Largest ever study into cannabis use investigates risk of paranoia and poor mental health in the general population

New research from the Institute of Psychiatry, Psychology & Neuroscience (IoPPN) at King’s College London, in partnership with the University of Bath, has found that the reasons why a person chooses to use cannabis can increase their risk of developing paranoia. 

The use and potency of cannabis is increasing worldwide, and dependence and cannabis-induced psychosis are also greatly increasing as a result, especially in North America. Two new research papers, both using data from Cannabis & Me - the largest survey of its kind - have identified key risk factors associated with the more severe forms of paranoia in cannabis users. 

The first study, published today in the BMJ Mental Health, explored the relationship between why people first started using cannabis, and how this affected their subsequent use.  

3389 former and current cannabis users aged 18 and over responded to a survey examining their reasons for first and continued use, their weekly consumption of cannabis in THC units, and their mental health. 

Researchers established several key findings. Respondents who first started using cannabis to self-medicate an illness, including physical pain, anxiety, depression, or because they were experiencing minor psychotic symptoms, all demonstrated higher paranoia scores.  

This was in contrast to those respondents who tried cannabis for fun or curiosity, or with their friends, who reported the lowest average paranoia and anxiety scores.  

Dr Edoardo Spinazzola, a Research Assistant at King’s IoPPN and the study’s first author said, “Our study provides vital evidence on how the reason someone first starts using cannabis can dramatically impact their long-term health.  

“This research suggests that using cannabis as a mean to self-medicate physical or mental discomfort can have a negative impact on the levels of paranoia, anxiety, and depression. Most of these subgroups had average scores of depression and anxiety which were above the threshold for referral to counselling." 

Respondents were also asked to provide data on the frequency and strength of the cannabis they were using so that researchers could track their average weekly consumption of Tetrahydrocannabinol (THC) - the principle psychoactive component of cannabis. 

The researchers found that the average respondent consumed 206 units of THC a week. This might equate to roughly 10-17 ‘joints’ per week, if the user was consuming an expected 20 per cent THC content that is standard for the most common types of cannabis available in London. 

However, respondents who started using cannabis to help with their anxiety, depression, or in cases where they started due to others in their household who were already using cannabis, reported on average 248, 254.7, and 286.9 average weekly THC units respectively. 

Professor Tom Freeman, Director of the Addiction and Mental Health Group at the University of Bath and one of the study’s authors said, “A key finding of our study is that people who first used cannabis to manage anxiety or depression, or because a family member was using it, showed higher levels of cannabis use overall.  

“In future, standard THC units could be used in a similar way to alcohol units – for example, to help people to track their cannabis consumption and better manage its effects on their health.” 

In a separate study, published in Psychological Medicine, researchers explored the relationship between childhood trauma, paranoia and cannabis use.  

Researchers used the same data set from the Cannabis & Me survey, with just over half of respondents (52 per cent) reporting experience of some form of trauma.  

Analysis established that respondents who had been exposed to trauma as children reported higher average levels of paranoia compared to those who hadn’t, with physical and emotional abuse emerging as the strongest predictors.  

Researchers also explored the relationship between childhood trauma and weekly THC consumption. Respondents who reported experience of sexual abuse had a markedly higher weekly intake of THC, closely followed by those who reported experiencing emotional and physical abuse. 

Finally, the researchers confirmed that the strong association between childhood trauma and paranoia is further exacerbated by cannabis use, but is affected by the different types of trauma experienced. Respondents who said they had experienced emotional abuse or household discord1 were strongly associated with increased THC consumption and paranoia scores. Respondents reporting bullying, physical abuse, sexual abuse, physical neglect and emotional neglect on the other hand did not show the same effects.  

Dr Giulia Trotta, a Consultant Psychiatrist and Researcher at King’s IoPPN and the study’s first author said, “This comprehensive study is the first to explore the interplay between childhood trauma, paranoia, and cannabis use among cannabis users from the general population. 

