Aim: Denosumab and bisphosphonates are the main treatments for osteoporosis in residential aged care (RAC). Strict adherence to 6-monthly denosumab dosing is critical as delayed doses can result in bone resorption and vertebral fractures. If denosumab is ceased, then a bisphosphonate is recommended to avoid vertebral fractures. We aimed to examine adherence to denosumab dosing guidelines, practices of ceasing or switching to bisphosphonate treatment, and how these vary in Australian RAC. Method: Design: A retrospective cohort study using routinely collected electronic medication administration data. Setting, participants: 10,674 residents treated for osteoporosis with denosumab or bisphosphates in 413 RAC homes, 2018-2022. Main outcome measures: Non-adherence to denosumab treatment guidelines, defined as either a dosing interval of >210 days or cessation of denosumab without bisphosphonate replacement (residents with ≥12-month follow-up). Secondary outcomes were residents who ceased or switched from a bisphosphonate. Results: 9,281 residents (86.9%) were administered denosumab and 5,986 were administered more than one dose. In total, there were 15,040 intervals between consecutive denosumab doses and 14.8% (n=2,222) were longer than recommended by guidelines, affecting 20.0% (n=1,881) of residents on denosumab. Of residents who ceased denosumab, 98.2% (n=833) were not administered a bisphosphonate. Of residents who ceased a bisphosphonate, 58.9% (n=279) switched to denosumab, with a median of 8 days (IQR: 18) between doses. Conclusion: Significant gaps exist in treatment of osteoporosis with denosumab and bisphosphonates in RAC placing residents at risk of harm. Interventions to address these gaps should be urgently explored, leveraging digital systems and RAC pharmacist services.