Participants receive montelukast 7 mg intravenously (IV) until until a decision is made for one of the following: (1) discontinuation from the study, (2) discharge from the study site, or (3) admission to the hospital. All randomized participants will receive systemic corticosteroids (60 mg prednisone OR 50 mg prednisolone) orally immediately after the completion of the study drug administration. In addition, all participants will continue to receive standardized treatment in addition to study drug. Standardized treatment may consist of: (1) β-agonist administration beginning 20 minutes after study drug infusion, and every 20 minutes thereafter, as needed; (2) oxygen therapy; and (3) inhaled ipratropium every 60 minutes as needed.

Participants receive placebo IV until until a decision is made for one of the following: (1) discontinuation from the study, (2) discharge from the study site, or (3) admission to the hospital. All randomized participants will receive systemic corticosteroids (60 mg prednisone OR 50 mg prednisolone) orally immediately after the completion of the study drug administration. In addition, all participants will continue to receive standardized treatment in addition to study drug. Standardized treatment may consist of: (1) β-agonist administration beginning 20 minutes after study drug infusion, and every 20 minutes thereafter, as needed; (2) oxygen therapy; and (3) inhaled ipratropium every 60 minutes as needed.

Duration of Treatment: 1 dose