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A risk assessment published in Carcinogenesis looks at the carcinogenicity of e-cigarettes.
Prof Peter Hajek, Professor of Clinical Psychology and Director of the Health and Lifestyle Research Unit, Queen Mary University of London, said:
“The review’s conclusions are misleading. The authors specify early on that they are not comparing vapers and smokers. 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 it1. What matters is the comparison with smoking. The crucial bit of information that the review omits is that vaping exposes users to only a very small fraction of some of the carcinogens in tobacco smoke2, and to none at all of the rest.
“Vaping provides nicotine at levels similar to smoking, but importantly, despite what the authors of this review imply, nicotine is not a carcinogen. What the paper describes in sections that concern body responses rather than detection of chemicals are primarily various effects of nicotine, and frequently of nicotine overdose. This is mostly from studies on stressed laboratory animals subjected to involuntary, chronic, and extremely high doses. Effects thus elicited have no relevance for voluntary use of nicotine by humans.
“The authors also found several case reports of vapers who got cancer, but drinkers of orange juice and cyclists get cancer too; and in addition, most of these cases were long-term smokers before they switched to vaping.
“Misinforming smokers risks discouraging them from using e-cigarettes, which are one of the most effective methods that exist to help people stop smoking. Switching from smoking to vaping removes the major source of all smoking related diseases, including cancer.”
1 Soulet S and Sussmann R. Critical Appraisal of Exposure Studies of E-Cigarette Aerosol Generated by High-Powered Devices. Contributions to Tobacco & Nicotine Research, Volume 34 (2025): Issue 5 (December 2025) https://doi.org/10.2478/cttr-2025-0019
2 Stephens WE. Comparing the cancer potencies of emissions from vapourised nicotine products including e-cigarettes with those of tobacco smoke Tobacco Control 2018;27:10-17.
Prof Lion Shahab, Professor of Health Psychology and Co-Director of the UCL Tobacco and Alcohol Research Group, University College London, said:
“This narrative review is problematic for several reasons and makes extraordinary claims that are not borne out by the data.
“This review did not follow standard practice. No information is provided about how studies were selected, nor were any inclusion or exclusion criteria specified or a protocol preregistered. This increases the risk of selection bias, including studies that favour one interpretation over another and certainly does not provide a systematic assessment of the literature. In fact, it is quite unclear how data presented in Tables 1 and 2 were arrived at (including subjective ratings of “confirmatory” evidence).
“No objective criteria are established a priori as to what strength of evidence would be sufficient to claim that e-cigarettes use actually cause cancer. Without such agreed criteria, this review has little credibility and simply reflects an interpretation of the authors, not an objective assessment of the state of evidence. As a case in point, Table 2 provides an overview of “key characteristics” of carcinogens but fails to acknowledge that these are neither all necessary nor sufficient to prove carcinogenicity, quite apart from the fact that it is unclear where the evidence to populate this table has come from.
“The authors make no attempt to engage with studies they review critically (or, indeed, provide limitations of their own review). This is evident from the inclusion of several studies that purport to show that e-cigarettes cause cancer, which have been heavily criticised in the past due to methodological problems (including references 8, 66 and 116). The inclusion of specific case reports (references 58 and 59) further underscores the selective interpretation of authors, given that previous reviews of such case studies have cautioned against overinterpretation, highlighting that clinical evidence on this topic is limited and insufficient to support claims that e-cigarettes cause oral cancer1. This tendency to overclaim certainty is compounded by the fact that the authors make reference to nicotine potentially causing cancer, when it is not even a recognised carcinogen by IARC.
“While it is clear that e-cigarettes expose users to harmful chemicals, which may lead to later disease, I would urge against sensationalisation of evidence. No-one would argue that e-cigarettes are entirely risk-free. They should be used as a harm reduction product to help those who smoke to quit and reduce their risk of developing smoking-related diseases. They should not be used by someone who has never smoked. However, this review does not offer a ‘smoking gun’ that e-cigarettes cause oral or lung cancer, nor does it make an attempt at quantifying this risk, which is unsurprising because the evidence is simply not there to allow for such an estimation.”
1 e.g. https://pubmed.ncbi.nlm.nih.gov/37992145/
Dr Baptiste Leurent, Associate Professor in Medical Statistics at UCL, said:
“It is important to note from the start that this not a new study, nor a structured systematic review. It is not formally new research, but could be considered as an opinion piece or a review combining different studies on the topic.
“I believe it might be of interest to public health researchers but could be misleading if presented to the public as providing evidence of an association between vaping and cancer.”
