Talk to people who use medical cannabis for pain, and you’ll hear a common story: it really helps. While patients know that cannabis can provide a level of relief conventional pharmaceuticals can’t match, those who write policies and prescriptions remain sceptical, insisting that “more research is needed”. But what happens when that research takes place, finds clear evidence that cannabis does work, and then totally ignores it?
A new study has identified a worrying pattern in published studies and research papers. Scientists are conducting reviews, finding statistically significant evidence that controversial treatments might work, before refusing to recommend them in their conclusions. The phenomenon, termed “reverse spin bias”, appears in more than one-third of recent cannabis systematic reviews and raises the question. Is ideology, not evidence, shaping the future of our healthcare?
Published in Research Integrity and Peer Review, the study was led by Renée O’Leary at the Centre of Excellence for the Acceleration of Harm Reduction (CoEHAR) in Italy. The team examined 29 systematic reviews on medical cannabis for pain published between 2021 and 2025. In ten of them, researchers reported findings showing cannabis worked, but then recommended against its use or made no recommendation at all. Studies would report clear statistical benefits, only to conclude that the evidence was “insufficient”, “inconsistent”, or plagued by “unknown future harms”.
What reverse spin bias looks like
Where regular spin bias involves researchers hyping up nonsignificant results to make them sound meaningful, reverse spin does the opposite. It talks down statistically significant findings. In their study, O’Leary and her team found researchers using a playbook of tricks to dismiss their own findings.
One systematic review found cannabis significantly more effective than placebo for chronic pain, but concluded that the “limited evidence” meant they couldn’t recommend it. Another reported that cannabis helped reduce pain intensity, then warned readers about hypothetical risks that weren’t documented in any of the studies they’d reviewed. A third found benefit but described it as “small”, minimising the clinical relevance for people living with chronic pain, where even modest improvements can be life-changing.
The most damning example came from the International Association for the Study of Pain. Their 2021 paper assessed 57 systematic reviews on cannabis and cannabinoid analgesia, rating eight as moderate or high quality. But when it came to the evidence summary the stated that there were ZERO moderate or high-quality reviews. In the minds of the authors, those positive findings simply didn’t exist.
Five ways researchers dismiss their own findings
The CoEHAR study identified five mechanisms researchers use to introduce reverse spin bias by comparing systematic review findings with the authors’ recommendations:
Discounting the evidence base: The most common tactic involved describing the evidence as “inconsistent”, “low quality”, or “insufficient” without justification. Some reviews labelled their evidence base this way despite including dozens of studies with consistent findings. Others claimed the evidence was limited but never explained what threshold would constitute “sufficient” evidence.
Discrediting primary studies: Individual studies showing benefit were dismissed as “not high quality” or producing only “small” effects. This happened even when formal risk-of-bias assessments rated studies favourably. Calling improvements “small” downplays their significance, especially in the case of pain.
Appealing to fear: Several reviews invoked “unknown future harms” to justify not recommending cannabis, despite no documented safety issues in the studies they examined.
Dismissing cannabis a priori: Some researchers rejected cannabis for reasons unrelated to their findings. One review team cited regulatory concerns. Another flagged ideological opposition, discounting it before the evidence was even properly considered.
Omitting findings entirely: In some reviews, positive results in subgroups simply didn’t make it into the conclusions. The data existed, but it just disappeared.
Why would scientists do this?
According to the CoEHAR team, there are three main drivers of reverse spin bias.
The first is publication pressure. Since at least 2000, anti-smoking organisations have actively lobbied journals not to publish research supporting tobacco industry products “regardless of the scientific merits of the paper”. For cannabis, prohibitionist attitudes remain entrenched in many medical journals, potentially making authors wary of drawing pro-cannabis conclusions even when the data supports them.
Secondly, researchers may feel pressure to align their conclusions with dominant views in their field, especially when those views treat cannabis scepticism as the default “responsible” position. Standing behind findings that challenge orthodoxy requires researchers to risk their professional reputations.
Finally, the paper suggests that study authors might be experiencing confirmation bias. When researchers have already stated a position against cannabis, changing their tune based on new evidence means admitting they were wrong. Reverse spin allows them to acknowledge the data whilst maintaining their prior stance.
What this means for UK cannabis patients
Cannabis patients in the UK already face stigma, from GPs to police, employers to landlords. While access is increasing, it is only through private prescriptions, creating a two-tier solution where patients can receive cannabis treatment, but only if they can afford it.
“Wider investigations of reverse spin bias are necessary because reverse spin could occur in any context where review authors favour null or negative conclusions.”
For a treatment already fighting decades of prohibition, reverse spin bias becomes another barrier to legitimacy. The NHS regularly cites “insufficient evidence” when refusing to prescribe or accept medical cannabis. Tabloids like the Daily Mail run front-page stories demonising cannabis patients. When researchers fail to stand behind their findings, this negativity compounds, fuelled by the perception that the medical evidence remains shaky and that cannabis patients are lying, exploiting loopholes or feigning illness.
Part of a wider pattern
This pattern isn’t unique to cannabis. O’Leary’s team found it in systematic reviews of e-cigarettes for smoking cessation, another socially controversial intervention. But controversy alone shouldn’t be a legitimate ground to discount data. As the CoEHAR study speculates, this pattern likely repeats wherever treatments clash with dominant moral or political narratives: safe consumption sites for drug users, managed alcohol programmes, and other harm reduction interventions. “Wider investigations of reverse spin are necessary because reverse spin could occur in any context where review authors favour null or negative conclusions.”
Science should be driven by findings, not business or politics, and O’Leary’s team hopes that naming the phenomenon will help. Aware of the issue, editors and peer reviewers may now spot the disconnect between findings and recommendations.
At leafie, we review and report on studies, papers and reviews covering cannabis and psychedelics daily, and almost all come with that tired, cautious caveat that more research is needed. It’s time the conversation shifted from this base covering attitude to standing behind the research. Cannabis is one of the most studied substances in human history, covered in over 53,000 peer-reviewed papers. If researchers keep finding benefits but fail to recommend the treatment, the problem stops being about the findings; it becomes about politics and personal preconceptions.
The evidence that cannabis works exists. Patients know this, and the data proves it. With the reverse spin bias problem in the spotlight, the question is now whether researchers will stand behind their findings rather than trying to hide them.
