Two more published studies claiming to link vaping to cancer have come under fire for what experts identify as major methodological flaws—resulting in one of them now being retracted. Another, new example of such claims has meanwhile garnered global attention—and more scathing criticism. Many researchers’ determination to prove a vaping-cancer link remains undimmed, but they often seem willing to cut corners and overclaim in their attempts to do so.
The newly retracted study was published by the Journal of Cancer Policy in 2025, titled “Evidence on vaping e-cigarettes as a risk factor for cancer: A systematic review.” It had concluded that “e-cigarette use may be associated with an increased risk of certain cancers, including cervical and breast cancer,” though it admitted the evidence was “inconclusive.”
“Following publication, substantial concerns were raised regarding the methodological integrity, accuracy, and scientific validity of the review,” states the journal’s retraction notice, published on March 19. “A detailed evaluation of the article, including issues documented in a subsequent correspondence identified multiple serious flaws that materially affect the reliability of the findings and conclusions.”
The editors’ retraction notice went on to list a damning range of issues, including: “Undisclosed deviations from the registered and published protocol,” methodological inconsistencies, “Misclassification of study designs,” inclusion of one past study that had already been retracted, “Internal contradictions and numerical discrepancies,” and “Conclusions that are not supported by the heterogenous and methodologically limited evidence presented.”
“It’s hard not to wonder, did the authors not find the results they expected, and then modified their search criteria until they found what they were looking for?”
Dr. Arielle Selya, a behavioral scientist with a background in adolescent substance use and addiction, said the reasons given for the retraction were those that other researchers had already pointed to in a criticism of the study published in December.
A systematic review, she explained, requires the authors to formally pre-register their protocol: search parameters and criteria for including or excluding primary articles. With any deviations from the pre-registered plan, the authors are obliged to explain and justify them.
The authors’ original plan was to review only human studies with clinical health outcomes. “But the study ended up massively broadening the search criteria to include biomarker studies, animal studies, conference abstracts which were not peer-reviewed, and data from people who smoked,” Selya told Filter.
“As a result,” she continued, the published study reviewed “evidence with unclear relevance to human clinical outcomes, and results that may be confounded by cigarette smoking. Worse, the severe deviations from the initial protocol introduced bias into the process.”
Since the authors failed to disclose or explain their deviations, it’s not known why these changes were made, but Selya has her suspicions: “It’s hard not to wonder, did the authors not find the results they expected in the initial search, and then modified their search criteria until they found what they were looking for?”
“There are massive discrepancies in numbers, for example reporting a total of 1,555 smokers in one place and 98,000 in another—and a retracted study was not only included in the review, but rated as high-quality,” she noted.
“Retractions remain rare—even though the flaws that were the basis for retraction also exist in hundreds of other studies.”
Tobacco harm reduction experts have expressed their dismay that such a study was allowed to influence the public debate. “This is a stark reminder that misinformation can travel far faster than scientific corrections,” stated Dr. Delon Human, who leads the Health Diplomats project Smoke Free Sweden, of the retraction.
The time it takes for a flawed study to be retracted—when all the while, its “findings” may be shouted from the rooftops—is one problem. Another is that retractions simply don’t happen as often as they should.
“Retractions remain rare—even though the flaws that were the basis for retraction also exist in hundreds of other studies,” Selya said.
One case in point, experts say, is a study published in January, in the journal Frontiers in Oncology.
This case-control study, titled “Vaping, smoking and risk of early onset lung cancer,” looked at vaping and smoking habits among 256 adults under 50 years old who were diagnosed with lung cancer. For comparison it also used data from 2,921 control subjects without cancer, matched by age, sex, race and location.
“The odds ratio for those who both vaped and smoked was 2.8 times higher than for smoking alone,” the study concluded.
Dr. Michael Siegel, a professor of public health and community medicine at the Tufts University School of Medicine, took to his blog to criticize this one.
“I do not believe the study conclusions are supported by the findings,” Siegel told Filter.
“It is the symptoms that likely led them to start vaping, not vaping which led them to develop cancer. This is called ‘reverse causation’ and it is a fatal flaw.”
Noting that “one should probably not draw a causal conclusion based on a single case-study,” Siegel pointed out that, “the study did not attempt to go back in time to determine a person’s smoking and vaping history—it essentially recorded the person’s vaping and smoking at the time of presentation.”
Not being able to verify how long a person had been vaping prior to the development of their cancer would seem a pretty important omission for a study purporting to link the two.
“It would take a substantial amount of time for this to occur,” Siegel said, “and the more likely explanation for the findings is that smokers who develop lung cancer or significant symptoms that precede lung cancer are more likely to try and quit using e-cigarettes.”
