis?


The leaflet tries to explain how common cannabis is throughout the world

Cannabis sativa and cannabis indica are members of the nettle family that have grown wild throughout the world for centuries. Both plants have been used for a variety of purposes including hemp to make rope and textiles, as a medical herb and as the popular recreational drug.

This is just more sloppy writing. Of course these plants have existed for many millennia, not just for “centuries” and have been also been used for all these purposes for millennia (the earliest recorded use of cannabis is 2700 BC in the pharmacopoeia of Shen Nung and archaeological evidence exists for use way before that.

It then explains the different types of cannabis, hash and herbal, fair enough. But then we get this

Skunk refers to a range of stronger types of cannabis, grown for their higher concentration of the main active ingredient, namely THC (tetrahydrocannabinol). The name refers to the pungent smell they give off while growing.

This is wrong on so many levels. This leaflet is supposed to be factual so it should explain that “Skunk” is a strain of cannabis developed by cross breeding, it is not a “range of stronger types of cannabis”, there are in fact many Skunk strains high in THC and some skunks can be well balanced in terms of THC and CBD.

The term “skunk” as used by the media and politicians and understood by most people used to mean the any type of allegedly high potency herbal cannabis sold on the street, which is what dealers are supposed to call it. Whatever the actual potency of street cannabis, this so-called “skunk” has a range of problems caused by its illegality such as the over-use of pesticides which may itself cause medical problems, but this isn’t touched on.

They can be grown either under grow-lights or in a greenhouse, often using hydroponic (growing in nutrient rich liquids rather than soil) techniques. There are hundreds of other varieties of cannabis with exotic names such as AK-47, Knock Out or Destroyer.

And far less violent names such as “Kush” or “Blueberry”, but they don’t sound as streetcred.

Over the last 15 years, skunk has invaded the street market and its THC content is about 2-3 times higher than the ‘traditional’ cannabis used in earlier years. In the UK, most sold materials is home grown because of a loop hole in the law making it legal to buy seeds over the internet.

The claim that “most sold materials is home grown” (we know what they mean) is impossible to prove and almost certainly wrong, the cannabis industry in the country is fuelled in no small part by large scale grow farms which don’t grow from seeds but use cuttings. Seeds can be bought in many more places than just “the internet” as well. More importantly they talk about “strength”, when the real issue, if it is an issue, is “potency”, the ratio of THC and CBD. So-called “skunk” is claimed to be low in CBD which has anti psychotic properties. This is a seriously bad mistake for this leaflet to make.


How is it used?


The way cannabis is consumed is described, although vapes don’t get a look-in.


What is its legal status in the UK?


This is supposed to be a leaflet about the medical impact of cannabis, especially with respect to the mental health impact. The law is really irrelevant. Cannabis is prohibited, I think most people understand that so why waste space with this information other than to drive home the prohibition message?


How does it work and what is the chemical make-up of cannabis?


There are about 400 chemical compounds in an average cannabis plant. The four main compounds are called delta-9-tetrahydrocannabinol (delta-9-THC), cannabidiol (CBD), delta-8-tetrahydrocannabinol and cannabinol. Apart from CBD, these compounds are psychoactive, the strongest one being delta-9-tetrahydrocannabinol. The stronger varieties of the plant contain little CBD, whilst the delta-9-THC content is a lot higher.

This hasn’t been changed since the original and is still wrong, as this blog wrote last time:

Well, no, that is just wrong. CBD was perhaps not regarded as psychoactive at one time, but it most certainly is now. Psychoactive is defined as “capable of affecting mental activity: a psychoactive drug”

CBD has anti psychotic properties and interacts with THC to give the “Cannabis experience”. It may not produce an intoxication, but it most definitely has an effect on mental activity and mood. For the RC Psych to make such a basic error on a leaflet like this is inexcusable.

The comment about strength is also very wrong for another reason; it is possible to have very strong cannabis which contains high levels of CBD and it’s also possible to have weak high potency cannabis with low levels of CBD, such as plants which have had most of the resin removed to make hash. If enough of it is smoked, it will have the same effects as high potency cannabis.

