Magnesium sulfate acts as a smooth-muscle relaxant and causes bronchodilation, which might be helpful in treating asthma. However, magnesium relaxes all smooth muscle including the heart which can lead to bradycardia and hypotension -- undesirable side effects in a patient already in distress!
Magnesium sulfate administered by itself (either IV or nebulized) does not improve pulmonary function sufficient to overcome an asthma attack. Given in combination with traditional treatment IV magnesium does improve pulmonary function in patients with moderate to severe asthma. Magnesium is not recommended for treatment of mild to moderate asthma.
Nebulized magnesium is an attractive alternative that might decrease cardiovascular side effects, but has not been shown to have the same benefits as IV magnesium.
Regardless of how sick the asthmatic appears, the first line treatment is always bronchodilators and steroids. Magnesium may be used as an adjunct to traditional treatment to improve pulmonary function when the cardiovascular side effects can be tolerated.
Aggarwal P et al. (2006). Comparison of nebulised magnesium sulphate and salbutamol combined with salbutamol alone in the treatment of acute bronchial asthma: A randomized study. Emerg Med J, 23; 358-62.
Silverman, R.A., et al. (2002). IV magnesium sulfate in the treatment of acute severe asthma. Chest, 122(2); 498-97.