“Thus, dual use of e-cigarettes and tobacco cigarettes is probably a reflection of their symptoms rather than the reverse,” he concluded. “In other words, it is the symptoms that likely led them to start vaping, not vaping which led them to develop cancer. This is called ‘reverse causation’ and it is a fatal flaw of the study.”
Selya agreed that this is an example of a “reverse-directionality problem” that has been discussed in other studies.
“The association is partly (or mostly) due to people who developed cancer before they ever started vaping,” she commented. “But the study doesn’t separate those cases and the association is ‘blamed’ on vaping.”
Like the retracted Journal of Cancer Policy study, the Frontiers in Oncology study also referenced a past paper that had been retracted.
For researchers like Selya and Siegel, trying to critique all of the vast, continually emerging volume of potentially flawed studies about vaping is an impossible game of whack-a-mole.
“We can hardly say that e-cigarettes are somehow safer than conventional cigarettes,” claimed the lead author.
Just at the end of March, yet another review, published in the Carcinogenesis journal, concluded that, “Nicotine-based e-cigarettes are likely to be carcinogenic to humans who use them causing an indeterminate burden of oral cancer and lung cancer.”
It immediately generated global media coverage, from the Australian Broadcasting Corporation to Britain’s Independent and the Los Angeles Times in the United States. And the authors have leant into this.
“This particular appraisal establishes that cancer is on the horizon for those who vape,” lead author Dr. Bernard Stewart said in an accompanying YouTube video. “We can hardly say that e-cigarettes are somehow safer than conventional cigarettes … the regulators now have additional evidence to discourage the availability of e-cigarettes.”
Stewart called his review a “definitive determination”—despite its consisting of “monitoring of biomarkers of exposure and biomarkers of harm implicating tumorigenesis,” rather than substantive epidemiological evidence.
Other scientists rushed to critique that characterization.
“The authors specify early on that they are not comparing vapers and smokers,” stated Dr. Peter Hajek, a professor of clinical psychology at Queen Mary University of London. “This allows them to present a detection of any level of a suspect chemical, however negligible, as ‘carcinogenic.’ Modern sensitive methods can detect tiny traces of chemicals with no relevance for health, but it is the dose that makes the poison. Higher doses have been reported in studies which fry e-liquid at very high temperatures, but this has nothing to do with how vapers use it.”
“This narrative review is problematic for several reasons and makes extraordinary claims that are not borne out by the data,” said Dr. Lion Shahab, professor of health psychology at University College London. He detailed various problems with the paper, a “tendency to overclaim certainty,” and the authors’ reference to the possibility of nicotine causing cancer when no reputable evidence suggests it does.
But these critical expert voices will inevitably be drowned out by the wave of eye-catching headlines taking the review at face value.
“This reflects a general bias against tobacco harm reduction. Researchers, like anyone else, are influenced by mainstream opinions and assumptions.”
What makes some researchers so quick to take shortcuts that generate shoddy science, or to overclaim based on very limited findings?
“This reflects a general bias against tobacco harm reduction,” Siegel said. “Researchers, like anyone else, are influenced by the mainstream opinions and assumptions of the times. I see it as an issue of unconscious bias.”
Similarly, Selya said that while some researchers may be intentionally deceptive, “others may be following funding incentives and absorbing the predominant narrative without thinking too closely or even being aware of the problems. And unfortunately, this perpetuates itself in the literature when the flaws rarely get corrected in the scientific record.”
The “ethical issue,” Siegel added, comes when problematic research is used to flood the media with false or exaggerated claims.
People who smoke are constantly exposed to this noise. The tragedy is, it’s likely to dissuade many from replacing deadly cigarettes with something that may not be totally without harm, but is without question much safer.
Photograph (cropped) by Aakash Dhage on Unsplash
Dr. Selya is an employee of Pinney Associates, Inc., which consults to Juul Labs and Philip Morris International (PMI) on non-combustible nicotine products. She also individually provides consulting services on behavioral science to the Center of Excellence for the Acceleration of Harm Reduction (CoEHAR) through ECLAT Srl, which received funding from the Foundation for a Smoke-Free World (now Global Action to End Smoking). Her commentary in this article is her own and these funders had no involvement. The Influence Foundation, the nonprofit which operates Filter, has received unrestricted grants from Juul Labs, Inc and PMI, as well as grants from Global Action to End Smoking. Joe Gitchell, the CEO of Pinney Associates, has made personal donations to The Influence Foundation. Filter’s Editorial Independence Policy applies.
