Cannabis and Psychosis

I took some time out last week to travel up to Northumberland with my wife and meet up with some friends for a long weekend away. On the Friday we wandered around the area in a car looking for things to do and found ourselves at Alnwick Castle Gardens.  If you ever find yourself in the area I would highly recommend a visit. It is a taste of what a modern Versailles might look like. The site is dominated by a magnificent water feature and the grounds are full of other fascinating water-based sculptures.

The main point of interest to me however was what they called the Poison Garden. This was a secure area where the charity which runs the site grow different poisonous plants and controlled drugs for educational purposes, under a licence from the Home Office.

The tour guide was marvellous and I learned a lot about how to produce ricin, the perils of ingesting rhubarb leaves and how dangerous it can be to trim a laurel in anything but a brisk wind. Things got particularly exciting when he showed us their cannabis plants. That’s not because of any habits I have, rather it is because I spend a lot of time talking about cannabis in my role as a psychologist and mental health awareness trainer. I spoke to the guide after the tour and the conversation led me to revisit what research tells us about the drug.

The official line that the government takes is that cannabis can raise the risk of producing the symptoms of psychosis in people who have a certain vulnerability. When we talk about this we point out that the kind of cannabis being smoked these days is a very different animal from that that was “not inhaled” by the likes of Bill Clinton in the 1960s and 70s. Compared to that the modern equivalent is akin to a super strength lager, anywhere between 2 to 7 times stronger (“Learn About Marijuana: Factsheets: Potency of Cannabis”, 2016). Often, the kind of cannabis you will hear about is called Skunk, but this is only one of a whole group of higher strength versions of the drug that are available now, mainly grown in this country.

The extra strength comes from the fact that the newer strains contain a far higher content of tetrahydrocannabinol or THC. This is the component which produces the high but which can also produce problems for the user in terms of loss of motivation, memory problems paranoia and even hallucinations; for instance, that time is slowing down (“Cannabis | FRANK”, 2016). Studies have revealed that the old school version of cannabis produced these symptoms in about 1 in 15 users. However similar studies have found that these super strength versions produce the symptoms in as many as 1 in 4 (Knox, 2016).

This is not only because of the higher THC content. Cannabis normally also contains a component called cannabidiol. There is evidence that this can balance out THC and produce an anti-psychotic effect (Leweke, Mueller, Lange, & Rohleder, 2016). In the cannabis that was traditionally used THC and cannabidiol were present in equal parts, about 4% each. Skunk and the like often contain 14 to 15% THC. Cannabidiol is hardly present at all (Coghlan, 2016).

So Skunk and similar higher strength versions of cannabis are far more likely to produce symptoms such as hallucinations, memory problems, paranoia and lack of motivation in those that use them; all symptoms associated with psychosis. But the story does not end there. The human body cannot process THC in such high concentration and so it stores it in the fat cells. This is then released over the following 4-6 weeks and although this will not produce a high it can still produce the negative symptoms listed above, with men being far more vulnerable to this because of the way their bodies store fat (Knox, 2016). Of course the reality for most users is that they do not leave 4-6 weeks between uses and so they are continually topping their THC up.

For me this raises questions as to what can actually be happening when a young person comes into contact with Mental Health Services. They may have symptoms consistent with psychosis even if they have not used cannabis for several weeks. Mental Health Services and Drug and Alcohol Services are for the most part very separate entities in the NHS and professionals in the former may lack the expertise to be able to spot that what is happening for the young person may be consistent with their cannabis use. It makes me wonder how many go on to being medicated with anti-psychotic drugs when a wiser course of action might be to help them reduce and manage their cannabis use?

A decade ago Professor Richard Bentall called for a move away the disease based model that gave us terms such as schizophrenia, towards one based on looking at an individual’s symptoms (Bentall, 2006). This would be a truly person-centred approach and perhaps a far more common-sense strategy to dealing with what is a growing problem.


Bentall, R. (2006). Madness explained: Why we must reject the Kraepelinian paradigm and replace it with a ‘complaint-orientated’ approach to understanding mental illness. Medical Hypotheses, 66(2), 220-233.

Cannabis | FRANK. (2016). Retrieved 26 September 2016, from

Coghlan, A. (2016). Skunk’s psychosis link is only half the cannabis story. New Scientist. Retrieved 23 September 2016, from

Knox, J. (2016). Poison Garden Interview – Cannabis. Alnwick Castle Gardens -The Poison Garden, Alnwick, Northumberland, UK.

Learn About Marijuana: Factsheets: Potency of Cannabis. (2016). Retrieved 23 September 2016, from

Leweke, F., Mueller, J., Lange, B., & Rohleder, C. (2016). Therapeutic Potential of Cannabinoids in Psychosis. Biological Psychiatry, 79(7), 604-612.

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