Zafirlukast is a leukotriene (loo-koe-TRY-een) inhibitor. Leukotrienes are chemicals your body releases when you breathe in an allergen (such as pollen). These chemicals cause swelling in your lungs and tightening of the muscles around your airways, which can result in asthma symptoms.

Zafirlukast is used to for chronic treatment of asthma, and to prevent asthma attacks in adults and children as young as 5 years old.

Do not give this medication to a child without a doctor's advice.

Zafirlukast may also be used for purposes not listed in this medication guide.

Zafirlukast will not work fast enough to treat an asthma attack that has already begun. Use only a fast-acting inhalation medicine to treat an asthma attack. Talk with your doctor if any of your asthma medications do not seem to work as well in treating or preventing attacks.

It may take up to several weeks before your symptoms improve. Keep using the medication as directed and tell your doctor if your symptoms do not improve after several weeks of treatment.

Call your doctor right away if you feel that this medicine is not working as well as usual, or if it makes your condition worse. If it seems like you need to use more of any of your medications in a 24-hour period, talk with your doctor.

You should not use this medication if you are allergic to zafirlukast, or if you have liver disease (including cirrhosis).

To make sure you can safely take zafirlukast, tell your doctor if you have liver disease.

The chewable tablet form of this medication may contain up to 0.842 milligrams of phenylalanine. Talk to your doctor before using this form of zafirlukast if you have phenylketonuria (PKU).

FDA pregnancy category B. This medication is not expected to be harmful to an unborn baby. Tell your doctor if you are pregnant or plan to become pregnant during treatment.

Zafirlukast can pass into breast milk and may harm a nursing baby. Do not breast-feed while you are taking zafirlukast.