An antiasthma drug. Zileuton (Zyflo) is a type of antiasthma drug called a leukotriene inhibitor, used by people with mild to moderate asthma to decrease symptoms and the number of acute asthma attacks. Zileuton cannot relieve an asthma attack that has already started. It works by inhibiting leukotrienes, substances in the body that are associated with the inflammation and tightening of airway muscles in the lung that are characteristic of asthma.