Long-acting medications are widely used to provide convenient ways of managing diseases, but they may cause serious harm to patients when prescribed erroneously. We present a case of hypocalcaemia as a result of therapeutic duplication of 2 long-acting bisphosphonates prescribed within days of each other by different physicians. We describe how we prevented similar medication errors through improvements in medical informatics systems. This case emphasizes the need for enhancements in medical informatics systems to avoid therapeutic duplication of long-acting medications in the interest of patient safety.