BACKGROUND-AIM:
The number of studies on the health risk of e-cigarette use is steadily increasing. This paper aims to briefly evaluate the current scientific knowledge on the harmfulness of e-cigarettes.

METHODS:
A brief narrative summary of comprehensive systematic reviews, meta and pooled data analyses, and results of other clinical and epidemiological studies was made. The research data published since 2010 in peer-reviewed, highly-impacted scientific journals and in research reports of well-known research and health institutions were searched on major scientific databases, including PubMed, EMBASE, Web of Science, Cochrane Library, Scopus.

RESULTS:
Key scientific findings on the health risk of e-cigarette use include research data on: 1/ the increased risk of nicotine intoxication and dependence (especially, in users of disposable products), 2/ negative impact on brain function and psychoneurological development of fetus, newborn and child, 3/ the risk of cigarette smoking and drug use (“gate effect”), 4/ the high content of carcinogens (formaldehyde, acetylaldehyde, heavy metals) when e-cigarette battery is overheated, 5/ the risk of mutagenic changes when metabolism of benzo(a)pyrene to genotoxic products is enhanced, 6/ the presence of bladder cancer biomarkers in urine samples of e-cigarette users, 7/ the risk of increased heart rate and blood pressure, viscosity, clots and vessels, 8/ the risk of lung injury, including EVALI when THC is added to e-liquid, 9/ the risk of broncholitis (impact of diacetyl) and other pulmonary diseases (impact of carbonyls), 10/ the risk of serious accidents, injury and burns when e-cigarette battery is overheated and may explode, 12/ no significant difference in the risk of cardiovascular diseases and metabolic dysfunction between e-cigarette users and cigarette smokers, 13/ substantial increase (20% to 40%) in the risk of almost all health outcomes in dual users of e-cigarettes and conventional cigarettes when compared with the risk observed in cigarette smokers.

CONCLUSIONS:
E-cigarettes may contain substances with no safe dose of strong psychoactive, carcinogenic, cardiovascular and pulmonary toxic properties. Prolonged, uninterrupted e-cigarette vaping, overheating the e-cigarette battery, vaping of e-cigarettes manufactured of low-quality components or bought in unknown sources (especially, disposable products) may enhance the content of these substances to the amount observed in conventional cigarettes and significantly contribute to both acute and chronic health outcomes. The risk of negative health outcomes tends to be particularly high among poly-tobacco and poly-substance abuse users. There is an urgent need to continue studies on the health risk of e-cigarette vaping, to educate e-cigarette users in this domain and to regulate these products accordingly with their health risk level, potentially as strictly as cigarettes and other tobacco products.