ABSTRACT Objectives This study aimed to compare the efficacy and safety of an initial 12 mg versus 6 mg adenosine dose for sinus rhythm conversion in patients presenting with supraventricular tachycardia (SVT) in the emergency department (ED). Methods This prospective observational study was conducted between February 2025 and January 2026. Adult patients (≥ 18 years) presenting to the ED with hemodynamically stable SVT confirmed by 12‐lead electrocardiography (ECG) were included. Patients were categorized into two groups according to the initial adenosine dose administered (6 mg vs. 12 mg). To address potential selection bias, 1:1 propensity score matching (PSM) was performed. The primary outcome was successful first‐dose sinus rhythm conversion. Secondary outcomes included adenosine‐related adverse effects and SVT recurrence during the ED stay. Results A total of 142 patients were analyzed ( n = 71 per group). First‐dose conversion was significantly higher with 12 mg compared with 6 mg (83.1% vs. 52.1%). In the PSM cohort ( n = 104), the 12 mg dose maintained its superior efficacy (82.7% vs. 53.8%). The PSM‐adjusted odds ratio for successful conversion was 4.12 (95% confidence interval [CI]: 1.85–9.14), with a number needed to treat (NNT) of 3.8 (95% CI: 2.5–8.3). SVT recurrence was numerically lower in the 12 mg group (1.4% vs. 9.9%). Adverse effects were similar between groups. Conclusions An initial 12 mg adenosine dose was associated with higher first‐dose sinus rhythm conversion than 6 mg, while adverse effects were similar between groups.