Massive umbrella review maps the strength of evidence for cannabis risks and benefits | Research

The takeaway

An umbrella review of 101 meta-analyses found convincing evidence linking cannabis to psychosis and car crashes, moderate evidence for some therapeutic benefits, but weak evidence for most other claimed associations.

Clinicians, policymakers, researchers, and anyone wanting a comprehensive evidence summary on cannabis risks and benefits.

101 meta-analyses reviewed; few associations reached convincing evidence level

What the researchers found

Of hundreds of reported associations, only a handful met the threshold for convincing or highly suggestive evidence. Harmful associations (psychosis, motor vehicle accidents, low birth weight) had stronger evidence than most therapeutic claims.

Why it matters

The cannabis literature is vast and contradictory. This umbrella review cuts through the noise by systematically rating the credibility of every major claimed risk and benefit, giving the clearest available picture of what the evidence actually supports.

The numbers in context

101 meta-analyses reviewed. Convincing harmful associations: psychosis incidence, motor vehicle accidents, low birth weight. Most therapeutic associations graded as low or very low certainty by GRADE.

How the study worked

Umbrella review of systematic reviews with meta-analyses from observational studies and RCTs. Credibility graded as convincing, highly suggestive, suggestive, weak, or not significant. RCT evidence graded using GRADE. Quality assessed with AMSTAR 2.

What this study cannot tell us

Umbrella reviews are only as good as the underlying meta-analyses, many of which had methodological weaknesses. Rapidly evolving cannabis products (high-potency, concentrates, edibles) may not be captured in older studies. Publication bias may affect underlying reviews.

How to read the evidence

Umbrella review is the highest level of evidence synthesis. Findings are limited by quality of underlying meta-analyses, but the methodology is rigorous.

When this study was published

Published 2022. Literature search through February 2022.

The bigger picture

Both cannabis advocates and opponents tend to cherry-pick studies. This comprehensive evidence map shows the reality is nuanced: some risks are well-established, some benefits are real but modest, and most claims in either direction rest on weak evidence.

Questions still open

How do the risk-benefit profiles differ across specific cannabinoid formulations and delivery methods? Will future high-quality RCTs shift any of the current evidence grades substantially?

Common questions

What risks of cannabis have the strongest evidence?

The umbrella review found convincing evidence linking cannabis use to increased risk of psychosis, motor vehicle accidents, and low birth weight in offspring. These associations met the highest credibility threshold across multiple meta-analyses.

What about medical benefits?

Some therapeutic benefits had moderate evidence, particularly for chronic pain and chemotherapy-related nausea. However, most claimed benefits were graded as low or very low certainty under the GRADE framework, meaning the evidence is not as strong as often assumed.

Read the original research

Balancing risks and benefits of cannabis use: umbrella review of meta-analyses of randomised controlled trials and observational studies.

BMJ (Clinical research ed.), 382, e072348

View on PubMed ↗Publisher’s paper ↗

Citation

Solmi, Marco; De Toffol, Marco; Kim, Jong Yeob; Choi, Min Je; Stubbs, Brendon; Thompson, Trevor; Firth, Joseph; Miola, Alessandro; Croatto, Giovanni; Baggio, Francesca; Michelon, Silvia; Ballan, Luca; Gerdle, Björn; Monaco, Francesco; Simonato, Pierluigi; Scocco, Paolo; Ricca, Valdo; Castellini, Giovanni; Fornaro, Michele; Murru, Andrea; Vieta, Eduard; Fusar-Poli, Paolo; Barbui, Corrado; Ioannidis, John P A; Carvalho, Andrè F; Radua, Joaquim; Correll, Christoph U; Cortese, Samuele; Murray, Robin M; Castle, David; Shin, Jae Il; Dragioti, Elena. (2023). Balancing risks and benefits of cannabis use: umbrella review of meta-analyses of randomised controlled trials and observational studies.. BMJ (Clinical research ed.), 382, e072348. https://doi.org/10.1136/bmj-2022-072348