January 7, 2020

expert reaction to study on smoking/vaping and stroke risk

A study, published in the American Journal of Preventive Medicine, reports that use of e-cigarettes and traditional cigarettes increases risk of stroke. 

Prof John Britton, Director of the UK Centre for Tobacco & Alcohol Studies and Consultant in Respiratory Medicine, University of Nottingham, said:

“This cross-sectional survey shows an association between smoking and stroke. This is entirely expected. It also shows no increase in risk of stroke among people who vape, but have never smoked, thus disproving the authors initial hypothesis that vaping would increase stroke risk.

“The findings of higher stroke risk among vapers who are still smoke or are former smokers are likely to be biased by reverse causation, whereby people who feel unwell or have already had a stroke try to quit smoking.

“These findings arise from subgroup analysis involving multiple hypothesis tests and are therefore unreliable. Contrary to the authors’ claim, this study provides no evidence that vaping increases the risk of stroke accrued from smoking tobacco.”

Prof Peter Hajek, Director of the Tobacco Dependence Research Unit, Queen Mary University of London, said:

“In this study of people of up to 44 years of age, those who both smoked and used e-cigarettes were more likely to have a history of stroke than those who only smoked. No details are provided on the timing of the stroke but it seems likely that smokers who suffered a stroke were more likely to try to switch to vaping than those who did not.

“While the paper itself is careful in interpreting the finding, the press release is grossly misleading. The study provides no justification for the claim that vaping increases the risk of stroke.”

Dr Lion Shahab, Senior Lecturer in Epidemiology & Public Health, University College London (UCL), said:

“This paper describes a cross-sectional analysis of the association of risk of stroke with the use of combustible and non-combustible nicotine delivery devices.

“As would be expected, results suggest that use of combustible cigarettes increases the likelihood of stroke compared with not using anything. The paper also finds that use of e-cigarettes among people who used to smoke does not reduce the risk of stroke compared with current cigarette smoking. The key finding that the paper highlights is that current dual users of cigarettes and e-cigarettes have a higher risk of stroke than those who only smoke cigarettes.

“It is argued that this increase in risk may be due to the effects of ingredients of e-cigarettes apart from nicotine. However, this interpretation seems to be contradicted by the fact that in this study e-cigarette users who never smoked had no higher risk of stroke than non-smokers and a reduced risk compared with cigarette smokers, consistent with no detrimental health effects of e-cigarettes.

As for all cross-sectional analyses, both temporal sequence and confounding represents a major problem for the interpretation of results. The key problem for this analysis is that it is unclear if whether dual use of e-cigarettes or switching to e-cigarettes from cigarettes was a result of stroke or preceded it. It is entirely possible that the group of current or former smokers took up e-cigarettes precisely because they had a health scare, which would result in the observed association.

“An additional problem is that the analysis comparing dual users with cigarette users does not include a key predictor of stroke risk: dependence and length of smoking (commonly measured in pack years). It is entirely possible that dual users chose to use e-cigarettes in addition to cigarettes because they are more dependent, as has been found in other studies, which would suggest that over their lifetime this group of people may have been exposed to more harmful substances from cigarette smoking, increasing their risk of stroke. While this paper highlights the need to continue studying the potential health effects of e-cigarette use, the results should be interpreted with caution as the observed associations may be simply due to unmeasured confounding and reverse causality.”

‘Risk of Stroke With E-Cigarette and Combustible Cigarette Use in Young Adults’ by Tarang Parekh et al. was published in the American Journal of Preventive Medicine at 5am UK TIME on Tuesday 7 January 2020. 

DOI: 10.1016/j.amepre.2019.10.008

Declared interests

Prof Hajek: “No conflict”

Dr Shahab: “No conflict to declare”

None others declared