The role of intravenous (IV) or nebulised magnesium sulphate (MgSO ) in the treatment of severe acute asthma in adults is unclear. A controversy exists regarding its efficacy. In children MgSO has a more evident clinical effect, but the child population has not been considered in this work. The applicability of the results from randomized clinical trials (RCTs) involving MgSO in adult population is questioned in the optimal treatment of asthma exacerbations. According to the newest guidelines from the Global Initiative for Asthma (GINA), optimal treatment in the emergency department (ED) is based on short-acting beta -agonists (SABA), oral or IV corticosteroids (CS), short acting muscarinic antagonists (SAMA) and the controlled oxygen therapy. Further improvements with IV or nebulised MgSO were assessed in a recent large multicentre, double-blind, placebo-controlled randomized 3 Mg trial, which failed to demonstrate clinical benefit. Several other RCTs found some benefit with MgSO , although the majority lacked some treatment options that are used in the optimal treatment of asthma exacerbations. Therefore, we reviewed the limitations of RCTs of IV or nebulised MgSO in adults with acute asthma, with the aim to answer in which subpopulation MgSO could be beneficial.