Shedding light on the smoke: unravelling the impact of smoking cessation on psoriasis development

The relationship between smoking and psoriasis has been under investigation since the 1980s, with higher rates of people who smoke reported among patients with ­psoriasis compared with controls.1 This is particularly evident in patients with palmoplantar pustulosis (PPP), where even small studies found improvements in PPP after smoking cessation.2 Since then, the association between smoking and psoriasis has been confirmed in both large cross-sectional and prospective studies showing increased risk of prevalent and incident psoriasis.3 Furthermore, Groot et al. found that prenatal tobacco exposure was associated with psoriasis in childhood.4 A meta-analysis suggested that smoking lowers treatment efficacy in patients with psoriasis.5 Adapting the principles of Sir Austin Bradford Hill,6 it seems plausible that smoking could be a causal risk factor for developing psoriasis. Notably, smoking is associated with a broad range of unfavourable lifestyle factors and comorbidities. Despite adjusting for potential confounders, residual and/or unmeasured confounding might still obscure the true nature of this association. In recent years, Mendelian randomization studies have used genetic variants as a surrogate for smoking, in order to exclude bias from confounders and reverse causation. Interestingly, these studies have shown conflicting results, probably owing to different choice of genetic variants as the instrument.7,8 Thus, the true nature of the relationship between smoking and psoriasis remains to be fully elucidated.