OBJECTIVES: Medication adherence is a problem in the treatment of osteoporosis. The aim of this study was to explore osteoporosis medication adherence in a Swedish population. METHODS: The study was based on the Swedish Prescription Register and included all patients (>50 years) who filled prescriptions for one or several osteoporosis treatments (alendronate, risedronate, etidronate, strontium, and/or raloxifene) between 2005 and 2008. A washout-period of 5 months was used to include treatment-naïve patients. Inpatient care data and death dates were extracted from the National Patient Register and the Causes-of-Death Register. Patients with secondary osteoporosis were excluded via ICD-10 codes. Medication Possession Ratio (MPR) was used to measure compliance in persistent patients (switching allowed). Treatment persistence and its determinants were investigated using failure time analysis with a permissible treatment gap of 2 months. RESULTS: The final study population included 53,364 patients receiving treatment for osteoporosis, with a mean age of 71 years. Average follow-up of was 314 days (max 1126). A total of 91%, 74%, 62%, 46%, 29%, and 16% remained on treatment (switching allowed) after 1 month, 3 months, 6 months, 1 year, 2 years, and 3 years, respectively. Average time on treatment was estimated at 461 days. Average MPR in persistent patients was 94.19% (CI95% 94.12-94.26). Weekly regimens were associated with better persistence than daily regimens (HR=0.554, p<0.001). Patients with prevalent co-morbidities were more likely to terminate treatment (HR=1.12, p<0.001) and women had better persistence than men (HR=0.866, p<0.001). CONCLUSIONS: Less than half of the patients included in the study remained on treatment after 1 year and only 16% after 3 years. Dosing regimens and prevalent comorbidities were important factors for the estimated discontinuation rates. Osteoporosis medication persistence in Sweden is far from optimal and consequently a serious problem in the medical management of osteoporotic patients.