The aim of this study was to analyse a possible synergistic effect between back pain and common mental disorders (CMDs) in relation to future disability pension (DP).

All 4 823 069 individuals aged 16–64 years, living in Sweden in December 2004, not pensioned in 2005 and without ongoing sickness absence at the turn of 2004/2005 formed the cohort of this register-based study. Hazard ratios (HRs) and 95% confidence intervals (CIs) for DP (2006–2010) were estimated. Exposure variables were back pain (M54) (sickness absence or inpatient or specialized outpatient care in 2005) and CMD (F40-F48) [sickness absence or inpatient or specialized outpatient care or antidepressants (N06a) in 2005].

HRs for DP were 4.03 (95% CI 3.87–4.21) and 3.86 (95% CI 3.68–4.04) in women and men with back pain. HRs for DP in women and men with CMD were 4.98 (95% CI 4.88–5.08) and 6.05 (95% CI 5.90–6.21). In women and men with both conditions, HRs for DP were 15.62 (95% CI 14.40–16.94) and 19.84 (95% CI 17.94–21.94). In women, synergy index, relative excess risk due to interaction, and attributable proportion were 1.24 (95% CI 1.13–1.36), 0.18 (95% CI 0.11–0.25), and 2.08 (95% CI 1.09–3.06). The corresponding figures for men were 1.45 (95% CI 1.29–1.62), 0.29 (95% CI 0.22–0.36), and 4.21 (95% CI 2.71–5.70).

Co-morbidity of back pain and CMD is associated with a higher risk of DP than either individual condition, when added up, which has possible clinical implications to prevent further disability and exclusion from the labour market.