Dr Stephen Burgess, Statistician at the MRC Biostatistics Unit, University of Cambridge, said:
“There may never be a day when evidence for the safety or harm of vaping is fully conclusive. This review argues that the state of evidence about vaping has transitioned from ‘more evidence needed’ to ‘there are clear and demonstrable pathways by which vaping may lead to higher cancer risk’. It does this by looking for ‘smoking guns’: potential cancer causing mechanisms that are elevated in vapers versus non-vapers. As such, the authors demonstrate clear evidence to support the first half of the question: does vaping increase markers which have been proposed as potential or likely drivers of cancer? Several potential pathways are implicated: DNA damage, oxidative stress, epigenetic changes, and bloodstream levels of cotinine, volatile organic compounds, and nickel. However, evidence linking these mechanisms to cancer outcomes in humans is more variable, and evidence linking these mechanisms to quantifiable increases in the risk of specific cancers is absent.
“This paper shows that there are several ‘smoking guns’: candidate mechanisms by which vaping may increase cancer risk. However, conclusive evidence for a causal effect of vaping on cancer risk, and in particular an estimate for the potential harm of vaping in practice, is likely to always be elusive.”
Dr Gavin Stewart, Campbell Collaboration Methods group Co-Chair and statistical editor, and Reader in Interdisciplinary Evidence Synthesis at Newcastle University, said:
“This work does not meet methodological expectations for evidence synthesis and consequently, its conclusions should be treated with circumspection. Both Cochrane and Campbell provide detailed methodological requirements for robust evidence synthesis – none of which is met by this work. The manner is which data was acquired and studies selected is opaque, there was no uniform transparent critical appraisal of evidence, synthesis is limited to description of reported study findings, quantitative data was treated qualitatively with no justification. There is no attempt to consider overall risk of bias, precision, inconsistency, selective reporting or publication biases as would typically be required to develop evidence-based guidance. It is therefore impossible to discern if the conclusions are based on robust evidence or no better than opinion.”
Prof John Britton, Emeritus Professor of Epidemiology, University of Nottingham, said:
“It has been recognised for nearly two decades that e-cigarette vapour contains substances that can cause cancer and oxidative damage to organs such as the lung. It is not surprising that, having summarised evidence published over the past decade, this study reaches the same conclusion. What the study does not do however is to advance knowledge by estimating quantitatively how big a risk to vapers these exposures represent.
“It is evident from multiple studies published to date that the levels of exposure to carcinogens and oxidants sustained by vapers are very low, indicating that the risks of vaping are likely to be very small. Smokers, in particular, can remain reassured that vaping is far less harmful than smoking.”
Prof Peter Shields, Emeritus Professor of Medical Oncology at the Ohio State University, said:
“This paper really does not add anything we do not already know, but selectively cites studies (and omits many others that refute the authors statements throughout) that leads the reader down the wrong path.
“None of their mechanistic studies (selectively cited) has been validated as actual risk predictors in people, and the authors failure to recognize that leads to harmful conclusions.
“Apart from whatever ‘qualitative risk assessment’ means, we did not need a paper to say that it would be ill-advised for never-smokers to begin vaping. Where they missed the boat, and did not cite, are the numerous studies showing substantially reduced carcinogenic exposures when people who smoke switch to vaping (including bronchoscopy studies and those or oral cells). There is a wide consensus that this is good for the individual and public health. This paper inappropriately does not address that, and hopefully the media will not echo the bad conclusions.”
Prof Stephen Duffy, Emeritus Professor of Cancer Screening at Queen Mary University of London, said:
“In the absence of rigorous epidemiological evidence of cancer in humans caused by vaping, this paper reviews research on contents of vaping compounds, biomarker studies in humans, and animal evidence. It is likely that there are potential carcinogenic effects of vaping substances, in common with so many things that people eat, drink or inhale.
“However, the tone of the conclusion suggests that this indirect evidence means that we cannot assume that vaping is safer than combustion smoking – and this seems to be an overinterpretation. Vaping does not involve exposure to the combustion products in smoking which have massive carcinogenic effects. It would require quite a stretch of the imagination to envisage how vaping compounds could match the cancer-causing effects of combustion smoking.”
‘The carcinogenicity of e-cigarettes: a qualitative risk assessment’ by Bernard W. Stewart et al. was published in Carcinogenesis at 3pm UK time on Monday 30 March 2026
Declared interests
Baptiste Leurent: “No conflict of interest”
Stephen Burgess: “I have no relevant conflict of interest to declare.”
Peter Shields: “In the past (>5 years ago) I have served as a consultant and expert witness in tobacco litigation on behalf of plaintiffs.”
John Britton: “None.”
Lion Shahab: “No relevant COI.”
Gavin Stewart: “I have no conflicts of interest to declare.”
Stephen Duffy: “I have no conflict of interest.”
Peter Hajek: “No COI.”
For all other experts, no reply to our request for DOIs was received.