The leaflet doesn’t mention the role of CBD in the effect of cannabis either, whereas in fact the “stoned” feeling is very different with high THC or more balanced cannabis. As stated CBD has anti psychotic properties, but why mention that in a leaflet about the psychotic effects of cannabis?


What are its effects?


This bit is just embarrassing. It lists the “pleasant effects” and the “unpleasant effects” and of course, the unpleasant effects greatly outnumber the pleasant effects. Indeed, the only pleasant effects from cannabis apparently are

A ‘high’ – a sense of relaxation, happiness, sleepiness, colours appear more intense, music sounds better.

While true, that barely begins to scratch the surface of what cannabis does. Of course, the unpleasant list is very big – indeed it takes up pretty much the rest of the leaflet.

Even though THC can produce relaxation, if higher amounts are consumed, it can have the opposite effect by increasing anxiety. Some cannabis users may have unpleasant experiences, including confusion, hallucinations, anxiety and paranoia, depending on their mood and circumstances.

Some users may experience psychotic symptoms with hallucinations and delusions lasting a few hours, which can be very unpleasant. Even though these unpleasant effects do not last long, since the drug can stay in the system for some weeks, the effect can be more long-lasting than users realise.

The previous version claimed

Around 1 in 10 cannabis users have unpleasant experiences,

Which is perhaps more honest. It goes on

Long-term use can have a depressant effect and reduce motivation. Some researchers also suggest that long-term use can lead to irreversible, but minor cognitive deficits.

Nowhere in this does it make the point these negative effects are rare as it used to do in the earlier version and the claim “since the drug can stay in the system for some weeks, the effect can be more long-lasting than users realise” is simply alarmist.

Long term heavy use might have these depressant and demotivational effects in some users, although it’s disputed. But then long term heavy use of anything is probably going to have some kind of negative impact and on the scale of things, this is pretty minor. Try being a long term heavy drinker for example. As regards brain structure changes, yes, it’s been claimed by some, but it’s not proven and certainly isn’t proven to be harmful.


Education and learning


There have also been suggestions that cannabis may interfere with a person’s capacity to:

  • concentrate
  • organise information
  • use information.

This effect seems to last several weeks after use, which can cause particular problems for students.

The phrase “There have been suggestions” is telling. There are also “suggestions” that cannabis aids creativity, so why not mention that as well? It’s just as factual.

However, a large study in New Zealand followed up 1265 children for 25 years. It found that cannabis use in adolescence was linked to poor school performance, but that there was no direct connection between the two. It looked as though it was simply because cannabis use encouraged a way of life that didn’t help with schoolwork.

Probably this study which was widely criticised for failing to take into account various social factors. It’s also worth mentioning this study which finds no evidence that marijuana lowers IQ in teens, so why cherry pick studies like that? This information wasn’t included for example:

marijuana-using twins failed to show significantly greater IQ decline relative to their abstinent siblings. Evidence from these two samples suggests that observed declines in measured IQ may not be a direct result of marijuana exposure but rather attributable to familial factors that underlie both marijuana initiation and low intellectual attainment.




Cannabis is bad for working people apparently:

It seems to have a similar effect on people at work. There is no evidence that cannabis causes specific health hazards. But users are more likely to leave work without permission, spend work time on personal matters or simply daydream. Cannabis users themselves report that drug use has interfered with their work and social life.

So cannabis interferes with our role as wage slaves and makes us less productive? This is playing to the stereotype of the lazy stoner.

The last sentence is strange, “Cannabis users themselves report that drug use has interfered with their work and social life” – what, all of them have reported this? I suspect the vast majority would dispute that.

Then we get this almost hysterical section

Of course, some areas of work are more demanding than others. A review of the research on the effect of cannabis on pilots revealed that those who had used cannabis made far more mistakes, both major and minor, than when they had not smoked cannabis. The pilots were tested in flight simulators, not actually flying… The worst effects were in the first four hours, although they persisted for at least 24 hours, even when the pilot had no sense at all of being ‘high’. It concluded “Most of us, with this evidence, would not want to fly with a pilot who had smoked cannabis within the last day or so”.

