A review of 72 reviews on medical cannabis found mixed results and mostly low-quality evidence | Research

The takeaway

A scoping review of 72 systematic reviews on medical cannabis found mostly mixed or inconclusive results, with only one review rated as high quality. Mild harms were frequently reported and may outweigh benefits.

Anyone evaluating the evidence for medical cannabis, healthcare policymakers, and researchers designing future studies.

Only 1 of 72 systematic reviews was rated high quality

What the researchers found

Pain management was the most commonly studied condition. A small number of reviews found benefits for pain, but analysis quality varied widely. Adverse effects were reported in 83% of reviews comparing cannabis to placebo and 83% comparing to active drugs. Minor harms (drowsiness, dizziness) were common. Serious harms were reported in 36% of reviews. Only 1 of 72 reviews was rated high quality; 36 were moderate, 35 were low/critically low.

Why it matters

This is a review of reviews, providing a bird's-eye view of the entire medical cannabis evidence base. The finding that 71 of 72 reviews were not high quality is a damning assessment of the field's evidence standards.

The numbers in context

1,975 citations screened. 72 systematic reviews included. 1/72 rated high quality. 36 moderate quality. 35 low/critically low quality. Adverse effects in 83% of reviews vs. placebo. Serious harms in 36% of those reviews.

How the study worked

Scoping review of systematic reviews from multiple databases. Two reviewers selected and charted data from 72 included systematic reviews covering various medical conditions.

What this study cannot tell us

Scoping review methodology provides a broad overview but does not synthesize quantitative results. The quality of included reviews varied dramatically. The review was limited to systematic reviews, potentially missing primary research.

How to read the evidence

Moderate: scoping review of 72 systematic reviews provides broad evidence synthesis, but the underlying evidence quality was mostly poor.

When this study was published

Published in 2019.

The bigger picture

Medical cannabis has gained enormous cultural and legal acceptance, but this comprehensive overview reveals that the scientific evidence supporting it remains remarkably weak. The field needs rigorous, well-designed clinical trials.

Questions still open

Why is the quality of cannabis research so low? Are regulatory barriers preventing better studies? Would the results change if only high-quality evidence were considered? Which specific conditions have the strongest evidence?

Common questions

Does this mean medical cannabis doesn't work?

Not necessarily. The review found that most existing research is too low-quality to draw firm conclusions either way. A small number of reviews did find benefits for pain, but the overall evidence base is weak.

What were the most common side effects?

Drowsiness and dizziness were the most frequently reported. Serious harms were less common but appeared in over a third of the reviews that reported on adverse effects.

Read the original research

Benefits and harms of medical cannabis: a scoping review of systematic reviews.

Systematic reviews, 8(1), 320

View on PubMed ↗Publisher’s paper ↗

Citation

Pratt, Misty; Stevens, Adrienne; Thuku, Micere; Butler, Claire; Skidmore, Becky; Wieland, L Susan; Clemons, Mark; Kanji, Salmaan; Hutton, Brian. (2019). Benefits and harms of medical cannabis: a scoping review of systematic reviews.. Systematic reviews, 8(1), 320. https://doi.org/10.1186/s13643-019-1243-x