OBJECTIVES: To assess duloxetine utilization among commercially-insured fibromyalgia patients. METHODS: This study analyzed administrative claims for fibromyalgia patients aged 18-64 who initiated duloxetine in 2006. Initiation was defined as no duloxetine coverage in the prior 90 days, with the first duloxetine prescription dispense date as the “index date.” Patients were excluded if they had less than 30 duloxetine supply days in the 12-month post-index period, or diagnosis of diabetic peripheral neuropathic pain or depression in the 12 months pre-index period. All duloxetine patients were classified in five cohorts based on index dosage: <30mg, 30mg, 31-59mg, 60mg, and >60mg. Changes in dosage, average daily dosage (ADD), and adherence to duloxetine (medication possession ratio≥0.8 as high adherence) were compared across cohorts. Multivariate regression models were performed to examine the association between index dosage and healthcare costs, controlling for demographics and clinical characteristics.RESULTS: Of 4,869 fibromyalgia patients identified, 4.4% had an index dosage of <30mg, 22.4% of 30mg, 5.9% of 31-59mg, 60.4% of 60mg, and 7.0% of >60mg. 28% of total patients experienced any increase in dosage, while 15.9% experienced any decrease. Among those with any dosage change (n=1,651), patients with an index dosage of 31-59mg had the shortest duration before any dosage change (89 days), followed by those in the <30mg, >60mg, 30mg, and 60mg (95, 100, 104, and 139 days, respectively) cohorts. ADD increased with index dosage. Patients with <60mg index dosage were less likely to be adherent than those in the 60mg cohort (odds ratios ranged 0.61 to 0.78, all p<0.05). Patients in the >60mg cohort had higher total healthcare costs compared with those in the 60mg cohort (adjusted difference: $3,747, p<0.05). CONCLUSIONS: About one-third of duloxetine treated fibromyalgia patients experienced any dosage change. Duloxetine adherence and ADD, and health care costs differ by duloxetine index dosage.