My comment from 2009 stands:

Don’t know about you, but this almost reads like it was written by a child. Again, it has no bearing on cannabis and mental health whatsoever, what is it even doing here?

And then


What about driving?


Oh my god, what about driving indeed!

In New Zealand, researchers found that those who smoked regularly, and had smoked before driving, were more likely to be injured in a car crash. A recent study in France looked at over 10,000 drivers who were involved in fatal car crashes. Even when the influence of alcohol was taken into account, cannabis users were more than twice as likely to be the cause of a fatal crash than to be one of the victims

A terrible example of cherry picked studies, how about this one which finds

Results from the current study using the TRL driving simulator confirm the results from these previous studies. There was a reduction of average speed on simulated motorway driving when participants had the high or low doses of cannabis. This strongly suggests that the participants as drivers are aware of their impairment, but attempt to compensate for their impairment by driving more cautiously

And at last we get on to:


Mental health problems


Which is what this leaflet is supposed to be about. Notice the phrasing “mental health problems”

There is growing evidence that people with serious mental illness, including depression and psychosis, are more likely to use cannabis or have used it for long periods of time in the past.  Regular use of the drug has appeared to double the risk of developing a psychotic episode or long-term schizophrenia. However, does cannabis cause depression and schizophrenia or do people with these disorders use it as a medication?

Over the past few years, research has strongly suggested that there is a clear link between early cannabis use and later mental health problems in those with a genetic vulnerability – and that there is a particular issue with the use of cannabis by adolescents.

This hasn’t changed since last time and as I wrote then:

Interestingly they don’t look at overall rates of illness and ask “are rates of mental illness increasing in line with rates of cannabis use?” That is the sort of question this leaflet might have been expected to address and is surely the information people reading it will be looking for. Of course, there is no evidence of a rise so the point isn’t mentioned.




A study following 1600 Australian school-children, aged 14 to 15 for seven years, found that while children who use cannabis regularly have a significantly higher risk of depression, the opposite was not the case – children who already suffered from depression were not more likely than anyone else to use cannabis. However, adolescents who used cannabis daily were five times more likely to develop depression and anxiety in later life.

Again, unchanged from last time and the response is the same: A cherry picked bit of data and hardly representative of the knowledge base regarding cannabis and depression. For example research from Australia entitled Exploring the association between cannabis use and depression found

There was a modest association between heavy or problematic cannabis use and depression in cohort studies and well-designed cross-sectional studies in the general population. Little evidence was found for an association between depression and infrequent cannabis use.


If the relationship is causal, then on current patterns of cannabis use in the most developed societies cannabis use makes, at most, a modest contribution to the population prevalence of depression.


Psychoses – schizophrenia and bipolar disorder


This bit has changed a lot. Originally it stated

Three major studies followed large numbers of people over several years, and showed that those people who use cannabis have a higher than average risk of developing schizophrenia. If you start smoking it before the age of 15, you are 4 times more likely to develop a psychotic disorder by the time you are 26. They found no evidence of self-medication. It seemed that, the more cannabis someone used, the more likely they were to develop symptoms.
That was quite specific and implied a strong causal relationship. Now it says

There is now sufficient evidence to show that those who use cannabis particularly at a younger age, such as around the age of 15, have a higher than average risk of developing a psychotic illness, such as schizophrenia or bipolar disorder.

These studies also show that the risk is dose-related. In other words, the more cannabis someone used, the more likely they were to develop a psychotic illness. Furthermore, a study in Australia recently showed that those who used cannabis could develop the illness about 2.70 years earlier than those who did not.

It does not, however, imply a causal relationship as the earlier version did.

It goes on to discuss why teenagers should be more at risk of developing these conditions, which is fair enough but should really mention that Schizophrenia has always been an illness of mostly late teens/early twenties and that prevalence of the illness doesn’t seem to be increasing despite the rise of so-called “skunk.

It is good advice though that young people – early teens – shouldn’t use cannabis, so why not just give it as good advice?

