Objective: Although anaphylaxis can result in poor outcomes, studies on the factors influencing the biphasic reaction of anaphylaxis have been limited. In this study, we evaluated the clinical information of patients at high risk for the biphasic reaction of anaphylaxis. Methods: This retrospective observational study was conducted in the emergency department (ED) of a single tertiary center. We enrolled patients diagnosed with and treated for anaphylaxis from January 2014 to December 2020. We gathered the electronic medical data of the patients who satisfied the diagnostic criteria for anaphylaxis and were treated with epinephrine. The enrolled patients were divided into those showing either a uniphasic reaction or a biphasic reaction depending on the need for multidose epinephrine. Results: The cohort included 255 patients of anaphylaxis (48.6%, male). Of these patients, 59 (23%) showed a biphasic reaction. Patients displaying a biphasic reaction had a longer median time from their ED visit to the administration of the first dose of epinephrine compared to those showing a uniphasic reaction-8 (4-13) vs. 7 (3-12) minutes. Patients showing a biphasic reaction were statistically associated with a longer time from their visit to the ED to epinephrine administration (odds ratio [OR], 1.073; 95% confidence interval [CI], 1.011-1.140; P=0.021), lower mean arterial pressure (OR, 0.971; 95% CI, 0.951-0.991; P=0.006), lower pulse rate (OR, 0.973; 95% CI, 0.954-0.991; P=0.004), and a lower rate of food allergen (OR, 0.406; 95% CI, 0.169-0.976; P=0.044). Conclusion The delayed administration of epinephrine upon ED arrival was associated with biphasic reactions in addition to lower mean arterial pressure and pulse rate, and exposure to non-food allergen. This study presents evidence supporting the prompt use of epinephrine in patients with anaphylaxis. However, further prospective studies in this regard would be needed.