Phentolamine, also known as benzamazoline (regitine), is an adrenergic alpha-blocker that selectively blocks alpha receptors and counteracts the alpha-type effects of adrenaline. In patients with persistent blood pressure above 22.7/14.7 kPa (170/110 mmHg) and suspected pheochromocytoma, a significant drop in blood pressure can help in the diagnosis. Sometimes a significant drop in blood pressure can lead to serious complications such as myocardial infarction and cerebrovascular accident, which should be noted. If the blood pressure drops rapidly within 2 to 3 minutes of phentolamine injection, with a drop of 4.67/3.33 kPa (35/25 mmHg) or more compared to the pre-injection period, and lasts for 3 to 5 minutes, it is a positive reaction. The positive rate of this test is higher in patients with pheochromocytoma. However, hypertensive patients without pheochromocytoma who have taken sedatives and hypotensive drugs (especially reserpine) before the test may cause false positive reactions, so the above drugs should be stopped for 8 to 14 days before the test. Patients with pheochromocytoma may have false negative results due to secondary renal lesions that do not cause a significant drop in blood pressure. The systolic blood pressure drop after phentolamine injection in normal and general hypertensive patients generally does not exceed 4.00 kPa (30 mmHg).