However, it can be difficult to separate smoking from other factors, such as alcohol consumption and stressful life events, that may have a confounding effect.
A recent twin study further supports the idea that both environmental and genetic factors are involved. Questionnaire-based data on smoking habits and psoriasis were collected for 34,781 twins, aged 20–71 years, from the Danish Twin Registry. A co-twin control analysis was performed on 1,700 twin pairs discordant for lifetime history of smoking.
Tobacco consumption and childhood exposure to tobacco smoke (ETS) were found to be significantly associated with psoriasis. Smoking habits were associated with psoriasis in a dose-dependent manner.
Tobacco consumption increased the risk for psoriasis by more than twice in smokers with a history of more than five pack-years, even after adjusting for age, sex and childhood ETS.
Among twin pairs discordant for a lifetime history of smoking, a significantly increased risk for psoriasis in the smoking twin compared with the never-smoking co-twin was observed. The risk in dizygotic twins was significant and higher than in monozygotic twins, which suggests genetic confounding between smoking and psoriasis.
Lønnberg A, Skov L, Skytthe A et al. Int J Derm 2016; 55: e72–8