A major Lancet review found little evidence that cannabinoids help mental disorders and increased side effects | Research

The takeaway

A Lancet Psychiatry meta-analysis of 83 studies (40 RCTs, n=3,067) found scarce evidence that cannabinoids improve depression, anxiety, ADHD, Tourette syndrome, PTSD, or psychosis, with THC products doubling adverse event rates compared to placebo.

Psychiatrists, mental health patients considering cannabis, policymakers, and anyone evaluating evidence for cannabinoid mental health treatments.

83 studies, scarce evidence

What the researchers found

Pharmaceutical THC (with or without CBD) produced only a very small improvement in anxiety among patients with other medical conditions (SMD -0.25). THC worsened negative symptoms of psychosis. THC did not significantly improve any other mental health outcome. THC nearly doubled adverse events (OR 1.99) and nearly tripled withdrawals due to adverse events (OR 2.78). Very few RCTs examined CBD or medicinal cannabis specifically.

Why it matters

This is the most comprehensive meta-analysis of cannabinoids for mental disorders published in a top-tier psychiatric journal. Its sobering conclusions challenge the widespread assumption that cannabis-based medicines are broadly effective for mental health conditions.

The numbers in context

83 studies total (40 RCTs, n=3,067). Anxiety improvement: SMD -0.25 (very low quality). Psychosis negative symptoms worsened: SMD 0.36. Adverse events: OR 1.99. Withdrawals from adverse events: OR 2.78. Depression: 42 studies (23 RCTs, n=2,551). Anxiety: 31 studies (17 RCTs, n=605).

How the study worked

Systematic review and meta-analysis searching MEDLINE, Embase, PsycINFO, and Cochrane databases (1980-2018). Included RCTs and observational studies of any medicinal cannabinoid in adults for depression, anxiety, ADHD, Tourette syndrome, PTSD, or psychosis. GRADE quality assessment applied.

What this study cannot tell us

Most included RCTs examined THC-based pharmaceuticals, not CBD or whole-plant cannabis. Many conditions had very few RCTs (Tourette: 2 RCTs n=36; PTSD: 1 RCT n=10; ADHD: 1 RCT n=30). The evidence was rated very low to low quality by GRADE criteria.

How to read the evidence

Rated strong because this is a rigorous systematic review and meta-analysis published in Lancet Psychiatry with GRADE quality assessment, despite the underlying evidence being low quality.

When this study was published

Published in 2019. Additional RCTs of cannabinoids for mental health conditions may have been completed since.

The bigger picture

While public enthusiasm for cannabis as a mental health treatment is high, this Lancet meta-analysis reveals how thin the evidence actually is. The gap between patient expectations and scientific evidence is wider than commonly recognized, particularly for depression and PTSD.

Questions still open

Will dedicated CBD trials show better results than THC-based products? Are the negative results specific to pharmaceutical formulations or do they apply to whole-plant cannabis? Should regulatory frameworks discourage cannabinoid use for mental health indications?

Common questions

Do cannabinoids help depression or anxiety?

This major meta-analysis found no significant evidence that cannabinoids improve depression and only a very small, low-quality signal for anxiety improvement in people who also had other medical conditions like chronic pain.

What about CBD specifically for mental health?

Very few of the included RCTs examined CBD specifically. The negative findings mainly apply to THC-based pharmaceutical products. CBD requires its own dedicated clinical trials for mental health conditions.

Are there risks to using cannabinoids for mental health?

THC-based products nearly doubled adverse events and nearly tripled study dropouts due to side effects compared to placebo. One study found THC worsened negative symptoms of psychosis.

Read the original research

Cannabinoids for the treatment of mental disorders and symptoms of mental disorders: a systematic review and meta-analysis.

The lancet. Psychiatry, 6(12), 995-1010

View on PubMed ↗Publisher’s paper ↗

Citation

Black, Nicola; Stockings, Emily; Campbell, Gabrielle; Tran, Lucy T; Zagic, Dino; Hall, Wayne D; Farrell, Michael; Degenhardt, Louisa. (2019). Cannabinoids for the treatment of mental disorders and symptoms of mental disorders: a systematic review and meta-analysis.. The lancet. Psychiatry, 6(12), 995-1010. https://doi.org/10.1016/S2215-0366(19)30401-8