Zafirlukast belongs to a group of medications known as leukotriene receptor antagonists. It works by blocking chemicals produced by the body called leukotrienes. Leukotrienes cause the lining of the breathing passages of the lung to swell. When used regularly, zafirlukast helps to reduce inflammation in the lining of the airways to make breathing easier.

Zafirlukast is used for the prevention and chronic treatment of asthma in adults and children 12 years of age and older. It works over a long period of time to prevent asthma attacks but will not help to relieve an asthma attack when it is occurring. Asthma sufferers should always have their fast-acting asthma relief medication on hand to deal with attacks.

This medication may be available under multiple brand names and/or in several different forms. Any specific brand name of this medication may not be available in all of the forms or approved for all of the conditions discussed here. As well, some forms of this medication may not be used for all of the conditions discussed here.

Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to take this medication if their doctor has not prescribed it.

The recommended dose for adults and children aged 12 years and over is 20 mg twice daily taken on an empty stomach (at least one hour before or 2 hours after a meal). Continue to take this medication regularly, even when you are not having symptoms of asthma.

Many things can affect the dose of medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones listed here, do not change the way that you are taking the medication without consulting your doctor.

It is important to take this medication exactly as prescribed by your doctor. If you miss a dose, take it as soon as possible and continue with your regular schedule. If it is almost time for your next dose, skip the missed dose and continue with your regular dosing schedule. Do not take a double dose to make up for a missed one. If you are not sure what to do after missing a dose, contact your doctor or pharmacist for advice.

Store this medication at room temperature and keep it out of the reach of children.

Do not take zafirlukast if you:

• are allergic to zafirlukast or any ingredients of the medication
• have reduced liver function

Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent.

The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

• abdominal pain
• diarrhea
• headache
• muscle aches
• nausea
• rash
• trouble sleeping
• vomiting

Although most of the side effects listed below don't happen very often, they could lead to serious problems if you do not check with your doctor or seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

• general feeling of being unwell
• increased frequency of infections (e.g., lack of energy, fever or chills, difficulty breathing, cough)
• itching
• joint pain
• signs of bleeding (e.g., unusual nosebleeds, bruising, blood in urine, coughing blood, bleeding gums, cuts that don't stop bleeding)
• signs of liver problems (e.g., nausea, vomiting, diarrhea, loss of appetite, weight loss, yellowing of the skin or whites of the eyes, dark urine, pale stools)

Stop taking the medication and seek immediate medical attention if any of the following occur:

• flu-like illness, rash, pins and needles or numbness of arms or legs, and severe sinusitis
• signs of a serious allergic reaction (e.g., abdominal cramps, difficulty breathing, nausea and vomiting, or swelling of the face and throat)

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Before you begin taking a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should take this medication.

Asthma attacks: Zafirlukast should not be used to try to relieve acute asthma attacks. Have your rescue medication ready in case of an acute attack. Do not stop taking zafirlukast without consulting your doctor.

Liver problems: Zafirlukast is not recommended for people with reduced liver function. It may also cause severe liver problems. If you develop jaundice (signs include yellowing of the skin or eyes); flu-like symptoms; nausea; fatigue; loss of appetite; itching; and pain on the right side of your abdomen, under the ribcage, stop taking this medication and contact your doctor immediately.

Warfarin interaction: Taking warfarin together with zafirlukast significantly increases the time it takes blood to clot. People taking warfarin and zafirlukast should have their bleeding times monitored closely and have their anticoagulant dose (warfarin) adjusted accordingly.

Pregnancy: This medication should not be used during pregnancy unless the benefits outweigh the risks. If you become pregnant while taking this medication, contact your doctor immediately.

Breast-feeding: Zafirlukast passes into breast milk. If you are a breast-feeding mother and are taking this medication, it may affect your baby. Talk to your doctor about whether you should continue breast-feeding.

Children: The effectiveness and safety of zafirlukast for children under 12 years of age have not been established.

There may be an interaction between zafirlukast and any of the following:

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

• stop taking one of the medications,
• change one of the medications to another,
• change how you are taking one or both of the medications, or
• leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.