What the evidence shows 

Cannabis can make serious mental illness worse - not better. Here's what the research shows.

Some private clinics promote cannabis to people struggling with depression, anxiety, and other mental health conditions. Current clinical evidence indicates that cannabis may worsen certain psychiatric conditions and has limited proven benefit for many mental health indications. ¹

A recent international peer reviewed study, published in The Lancet concluded that cannabis is not an effective treatment for common mental health conditions despite the global surge in patients using it for that purpose. ¹⁴

Cannabis and psychosis: 

Cannabis use, particularly high-THC products, increases the risk of developing psychosis and schizophrenia ²

The risk is dose-dependent: the stronger the cannabis, the higher the risk ³

For people already diagnosed with psychotic disorders, cannabis worsens symptoms and interferes with treatment ⁴

Cannabis and depression: 

The British National Formulary lists depression and euphoric mood as common or very common side-effects of cannabis ⁵

Suicidal ideation is listed as an uncommon but "not unlikely" side-effect ⁵

There is no robust evidence that cannabis treats depression—and evidence it may cause or worsen it ⁶

Cannabis and bipolar disorder: 

Cannabis use disorder is associated with: 

  • Increased risk of both psychotic and non-psychotic bipolar disorder ⁷

  • Younger age at onset ⁸

  • Higher prevalence of psychotic symptoms ⁹

  • Poorer treatment adherence ¹⁰ 

People with bipolar disorder who use cannabis have worse outcomes across multiple domains. ¹¹

Cannabis use via prescription:

People with prescriptions show a higher probability of frequent use of processed, higher-potency products, and meeting a threshold for high-risk use. ¹²

What the experts say:

Professor Sir Robin Murray, one of the UK's leading psychiatrists at King's College Hospital, London: 

"The clinical evidence is clear - cannabis makes bipolar disorder worse. These clinics are prescribing substances that harm the very patients they claim to help." 

Dr. Pavan Chahl, consultant psychiatrist, told Oliver's inquest: 

"Under current BNF guidance, medicinal cannabis should not be prescribed to someone with a history of severe psychiatric disorder. On this evidence, Oliver Robinson should never have been prescribed medicinal cannabis." 

What medical cannabis is licensed for: 

Medical cannabis has proven efficacy for a narrow set of conditions: 

  • Severe, treatment-resistant epilepsy in children 

  • Nausea and vomiting caused by chemotherapy 

  • Muscle spasms in multiple sclerosis 

For these conditions, cannabis-based medicines are prescribed through the NHS, following rigorous clinical assessment and specialist consultation. 

What private clinics are prescribing for: 

Private cannabis clinics are prescribing unlicensed cannabis products for a wide range of conditions with poor or no evidence of efficacy, including: 

  • Depression 

  • Anxiety 

  • ADHD 

  • Insomnia 

  • Chronic pain 

  • "General wellbeing" 

659,293 unlicensed cannabis products were prescribed in 2024 - a 133% increase from 2023. ¹³

These trends underscore the need for stronger evidence standards, transparency, and safety oversight in private prescribing.

¹ - Moore, T.H., Zammit, S., Lingford-Hughes, A., et al. (2007). Cannabis use and risk of psychotic or affective mental health outcomes: a systematic review. The Lancet, 370(9584), 319–328. https://doi.org/10.1016/S0140-6736(07)61162-3

Maggu, G., Choudhary, S., Jaishy, R., et al. (2023). Cannabis use and its relationship with bipolar disorder: A systematic review and meta-analysis. Indian Psychiatry Journal, 32(2), 202–214. https://pmc.ncbi.nlm.nih.gov/articles/PMC10756590/

² - Di Forti, M., Quattrone, D., Freeman, T.P., et al. (2019). The contribution of cannabis use to variation in the incidence of psychotic disorder across Europe (EU-GEI): a multicentre case-control study. The Lancet Psychiatry, 6(5), 427–436. https://doi.org/10.1016/S2215-0366(19)30048-3

This landmark study found daily cannabis users had three-fold higher odds of psychosis, and daily users of high-potency cannabis (>10% THC) had five-fold higher odds compared to non-users.

³ - Marconi, A., Di Forti, M., Lewis, C.M., Murray, R.M., & Vassos, E. (2016). Meta-analysis of the association between the level of cannabis use and risk of psychosis. Schizophrenia Bulletin, 42(5), 1262–1269. https://doi.org/10.1093/schbul/sbw003

This meta-analysis of 66,816 individuals confirmed a dose-response relationship, with heaviest users having an OR of 3.90 (95% CI 2.84–5.34) for psychotic outcomes compared to non-users.

Di Forti, M., et al. (2009). High-potency cannabis and the risk of psychosis. British Journal of Psychiatry, 195(6), 488–491. https://pmc.ncbi.nlm.nih.gov/articles/PMC2801827/

⁴ - Schoeler, T., Petros, N., Di Forti, M., et al. (2016). Association Between Continued Cannabis Use and Risk of Relapse in First-Episode Psychosis: A Quasi-Experimental Investigation Within an Observational Study. JAMA Psychiatry, 73(11), 1173–1179. https://doi.org/10.1001/jamapsychiatry.2016.2427

This study found continued cannabis use after first-episode psychosis was associated with increased risk of relapse and psychiatric hospitalisation, with a dose-response relationship.

