Background: Studies of IV magnesium sulfate as a treatment for acute asthma have had mixed results, with some data suggesting a benefit for acute severe asthma, but not for mild-to-moderate asthma. In a multicenter cohort, this study tests the hypothesis that administration of magnesium sulfate improves pulmonary function in patients with acute severe asthma.
Design: Placebo-controlled, double-blind, randomized clinical trial.
Setting: Emergency departments (EDs) of eight hospitals.
Patients: Patients aged 18 to 60 years presenting with acute asthma and FEV1 
30% predicted on arrival to the ED.
Intervention: All patients received nebulized albuterol at regular intervals and IV methylprednisolone. Two grams of IV magnesium sulfate or placebo were administered 30 min after ED arrival. The primary efficacy end point was FEV1 at 240 min, and the data analysis was intent to treat.
Results: Two hundred forty-eight patients were included, and the mean FEV1 on ED arrival was 22.9% predicted. At 240 min, patients receiving magnesium had a mean FEV1 of 48.2% predicted, compared to 43.5% predicted in the placebo-treated group (mean difference, 4.7%; 95% confidence interval [CI], 0.29 to 9.3%; p = 0.045). A regression model confirmed the effect of magnesium compared to placebo was greater in patients with a lower initial FEV1 (p < 0.05). If the initial FEV1 was < 25% predicted, the final FEV1 was 45.3% predicted in the magnesium-treated group and 35.6% predicted in the placebo-treated group (mean difference, 9.7%; 95% CI, 4.0 to 15.3%; p = 0.001). If the initial FEV was 
25% predicted, magnesium administration was not beneficial; the final FEV1 was 51.1% predicted in the magnesium-treated group and 53.9% predicted in the placebo-treated group (mean difference, – 2.9%, 95% CI, – 9.4 to 3.7; p = not significant). Overall, the use of magnesium sulfate did not improve hospital admission rates.