OBJECTIVES: To assess the changes (targeting 35-50% decrease) in collagen type I c-telopeptide (CTX-1), a bone resorption marker, levels over 6 and 12 months according to exposure and adherence level to oral bisphosphonate therapy. METHODS: Clinical data were obtained from a cohort of patients with a recent fracture managed in a Fracture Liaison Service. Serum CTX-1 levels were measured at baseline, 6 and 12 months. Administrative data on pharmacy claims were obtained. Subjects were considered exposed if they had filled at least one prescription of risedronate (Ri) or alendronate (Al) 3 months before CTX-1 measurements. Patients without Ri/Al dispensation during the observation period comprised the unexposed group. The proportion of days covered (PDC) was measured to estimate adherence over the 3-month period before each CTX-1 measurement (PDC>