Introduction:
The tobacco control community has raised some concerns about whether studies on electronic cigarettes (e-cigarettes) published in scientific journals hide any conflicts of interest (COIs) and whether such reports are biased. This study assessed potential COIs in the e-cigarette scientific literature.

Methods:
We analysed all e-cigarette publications indexed in PubMed up to August 2014. We extracted information about the authors (affiliations, location, etc.), publication characteristics (type, topic, subject, etc.), results and conclusions, the presence of a COI statement (yes/no), and funding by and/or financial ties to pharmaceutical, tobacco, and/or e-cigarette companies. An algorithm to determine the COI disclosure status was created based on the information in the publication. Prevalence ratios (PRs) were calculated to identify associations with COI disclosure, controlling for several independent variables.

Results:
Of the 404 publications included in the analysis, 37.1% (n=150) had no COI disclosure statement, 13.4% had declared potential COIs with pharmaceutical companies, 3.0% with tobacco companies, and 10.6% with e-cigarette companies. The conclusions in publications with COIs, which were mainly ties to pharmaceutical companies, were more likely to be favourable to e-cigarettes (PR=2.23; 95%CI:1.43–3.46). Publications that supported the use of e-cigarettes for harm reduction (PR=1.81; 95%CI:1.14–2.89) or as a tool for smoking cessation (PR=2.02; 95%CI:1.26–3.23) were more likely to have conclusions that were favourable to e-cigarettes.

Conclusions:
One-third of the publications reporting studies on e-cigarettes did not have a COI disclosure statement, and this proportion was even higher in news articles, editorials, and other types of publications. Papers with conclusions that were favourable to e-cigarettes were more likely to have COIs. Journal editors and reviewers should consider evaluating publications, including the funding sources, to determine whether the results and conclusions may be biased and to determine whether there are any relationships with PTEC_COs.

Funding:
The group and this study is partially funded by the Spanish Society of Epidemiology.