Does a history of depression actually mediate smoking-related pain? findings from a cross-sectional general population-based study

Background
Smokers report more pain and worse functioning. The evidence from pain clinics suggests that depression affects this relationship: The association between smoking and chronic pain is weakened when controlling for depression. This study explored the relationship between smoking, pain and depression in a large general population-based cohort (Generation Scotland: Scottish Family Health Study).

Methods
Chronic pain measures (intensity, disability), self-reported smoking status and a history of major depressive disorder (MDD) were analysed. A multivariate analysis of covariance determined whether smoking status was associated with both pain measures and a history of depressive illness. Using a statistical mediation model any mediating effect of depression on the relationship between smoking and chronic pain was sought.

Results
Of all 24,024 participants, 30% (n=7162) reported any chronic pain. Within this chronic pain group, 16% (n=1158) had a history of MDD; 7108 had valid smoking data: 20% (n=1408) were current smokers, 33% (n=2351) former and 47% (n=3349) never smokers. Current smokers demonstrated higher pain intensity and pain-related disability scores compared with former and non-smokers (p<?0.001 for all analyses). From the mediation model, the effect on pain intensity decreased (p?<?0.001), indicating that the relationship between smoking and a history of depression contributes significantly to the effect of smoking on pain intensity. When applied to smoking-related pain disability,
there was no mediation effect.

Conclusions
In contrast to smokers treated in pain clinics, a history of MDD mediated the relationship between smoking and pain intensity, but not pain-related disability in smokers in the community.