OBJECTIVES: To assess the impact of constipation on opioid use patterns, resource utilization, and costs in patients on opioid therapy. METHODS: A retrospective study was conducted in patients who initiated opioid therapy between 1/1/99 and 12/31/05. Patients were identified from longitudinal insurance claims from US health plans. An index date was defined as the date of the 1st pharmacy claim for an opioid. Patients had ³30 days of opioid use and continuous plan coverage for ³6 months before and ³12 months after their index date. Outcomes were assessed over 12 months following the index date. Constipation was defined as evidence of ³1 ICD-9 code of 564.0 in the follow-up period. Opioid use patterns were compared between patients with constipation and 1:1 matched controls without constipation. Two- part semi-logarithmic multivariate regression models were estimated to assess the impact of constipation on all-cause resource utilization and costs. Smearing estimates were applied to interpret results of the semi-logarithmic models. RESULTS: We identified 39,485 patients of whom 2,519 (6.4%) had constipation. Most patients with constipation were female (66%) and ³45 years old (68%). Compared to controls, the constipation group had higher rates of concurrent use of ³2 opioids (36% vs 24%;p<0.001), discontinuation (31% vs 25%;p<0.001), and switching (45% vs 28%;p<0.001) between opioids. Patients with constipation were more likely to have inpatient admission (odds ratio [OR]=2.13;p<0.001), emergency (OR=2.25;p<0.001), outpatient visits (OR=9.45;p<0.001), skilled nursing (OR=2.26;p<0.001), hospice (OR=2.27; p<0.001) and home health services (OR=1.54;p<0.001). Patients with constipation had significantly higher all-cause costs for emergency ($1,277 vs $588), outpatient ($3,635 vs $1,861), nursing facility ($1,885 vs $613), home health ($1,909 vs $1,069) and prescription drug ($12,005 vs $6,809) services compared to patients without constipation. CONCLUSION: Presence of constipation in opioid-treated patients was found to have significant impact on opioid use patterns, health care utilization, and associated costs.