It finishes with this

It is also known that not everyone who uses cannabis, even at a young age, develops a psychotic illness. The available research shows that those who have a family history of a psychotic illness, or those who have certain characteristics such as schizotypal personality, or possibly have certain types of genes, may increase the risk of developing a psychotic illness following the regular use of strong cannabis.

Actually, these characteristics increase the risk of developing the illness anyway, with or without cannabis but it’s a fair point that using cannabis seems to make things worse.


Physical health problems


Even though the main risk to physical health from cannabis is probably from the tobacco that it is often smoked with, new research has found that the cannabis plant also contains cancerogenic mutagens that can affect people’s lungs.
So why no advice to Toke Pure?
They are correct that cannabis does contain carcinogenic chemicals and indeed smoking can damage your lungs, but why couldn’t they bring themselves to say that cannabis isn’t linked to lung cancer or COPD instead of using terms like “contains cancerogenic mutagens” which seems designed to scare rather than inform?


Is there such a thing as ‘cannabis psychosis’?


A total re-write: Previously it said

Recent research in Denmark suggests that yes, there is.

Now it says

Some people may develop temporary psychotic symptoms, such as hallucinations and delusions, which resolve themselves within hours or a few days without any help. People who experience these temporary effects do not normally come to the attention of psychiatric services.

Previously, if a person was known to be a heavy cannabis user, and came to psychiatric services with a psychotic condition, the term “cannabis psychosis” was used. Often these problems turned out to be long-lasting, and the person developed enduring severe mental illness. However, we have now found that these cases should be diagnosed according to person’s symptoms, including their use of cannabis which could be a possible trigger for psychosis.

It may be this group of people are particularly vulnerable to the effects of cannabis, and so should probably avoid it in the future.

Which is sort of a way of saying “no” to the question they posed? This seems reasonable advice though. It is also possible that heavy cannabis use (and a bad reaction to it) is a sign of an emerging illness – a “premorbid indication” to use shrink speak. A study from Denmark alluded to in the earlier version, Familial Predisposition for Psychiatric Disorder concluded

It is recommended that individuals with a cannabis-induced psychosis … be treated as though the condition is a first sign of schizophrenia, regardless of predisposition to a psychiatric disorder.


Is cannabis addictive?


Yes. Even though in the past cannabis was not thought to be addictive, current evidence now suggests that it can be, particularly if used regularly.
For about 1 in 10 users and even then the addiction is usually fairly mild. It fails to point this out of course.

What about skunk and other stronger varieties?


Here we get the “Skunk scare” warnings about high THC content, but absolutely no mention of CBD. It does contain this gem:
Even though low THC content in herbal cannabis can reduce anxiety, higher amounts of THC produces anxiety. They may be used by some as a substitute for Ecstasy or LSD.
Again, terrible writing. Does it imply high THC cannabis is used as a substitute for LSD or Ecstasy because it produces anxiety? I do wonder if they have any real experience of recreational drug use.
It is worth mentioning again that although high potency cannabis may cause problems for some people, the vast majority of consumers enjoy it without a problem.

Problems with cannabis use


At last they do admit
Many – perhaps most – people who use cannabis do enjoy it. But it can become a problem for some people
Perhaps most? The vast, vast majority enjoy cannabis with no problems, why can’t they tell the truth?
The leaflet finishes with advice on how to cut down or stop cannabis use, leaning heavily on Home Office publications including near toxic “Talk to Frank” website.
In summary this leaflet was a disgrace to the Royal Society when first issued in 2009 and hasn’t been improved over the past 8 years. It should be withdrawn.


UKCIA is a cannabis law reform site dedicated to ending the prohibition of cannabis. As an illegal drug, cannabis is not a controlled substance - it varies greatly in strength and purity, it's sold by unaccountable people from unknown venues with no over sight by the authorities. There is no recourse to the law for users and the most vulnerable are therefore placed at the greatest risk. There can be no measures such as age limits on sales and no way to properly monitor or study the trade, let alone introduce proper regulation. Cannabis must be legalised, as an illegal substance it is very dangerous to the users and society at large.