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World Psychiatry 22:1 February 2023 the potential of reverse causality). Despite the growing causal evidence, the neurobiological pathways through which smoking adversely affects mental health have yet to be ascertained. One plausible mechanism is related to neuroadaptations in nicotinic pathways in the brain which are associated with psychological withdrawal symptoms, such as depressed mood, agitation and anxiety. Withdrawal symptoms are alleviated by smoking but return when blood levels of nicotine decline at around 20 min after smoking, resulting in repeated changes in a smoker’s psychological state throughout the day, and perhaps also supporting the “self-medication hypothesis” around smoking and mental health. The fluctuations in mood state experienced by smokers could worsen mental health over time, and the associated biological effects of withdrawal-induced psychological symptoms could increase the risk of developing mental illness. Another potential biological pathway relates to inflammation and oxidative stress, which are both implicated in a range of mental health conditions. A large cohort study in 2021 confirmed that current smoking was associated with increased oxidative stress biomarkers, in a dose-response fashion. Alongside this, the observation that those who had quit smoking for >10 years had similar oxidative stress biomarker levels as never smokers indicates that the biological effects relevant to mental health are reversible, which is also consistent with the aforementioned evidence from RCTs showing that cessation improves mental health status in smokers. Continued research into the mechanistic pathways involved in the effects of smoking on mental health will serve to both confirm the nature of indicated causal relations, and increase our understanding of how cessation or other strategies can improve neurological and psychological outcomes in smokers (with or without diagnosed mental illness). Relatedly, the recent adoption of e-cigarettes across society calls for more research on how their use impacts mental health. While studies in psychiatric settings have suggested that ecigarettes may be a beneficial tool for helping people with mental illness to reduce tobacco use, and thus the adverse physical and mental health effects of smoking, other research in the general population has indicated that nicotine consumption in e-cigarette form may still impact adversely on psychological well-being. Further research is needed to establish a clear evidence base and consensus around the use of e-cigarettes with regards to mental health, in the general population as well as in psychiatric settings. Meanwhile, as the literature around the magnitude and mechanisms of the psychiatric effects of nicotine and tobacco smoking continues to evolve, promoting smoking cessation in populations with or at-risk for mental illness should be considered as an urgent priority anyway. In recent decades, public health initiatives in many Western societies have successfully reduced tobacco smoking across the general population. However, these initiatives have failed to reach some of most vulnerable members of society, resulting in disparities in tobacco smoking among mental health populations becoming even more apparent than ever. People with mental illness now smoke >40% of all cigarettes sold, and account for around half of all smoking-related deaths across the population, making this single health behavior a key driver of the premature mortality observed in people with severe mental illness. In summary, there is an increasingly strong triangulation of evidence from various study designs and populations that smoking adversely impacts on mental health, in terms of both enhancing the risk of mental illness, and increasing psychiatric symptoms in those with and without diagnosed conditions. While the research priorities lie with elucidating the causal mechanisms for the effects, the clinical priorities pertain more immediately to establishing and disseminating effective smoking cessation interventions within mental health care, in order to protect both the physical and mental health of smokers treated for mental illness.