Joint effects of back pain and mental health conditions on healthcare utilization and costs in Ontario, Canada: a population-based cohort study.

We assessed the joint effects of back pain and mental health conditions on healthcare utilization and costs in a population-based sample of adults in Ontario. We included Ontario adult respondents of the Canadian Community Health Survey between 2003 and 2012, followed up to 2018 by linking survey data to administrative databases. Joint exposures were self-reported back pain and mental health conditions (fair/poor mental health, mood, and anxiety disorder). We built negative binomial, modified Poisson and linear (log-transformed) models to assess joint effects (effects of 2 exposures in combination) of comorbid back pain and mental health condition on healthcare utilization, opioid prescription, and costs. The models were adjusted for sociodemographic, health-related, and behavioural factors. We evaluated positive additive and multiplicative interaction (synergism) between back pain and mental health conditions with relative excess risk due to interaction (RERI) and ratio of rate ratios (RRs). The cohort (n = 147,486) had a mean age of 46 years (SD = 17), and 51% were female. We found positive additive and multiplicative interaction between back pain and fair/poor mental health (RERI = 0.40; ratio of RR = 1.12) and mood disorder (RERI = 0.41; ratio of RR = 1.04) but not anxiety for back pain-specific utilization. For opioid prescription, we found positive additive and multiplicative interaction between back pain and fair/poor mental health (RERI = 2.71; ratio of risk ratio = 3.20) and anxiety (RERI = 1.60; ratio of risk ratio = 1.80) and positive additive interaction with mood disorder (RERI = 0.74). There was no evidence of synergism for all-cause utilization or costs. Combined effects of back pain and mental health conditions on back pain-specific utilization or opioid prescription were greater than expected, with evidence of synergism. Health services targeting back pain and mental health conditions together may provide greater improvements in outcomes.