S Calabria,1 E Cinconze,2 M Rossini,3 E Rossi,2 AP Maggioni,1,4 A Pedrini,1 M De Rosa2 1CORE, Collaborative Outcome Research, Bologna, Italy; 2Health Care Systems Department, CINECA, Interuniversity Consortium, Bologna, Italy; 3Rheumatology Unit, University of Verona, Verona, Italy; 4ANMCO Research Center, Firenze, ItalyPurpose: Osteoporosis is a chronic disease and an important health and social burden due to its worldwide prevalence. Literature and clinical experience report incomplete adherence to the therapy. This retrospective observational study aimed at assessing the adherence to first-line antiosteoporosis drugs (AODs; reimbursed by the National Health System, according to the Italian Medicine Agency recommendation number 79), alendronate or risedronate, with or without calcium and/or vitamin D supplements, in a real, Italian clinical setting.Patients and methods: Analyses were carried out on data present in the ARNO Observatory, a population-based patient-centric Italian database. From a population of 5,808,832 inhabitants with available data, a cohort of 3.3 million of patients aged ≥40 years was selected. New users of first-line AODs as monotherapy (accrual period, 2007–2009) were followed up over 3 years to assess adherence at 6, 12, and 36 months to AODs and to supplements and related determinants.Results: Approximately 40,000 new users were identified: mostly women, aged on average (standard deviation) 71±10 years. Alendronate was the most prescribed (38.2% of patients), followed by risedronate (34.9%) and alendronate with colecalciferol as a fixed-dose combination (25.8%). Adherence at the 6-month follow-up was 54%, and this constantly and significantly decreased after 1 year to 46%, and after 3 years to 33% (P50 years were more likely to adhere to treatment regimen (P5 drugs), cardiovascular, and neurological therapies were significantly associated with low adherence throughout the follow-up period.Conclusion: In a huge clinical practice sample, this study highlights suboptimal adherence to first-line AODs and to supplements and important determinants, such as concomitant therapies.Keywords: osteoporosis, adherence, determinants, bisphosphonates, colecalciferol