Marta di Forti explains her research into recreational cannabis use
Dr Marta Di Forti is a Medical Research Council Clinician Scientist fellow and Honorary Senior Lecturer at the Department of Social, Genetic, and Developmental Research, Institute of Psychiatry, King’s College London. She is also honorary consultant for Adult Psychiatry with the South London and Maudsley NHS foundation trust.
In this interview we find out more about her research into the effects of recreational cannabis use.
Why did you first develop an interest in researching a link between cannabis use and psychosis?
It was a combination of serendipity and clinical observation. When I started my higher training at the Maudsley I was struck by how frequently young patients suffering from their first episode of psychosis reported adolescence cannabis use. My patients’ families were very determined in advocating that the use of cannabis had played an important role in precipitating the illness in their loved one.
At the same time, I joined Professor Sir Robin Murray’s research team. He had become one of the leading figures in the research of the link between cannabis and psychosis, and had just published one of the landmark studies looking at this question. I was also very interested in completing a PhD investigating how genetic susceptibility shapes individual risk to psychosis following environmental exposure, and I thought cannabis use to be the most interesting and topical environmental exposure as it was also relevant to my clinical practise.
Your research makes a distinction between high potency cannabis and other types of cannabis. What is the difference?
The potency of a type of cannabis depends on the concentration/amount of delta 9 tetra-hydro-cannabinol, also known as THC, contained in that type of cannabis. The greater the amount of THC in a type of cannabis the greater is its potency.
In my research study with participants from the South London area we reported how frequently participants had used high potency cannabis, whose street name in London is "skunk".
This type of cannabis tends to contain a high proportion of THC – on average 14-15%.
Moreover, skunk contains very little of another important ingredient of cannabis, a cannabinoid called Cannabidiol (CBD). THC and CBD seem to produce opposite effects on cognition and in terms of the propensity to induce psychotic symptoms.
What effect does the high levels of THC present in ‘skunk’ have on people who use it?
Our epidemiological research has shown that participants who report using cannabis such as skunk, with high THC % and little or no CBD, are much more likely to develop psychotic disorders and come to the attention of the mental health services than those who never used cannabis. In addition, if they use skunk on a daily basis their likelihood to develop a psychotic disorder is 5 times greater compared to those who have never used cannabis. Indeed our research has shown that almost a quarter (24%) of new cases of psychotic disorder in our catchment area could be prevented if skunk didn’t exist.
Is this effect the same if people use other types of cannabis?
On the contrary, our research indicated that the use of hash/resin type cannabis (a more traditional type) from the South London area did not increase the risk to develop psychotic disorders. In 2008 the Home Office reported, around the time of our study, hash/resin as containing on average 4-5% of THC, and very interestingly, an equal amount of CBD, which might result in CBD counteracting THC’s effect on precipitating psychotic symptoms.Though our most recent potency paper (2018) indicates that even in hash/resin the amount of CBD is dropping.
Can you explain in more detail the link you identified between regularity of use of cannabis and the likelihood of developing psychosis.
We were the first, among the researchers investigating the causal association between cannabis use and psychosis, to focus on the importance of pattern of use. Indeed, we asked our study participants the question “what type of cannabis have you mostly used”.
All participants were asked about their pattern of cannabis use. When they started, what type they used and how frequently. Participants gave very clear and detailed reporting about the type of cannabis they used, as the different types can be distinguished by smell, taste and of course their recreational effects. This was not unusual - many of us who have tried alcohol could distinguish a glass of whiskey from a glass of beer, even with our eyes closed. The data we collected allowed us to have a better estimate of individuals’ degree of exposure /use of cannabis and its effects on Mental Health.
Are people of certain ages, genders or ethnicities more vulnerable to the risks of developing psychosis?
In our case control analysis we took into account the possible confounding effects of a variety of social demographic variables like age, gender, ethnicity and also consumption of other drugs, including stimulants. We were able to show a significant association between adolescence cannabis use and later onset of psychosis.
It has been suggested that using cannabis while the brain is undergoing important changes as it does during adolescence, might disrupt normal development and increase the risk of developing a psychotic disorder later.
Your research confirms a link between the AKT1 gene and cannabis use in the onset of psychosis. Could you explain a bit more about this interaction?
We wanted to address the question of why not all cannabis users, including not all cannabis users of high potency cannabis, develop psychosis.
In our case-control study we found that those who carried a rare genetic variant of the AKT1 gene were significantly more likely to develop psychosis following cannabis use, especially daily use, compared to the ones that had the more common variant of this gene.
We chose to try and replicate previously reported findings on AKT1 influencing individual vulnerability to cannabis use, because the AKT1 gene codes for a protein which is involved in regulating dopamine signalling. Hence, dopamine dysregulation has been consistently described as underlying the onset of psychotic symptoms, and some studies have suggested that cannabis use is associated with increased dopamine release.
Your keynote speech at our Members Day in November is titled ‘Cannabis use and psychosis: A cause or a cure?’ When do you think it could be used as a cure for illness?
I am primarily a researcher interested in the harmful effect of recreational cannabis use, but of course I would be foolish not to acknowledge the now building evidence supporting the therapeutic properties of some cannabinoids.
There is now evidence for instance that preparation high in CBD can be effective in reducing the number of seizures in some very severe form of childhood epilepsy. In UK, we already prescribe Sativex, a mix of equal concentrations of CBD and THC, for Multiple Sclerosis.
More recently Professor Phillip McGuire and his team from King’s College London published a trial showing that adding CBD to a conventional antipsychotic in a patient with a diagnosis of schizophrenia improved their symptom profile, also marginally their cognitive function and overall their general level of functioning, which is a very important marker of subjective and objective improvement.
Nevertheless, I think we need to be very careful about what is meant with the term: “medicinal cannabis”. The cannabis plant and therefore any sample from it that people consume, contains over 100 cannabinoids whose effects we are only beginning to fully understand. Smoking recreational street cannabis is like a blind date, you do not the outcome until the end, and so it could be if consuming any cannabis for medicinal purposes.
I believe that, when we start in this country to prescribe cannabis as a medicine, we should be very clear about how much, how frequently and to whom it is safe to prescribe cannabis.
Tell us a little more about yourself – what do you like to do when you’re not working?
I have a wonderful 3 and half year old black Labrador, Iona, which I walk on Wimbledon Common twice a day. I also enjoy going to the cinema and I like to walk and site-see around London. I am a member of the Natural History and Science museums, which I regularly visit with my Godson; and also the British Museum and the Royal Academy. My true passion however remains my job. I enjoy meeting patients and their families and listening to them gives sense to my research.
Dr Di Forti is the most widely cited expert in the field of cannabis use and mental illness. She will be giving the keynote address at Member’s Day this year, on the topic ‘Cannabis use and psychosis: A cause or a cure?’
To reserve your place go to rethink.org/membersday