Background: Some patients with normal cortisol levels may still exhibit adrenal insufficiency. To circumvent missed diagnoses, the 2022 guidelines from the American Association of Clinical Endocrinology (AACE) recommend the insulin tolerance test (ITT) for definitive diagnosis of adrenal insufficiency. Nevertheless, this test is not routinely conducted in Vietnam due to its time-intensive nature and concerns regarding hypoglycemic side effects. Methods: A cross-sectional study was conducted on 96 patients undergoing long-term treatment with 5 mg of prednisolone or an equivalent dose of glucocorticoids at Nguyen Tri Phuong Hospital’s outpatient clinic from May 2023 to February 2024. Results: All participants underwent the ITT, and the results revealed that the true prevalence of chronic adrenal insufficiency due to prolonged glucocorticoid treatment was 58.3%. During the ITT, cortisol levels in the adrenal insufficiency group were significantly lower than in the non-adrenal insufficiency group at all times. When investigating the adverse effects of ITT, we noted that 45.8% of patients experiencing moderate hypoglycemia symptoms, 0% experiencing severe hypoglycemia symptoms, and 54.2% exhibiting no hypoglycemia symptoms. Conclusion: The ITT is decisive in diagnosing chronic adrenal insufficiency in patients undergoing prolonged glucocorticoid therapy and is completely safe for clinical practice.