Zafirlukast is used to prevent and treat the symptoms of asthma. Do not use zafirlukast for an acute asthma attack. Zafirlukast is a type of leukotriene receptor antagonist (LTRA). Leukotriene receptor antagonists (LTRAs) work by blocking natural substances in the body called leukotrienes that cause swelling and constriction of the airways.