A groundbreaking investigation spearheaded by the Institute of Psychiatry, Psychology & Neuroscience (IoPPN) at King’s College London, in collaboration with the University of Bath, uncovers compelling evidence that the initial motivations behind cannabis usage significantly influence the risk of developing paranoia and related psychiatric symptoms. The research, drawing on data from the expansive Cannabis & Me survey — the largest of its kind globally — marks a pivotal advance in understanding how not just cannabis itself but the reasons for its use impact mental health trajectories.
While cannabis consumption and potency continue to surge internationally, paralleling rises in dependence and cannabis-induced psychosis, the nuances behind these trends remain underexplored. This duo of studies probes critical psychological variables by scrutinizing the relationship between first-time use rationales and ensuing patterns of cannabis consumption measured in standardized THC units. THC, or tetrahydrocannabinol, is the primary psychoactive constituent responsible for cannabis’s psychotropic effects, and its quantification provides a standardized framework analogous to alcohol units, facilitating more precise risk assessment.
The principal study, recently published in BMJ Mental Health, analyzed responses from 3,389 adults aged 18 and above with current or prior cannabis use histories. Participants completed detailed surveys capturing their initial reasons for cannabis experimentation, ongoing usage frequency, and weekly consumption expressed in THC units, alongside standardized assessments of paranoia, anxiety, and depression. Crucially, researchers discerned stark contrasts between users who initiated cannabis to self-medicate various conditions — such as chronic physical pain, anxiety disorders, depression, or prodromal psychotic symptoms — and those whose first interactions with cannabis were recreational or social.
Individuals using cannabis as an attempt to alleviate physical or psychological distress consistently scored higher on validated paranoia scales, with anxiety and depression levels commonly exceeding clinical thresholds that typically warrant therapeutic intervention. This contrasts with users motivated by curiosity or social engagement, who tended to report the lowest paranoia and anxiety symptomatology. The data implicate the intention behind usage as a potent modifier of cannabis’s mental health impact, emphasizing that initial self-medication attempts can inadvertently exacerbate psychopathological risk.
From a quantitative perspective, the average weekly THC intake among all respondents was approximately 206 units — equating roughly to 10-17 joints of cannabis standardized at 20% THC concentration, the prevalent potency in London’s market. Notably, those whose initial cannabis use was influenced by pre-existing anxiety or depression symptoms, or by cannabis use among household members, exhibited significantly elevated consumption levels, averaging between 248 and 287 THC units weekly. This escalation underscores a concerning pattern of intensified exposure among vulnerable groups, potentially fueling a reinforcing cycle of cannabis use and mental health deterioration.
Professor Tom Freeman, Director of the Addiction and Mental Health Group at the University of Bath, highlights the broader implications: “The adoption of THC standard units could revolutionize how cannabis consumption is monitored and managed clinically, much like alcohol units have shaped alcohol use awareness and public health strategies.” Such standardized metrics could enable personalized risk tracking, guiding harm reduction approaches and clinical decision-making.
Delving deeper, a companion study featured in Psychological Medicine explores the intricate nexus between childhood trauma, paranoia, and cannabis use. Utilizing the same extensive Cannabis & Me dataset, it was revealed that over half of the participants reported experiencing childhood trauma in various forms. Physical and emotional abuse emerged as the most robust predictors of heightened paranoia scores, with sexual abuse notably linked to the highest average weekly THC consumption—followed closely by emotional and physical abuse.
The research elucidates that the interplay between trauma exposure and paranoia is potentiated by cannabis use but varies substantially depending on trauma typology. Specifically, emotional abuse and household discord—characterized by familial conflict and disharmony—were strongly correlated with increased THC intake and paranoia severity. Conversely, experiences such as bullying, neglect (both physical and emotional), and certain other forms of abuse did not exhibit the same exacerbating effect. This granular differentiation provides critical insights into targeted intervention points for at-risk populations.
Dr. Giulia Trotta, Consultant Psychiatrist and lead author on the trauma-focused study, emphasizes the clinical ramifications: “Our findings underscore the necessity of early and thorough trauma screening in individuals presenting with paranoia, particularly those who use cannabis. The divergent impact of trauma types on cannabis’s effects suggests tailored treatment modalities must be considered to address these complex interrelations.”
Both studies reinforce the cautionary perspective held by clinicians like Professor Marta Di Forti, whose extensive experience managing patients in the South London and Maudsley NHS Foundation Trust’s Cannabis Clinic underscores the real-world consequences of unregulated cannabis use. Professor Di Forti remarks, “There exists a widespread misconception that cannabis is benign; however, our research reveals that self-medicating with cannabis can significantly jeopardize mental health, especially in those with prior trauma or psychiatric symptoms. Policy reforms regarding legalization must be accompanied by robust public education and healthcare infrastructures to mitigate adverse outcomes.”
The studies collectively illuminate the multifaceted risks intertwined with cannabis use, accentuated by personal history, motivations, and dosage. This nuanced understanding calls for evolving clinical frameworks and public health policies that transcend simplistic narratives, recognizing that the path from cannabis experimentation to psychopathology is not only about exposure but deeply influenced by individual vulnerabilities and usage contexts.
Funding for the Cannabis & Me project was generously provided by the Medical Research Council, facilitating a rigorously designed observational approach that integrates psychological, epidemiological, and substance use metrics. These influential findings contribute to a growing body of evidence that challenge existing paradigms, urging stakeholders in healthcare, policy, and community education to engage with cannabis-related mental health risks through an informed and multifactorial lens.
As cannabis use becomes increasingly normalized globally, this research serves as a pivotal reminder that consumption is not a monolithic behavior, and its impact on mental health is inextricably linked to why and how individuals engage with the substance. Future research is poised to refine THC unit standardization further and to develop predictive models that can identify those at elevated risk, representing a transformative leap toward more effective prevention and intervention strategies in mental healthcare.
Subject of Research: People
Article Title: Are reasons for first using cannabis associated with subsequent cannabis consumption (standard THC units) and psychopathology?
News Publication Date: 27-Aug-2025
References:
Spinazzola, Di Forti et al., BMJ Mental Health, 27 August 2025
Trotta, Di Forti et al., Psychological Medicine, 8 August 2025 (DOI 10.1017/S0033291725101190)
Keywords: Cannabis, Paranoia, Psychosis, Depression
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