A meta-analysis of 107 population-based studies found that dual use of e-cigarettes and cigarettes was associated with higher odds of several diseases than smoking alone.

Published in NEJM Evidence, researchers pooled 124 odds ratios from studies published through October 1, 2023, with more than 75% (84/107) published between 2020–2023. The analysis included both cross-sectional (76%) and longitudinal studies, with sample sizes ranging from 976–924 882 participants. Most studies were based on large US nationally representative surveys (PATH, BRFSS, NHANES, PRAMS, NHIS), with some international data from Korea, Sweden, China, and Kuwait.

Compared with cigarette smoking, current e-cigarette use showed similar odds of cardiovascular disease, stroke, and metabolic dysfunction. Odds were lower for asthma, chronic obstructive pulmonary disease, and oral disease.

Dual users—those using both e-cigarettes and cigarettes—had consistently higher odds of disease compared with exclusive smokers. These included cardiovascular disease (OR, 1.23), stroke (OR, 1.26), metabolic dysfunction (OR, 1.22), asthma (OR, 1.20), COPD (OR, 1.41), and oral disease (OR, 1.27).

Relative to nonusers, both e-cigarette users and dual users showed elevated odds across all outcomes. E-cigarette use was linked to 24%–47% higher odds for several diseases. Dual use was associated with up to a 3.29-fold increase in COPD odds and nearly double the odds for stroke and cardiovascular disease. Cigarette smoking compared with nonuse was also associated with increased odds of disease, ranging from 1.27–2.99.

All included studies were rated as having low risk of bias. Results were consistent regardless of study design, population characteristics, or adjustment for smoking history.

The researchers also accounted for national prevalence of dual use. In the United States, 39.1% of e-cigarette users in 2019 also smoked cigarettes. A Monte Carlo analysis suggested that this level of dual use increased the probability that e-cigarettes carry greater disease risk than smoking alone for several conditions.

The study also summarized 22 additional outcomes with fewer studies, such as preterm birth, sleep apnea, and COVID-19. Many showed greater odds among dual users than smokers alone.

"There is a need to reassess the assumption that e-cigarette use provides substantial harm reduction across all cigarette-caused diseases, particularly accounting for dual use," said Stanton Glantz, PhD, and colleagues at the University of California, San Francisco.

Authors noted that varying e-cigarette devices and lack of consistent data on use duration or intensity limited deeper conclusions. Since e-cigarettes have been on the market for less than 20 years, the full long-term health impacts may not yet be fully manifested. More longitudinal studies are needed.

Disclosures and full methods are available in the original study.