A 2025 systematic review confirmed: "High-potency THC use is linked to relapse, increased psychosis, and treatment resistance in individuals with schizophrenia." https://pmc.ncbi.nlm.nih.gov/articles/PMC12539040/

⁵ - https://www.farleys.com/case_study/oliver-robinson-inquest-concludes-prescription-of-medicinal-cannabis-contributed-to-death/

Note: A direct URL to the BNF monograph page cannot be provided as BNF content is behind a paywall (https://bnf.nice.org.uk). However, this claim was confirmed as testimony under oath at the Oliver Robinson inquest by expert Consultant Psychiatrist Dr Pavan Chahl, as documented by Farleys Solicitors

⁶ - Lev-Ran, S., Roerecke, M., Le Foll, B., George, T.P., McKenzie, K., & Rehm, J. (2014). The association between cannabis use and depression: a systematic review and meta-analysis of longitudinal studies. Psychological Medicine, 44(4), 797–810. https://doi.org/10.1017/S0033291713001438

This meta-analysis found cannabis use was associated with a modestly increased risk of developing depression (OR 1.17; 95% CI 1.05–1.30).

Mammen, G., Rueda, S., Englesbe, M.J., et al. (2025). The association between cannabis and depression: an updated Systematic Review and Meta-analysis. Psychological Medicine, 55, e56. https://doi.org/10.1017/S0033291725000170

This updated meta-analysis found an OR of 1.29 (95% CI 1.13–1.46) for depression among cannabis users. It noted high heterogeneity across studies and stated the evidence was consistent with a positive association between cannabis use and depression development.

⁷ - Hjorthøj, C., Posselt, C.M., & Nordentoft, M. (2023). Cannabis Use Disorder and Subsequent Risk of Psychotic and Nonpsychotic Unipolar Depression and Bipolar Disorder. JAMA Psychiatry, 80(8), 803–810. https://jamanetwork.com/journals/jamapsychiatry/fullarticle/2804862

This Danish cohort study of 6,651,765 individuals found CUD was associated with increased risk of psychotic bipolar disorder (HR 4.05; 95% CI 3.52–4.65) and nonpsychotic bipolar disorder in men (HR 2.96; 95% CI 2.73–3.21) and women (HR 2.54; 95% CI 2.31–2.80).

⁸ - Lagerberg, T.V., Sundet, K., Aminoff, S.R., et al. (2011). Excessive cannabis use is associated with earlier age at onset in bipolar disorder. European Archives of Psychiatry and Clinical Neuroscience, 261, 397–405. https://doi.org/10.1007/s00406-011-0188-4 (cited in the systematic review at )

Martínez-Arán, A., et al. (2025). Cannabis and tobacco use in bipolar disorder: Associations with clinical characteristics. Journal of Affective Disorders. This study confirmed cannabis use in BD I patients is "linked to a younger age at the time of admission and at their first episode." https://www.sciencedirect.com/science/article/abs/pii/S0165032725005877

⁹ - Hjorthøj, C., et al. (2023), as above. CUD was associated with significantly higher risk of psychotic bipolar disorder (HR 4.05) compared to nonpsychotic bipolar disorder (relative HR 1.48; 95% CI 1.21–1.81).

Martínez-Arán, A., et al. (2025), as above, explicitly found "a higher incidence of psychotic symptoms" in BD I patients who use cannabis.

¹⁰ - Martínez-Arán, A., et al. (2025), as above, found cannabis use in BD I patients was associated with "a greater tendency to stop treatment".

¹¹ - Maggu, G., et al. (2023), as above, noted that "cannabis abuse is a predictor of increased psychosocial difficulties, poor treatment adherence, longer duration, and increased severity of the bipolar disorder" citing Strakowski et al. (2000), Strakowski et al. (1998), and Tohen et al. (1990).

¹² - Wadsworth, E. et al. (2026) - https://www.medrxiv.org/content/10.64898/2026.01.19.26344380v1.full.pdf

¹³ - WRD News (2026): reported the increase as 133%, not 134%. "Unlicensed cannabis prescriptions in the UK surged from 282,920 in 2023 to 659,293 in 2024. That represents nearly 10 tonnes of cannabis, a 133% increase in just one year." https://wrdnews.org/cannabis-prescriptions-benefits-claimants-get-discounts

¹⁴ - Wilson J, Dobson O, Langcake A et al. The efficacy and safety of cannabinoids for the treatment of mental disorders and substance use disorders: a systematic review and meta-analysis. The Lancet Psychiatry, 2026; - https://www.thelancet.com/journals/lanpsy/article/PIIS2215-0366(26)00015-5/fulltext