“We have not only established a clear association between trauma and future paranoia, but also that cannabis use can further exacerbate the effects of this, depending on what form the trauma takes.  

“Our findings will have clear implications for clinical practice as they highlight the importance of early screening for trauma exposure in individuals presenting with paranoia.” 

Professor Marta Di Forti, Professor of Drug use, Genetics and Psychosis at King’s IoPPN, Clinical Lead at the South London and Maudsley NHS Foundation Trust’s Cannabis Clinic for Patients with Psychosis, and the senior author on both studies said, “There is extensive national and internation debate about the legality and safety of cannabis use.  

“My experience in clinic tells me that there are groups of people who start to use cannabis as a means of coping with physical and emotional pain. My research has confirmed that this is not without significant further risk to their health and wellbeing, and policy makers across the world should be mindful of the impact that legalisation , without adequate public education and health support, could have on both the individual, as well as on healthcare systems more broadly.” 

Cannabis & Me was possible thanks to funding from the Medical Research Council (MRC). 

Ends 

For more information, please contact Patrick O’Brien (Media Manager) on 020 7848 5377. 

Are reasons for first using cannabis associated with subsequent cannabis consumption (standard THC units) and psychopathology? (Spinazzola, Di Forti et al) (DOI ) was published in BMJ Mental Health. Published 27/08/25 

The impact of Childhood Trauma and Cannabis Use on Paranoia: A Structural Equation Model Approach (Trotta, Di Forti et al) (DOI 10.1017/S0033291725101190) was published in Psychological Medicine.  Published 08/08/25 

1. Household discord refers to living in a space where there is disharmony, conflict, or disagreement within a family unit 

About King’s College London and the Institute of Psychiatry, Psychology & Neuroscience  

King’s College London is amongst the top 35 universities in the world and top 10 in Europe (THE World University Rankings 2023), and one of England’s oldest and most prestigious universities.   

With an outstanding reputation for world-class teaching and cutting-edge research, King’s maintained its sixth position for ‘research power’ in the UK (2021 Research Excellence Framework). 

King's has more than 33,000 students (including more than 12,800 postgraduates) from some 150 countries worldwide, and some 8,500 staff. The Institute of Psychiatry, Psychology & Neuroscience (IoPPN) at King’s is a leading centre for mental health and neuroscience research in Europe. It produces more highly cited outputs (top 1% citations) on psychiatry and mental health than any other centre (SciVal 2021), and on this metric has risen from 16th (2014) to 4th (2021) in the world for highly cited neuroscience outputs. In the 2021 Research Excellence Framework (REF), 90% of research at the IoPPN was deemed ‘world leading’ or ‘internationally excellent’ (3* and 4*). World-leading research from the IoPPN has made, and continues to make, an impact on how we understand, prevent and treat mental illness, neurological conditions, and other conditions that affect the brain. 

www.kcl.ac.uk/ioppn | Follow @KingsIoPPN on Twitter, Instagram, Facebook and LinkedIn 

About the University of Bath 

The University of Bath is one of the UK's leading universities, recognised for high-impact research, excellence in education, an outstanding student experience and strong graduate prospects. 

• We are ranked in the top 10 in all of the UK’s major university guides.  

• The University achieved a triple Gold award in the last Teaching Excellence Framework 2023, the highest awards possible, for both the overall assessment and for student outcomes and student experience. The Teaching Excellence Framework (TEF) is a national scheme run by the Office for Students (OfS). 

• We are also ranked among the top 10% of universities globally, placing 132nd in the QS World University Rankings 2026.  

Research from Bath is helping to change the world for the better. Across the University’s three Faculties and School of Management, our research is making an impact in society, leading to low-carbon living, positive digital futures, and improved health and wellbeing. Find out all about our Research with Impact: https://www.bath.ac.uk/campaigns/research-with-impact/