A randomized, controlled, double-blind clinical trial was conducted in three university hospital emergency departments (EDs) between August 2009 and December 2014 to determine the advantage of two different doses magnesium sulfate (MgSO4) vs placebo in rate control of rapid atrial fibrillation (AF) managed in the ED. For this investigation, they recruited patients > 18 years with rapid AF (>120 beats/min) and randomized to 9 grams of intravenous MgSO4 (high-dose group, n = 153), 4.5 g of intravenous MgSO4 (low-dose group, n = 148), or serum saline infusion (placebo group, n = 149), given in addition to atrioventricular (AV) nodal blocking agents. Findings suggested that MgSO4 seems to have a synergistic impact when combined with other AV nodal blockers resulting in improved rate control. They observed comparable effectiveness with 4.5 and 9 grams of MgSO4. However, a dose of 9 grams was related to more side effects. Minor adverse effects were noted and significantly more frequent with high-dose MgSO4.