A recent peer-reviewed paper has identified a previously unrecognised form of reporting bias that may be skewing how medical cannabis evidence is presented.
The paper, published in Research Integrity and Peer Review, introduces the concept of reverse spin bias – a pattern in which authors of systematic reviews “discount, downplay, or dismiss beneficial findings about a treatment despite their own evidence showing statistically significant effects.
This phenomenon was observed repeatedly in studies on e-cigarettes for smoking cessation and medical cannabis for pain.
Traditional spin bias in scientific literature usually refers to cases where authors make non-significant results appear more positive than they are. In contrast, reverse spin bias happens when authors undervalue or undermine statistically significant positive outcomes, often without methodological justification.
The study was led by Renée O’Leary and colleagues, who argue that this type of narrative shift has not been systematically documented before.
What does ‘reverse spin bias’ look like?
The researchers examined 29 recent systematic reviews on medical cannabis for pain and found that 10 exhibited reverse spin bias. In these cases, even though the review’s own data suggested potential benefit, the authors either stopped short of recommending cannabis, questioned its value, or framed the evidence in a way that overshadowed beneficial results.
This includes describing their evidence as ‘inconsistent’, ‘consisting of only low-quality studies’, or having an ‘insufficient number of primary studies, regardless of the number’.
Several mechanisms for reverse spin bias were identified in the study, including discounting the evidence, omitting findings, and discrediting primary studies.
Other approaches were designed to ‘provoke fear’ by making claims of ‘unknown, unnamed future harms’ to dismiss findings of treatment benefit, and language related to risks in conclusions, even where adverse effects data were not part of the review.
“We suspect that reverse spin bias has a similar function to spin bias, as a strategy to improve the chances of publication,” the authors write.
“For medical cannabis, the assumption that cannabis use has only harms is a position held by a number of journals, limiting the dissemination of contradicting evidence.”
The wider impact of reverse spin bias
In response to the paper, the UK charity, Drug Science, highlighted how reverse spin bias could influence broader drug and harm-reduction policy debates.
“Evidence does not simply inform decisions, but legitimises them,” explains Hannah Barnett, in a blog for Drug Science.
“When studies downplay the observed benefits of socially controversial interventions, policy can appear evidence-led while remaining norm-driven. Reverse spin bias may therefore create a feedback loop that reinforces stigma, delays harm reduction, and justifies inaction.
“This is particularly consequential in drug policy contexts, where moral narratives have long shaped public discourse.”
In the context of medical cannabis, failing to clearly report and interpret evidence accurately could delay access to potentially helpful treatment and leave clinicians and patients without a balanced understanding of the research.
The authors speculate that reverse spin bias could also be occurring in reviews on other socially contested treatments, such as safe opioid consumption sites or managed alcohol programs.
“Recognising this bias is not about advocating for specific treatments,” Barnett writes.
“It is about ensuring that drug policy is shaped by evidence as it exists, not as it is most comfortable to present.”
‘Stronger scrutiny’ needed to address reporting bias
As reverse spin bias could occur in any context where review authors favour negative conclusions, the authors say, wider investigations of this newly-identified phenomenon are needed.
“Editors and peer reviewers must be alert to discrepancies between the findings of biomedical systematic reviews and the treatment recommendations that their authors endorse,“ they conclude.
“While it may seem like a standard task, looking at our examples strongly suggests that this critical examination for reporting bias has been all too frequently omitted. By proposing a new term, reverse spin bias, we hope to bring stronger scrutiny to bear on these instances of reporting bias that are detrimental to evidence-informed clinical practice.”
