About That ‘Cannabis Causes Head and Neck Cancer’ Study

The news that a just-released study has revealed a connection between daily cannabis use and head and neck cancers was so irresistible that pretty much every media outlet whether cannabis-related or not was literally compelled to run with it, just as CNN did, and breathlessly herald the fatal results in a head-turning headline: Daily marijuana use linked to increased risk of deadly head and neck cancers, study finds.

But the use of the word “linked” was an immediate red flag for me. Having written about many studies on a myriad of subjects over the years, I’ve seen very few that actually find such absolute links, much less announce them, and indeed, the authors of this cohort study do not claim to have discovered a link, but an “association between cannabis-related disorder and the development of HNC in adult patients.” (italics added). The use of “association” in this way usually refers to a general relationship rather than a linear relationship, or correlation, between the variables being studied.

As well, the authors further allow in the conclusion, “The limitations of the database, further research should examine the mechanism of this association and analyze dose response with strong controls to further support evidence of cannabis use as a risk factor for HNCs.” One such limitation of the database is that “Cohort 1 (cannabis-related disorder group) included individuals who received a diagnosis of cannabis-related disorder…and had a reported outpatient hospital clinic visit…with no prior history of HNC. Cannabis-related disorders are defined by the excessive use of cannabis with associated psychosocial symptoms, such as impaired social and/or occupational functioning.”

Notwithstanding the authors’ likely opinion that more research is needed, or even just the general usefulness of knowing if such a risk actually exists, the question remains whether this study has in fact proven the necessity for further research into a range of cancers that could be inflamed by any number of individual and environmental factors. But even if it is not proven, does this study provide enough of an association between cannabis use and head and neck cancer to warrant the claim that more study is needed? In other words, how solid is this study?

I was of course not alone is seeing immediate red flags. According to Ben Caplan, MD, a Massachusetts physician who runs the medical cannabis CED clinic, the study is, to put it nicely, imperfect. In a just-published Substack post titled Untangling the Headlines: A Critical Look at the Study Linking Cannabis to Cancer, the good doctor adds the clarifying subhead, Why One Study Doesn’t Make Cannabis the New Tobacco.

Without stealing Dr. Caplan’s considerable thunder on the subject, I will include here a smattering of his intricate critique of a study that makes “bold, bottom-line claims.” These claims include:

Cannabis Use Linked to Higher Cancer Risk Cannabis as a Possible Cause of Cancer Increased Cancer Risk for Cannabis Users Cannabis Compared to Other Carcinogens Potential Public Health Concerns

Dr. Caplan both anchors and softens his criticism by taking the time to explain why it is necessary to “take a magnifying glass to the findings of any study, and certainly one that suggests a significant link between cannabis use and an increased risk of any relatively common disorder,” and notes respectfully, “While this study ‘adds to the conversation,’ the numerous limitations it contains should make us cautious about how much we trust its conclusions.”

Those limitations, each of which he again takes the time to explain individually, include the issue of reverse causality – “the possibility that people who are developing or already have undiagnosed cancers might be using cannabis to manage symptoms like pain or discomfort”; a missing mechanism of action – “a plausible biological explanation for how cannabis could cause these cancers”; flaws in the study’s design and interpretation –including detection bias and ignoring pre-existing conditions; coding as a source of participant identifiers – “the researchers rely on ICD-10 codes as a way to pick participants, and this presents a huge limitation.”; and putting relative risk in perspective compared with absolute risk – “This is the actual chance of developing a disease.”

The declared purpose of Dr. Caplan’s effort here is to untangle the headlines created by this study by putting the research into proper perspective. To that end, he concludes, “In context, while the study reports an increased relative risk, the absolute risk of developing these cancers remains low for most people, especially those who use cannabis infrequently or for medical reasons.”

Dr. Caplan’s Substack Critique of the Cannabis Use and Neck and Head Cancer study can be found here.