Diagnosing post-gastrectomy hypoglycemia is challenging, often relying on medical history with documented low plasma glucose levels. Here, we present three cases of patients who presented a high probability of post-gastrectomy hypoglycemia diagnosed and managed successfully using "theragnostic" continuous glucose monitoring and alpha-glucosidase inhibitors. In the first week, patients maintained their current lifestyle without medical intervention; in the second week, voglibose 0.2 mg before meals was prescribed. Continuous glucose monitoring data from the first week confirmed the diagnosis with multiple hypoglycemic events after postprandial peaks, whereas data from the second week showed reduced hypoglycemic events and lower glycemic variability, demonstrating voglibose's therapeutic effect. This report highlights the effective management of post-gastrectomy hypoglycemia using voglibose and theragnostic continuous glucose monitoring, showing its potential benefits and safety for similar cases.