Editorial Perspective: Smoking, vaping and mental health - a perspective on potential causal mechanisms.

This editorial perspective focuses on the complex relationship of cigarette smoking and e-cigarette use ('vaping') with mental health problems. It is challenging to reliably determine the causal nature of these associations because both (e-)smoking and mental health problems generally arise during adolescence, and both are highly multifactorial in their aetiology. While there is now scientific consensus that cigarette smoking is a causal risk factor for mental health problems, there is still a scarcity of causal research and conclusions with respect to e-cigarette use. In order to more reliably determine whether and how (e-)smoking affects mental health, it is important to better understand the potential causal pathways. Here, we discuss the main biological mechanisms that might explain causal effects of smoking and e-cigarettes on mental health, including (neuro-)inflammation, oxidative stress and nicotine binding. We showcase informative studies that have been conducted using sophisticated causally informative study designs and identify in which areas robust causal knowledge is especially lacking. In future work, evidence 'triangulation', where different types of research methods are integrated to look for converging results, seems to be the most promising approach to obtain reliable causal evidence.