Get emergency medical help if you have signs of an allergic reaction: hives, blisters, severe itching; difficult breathing; swelling of your face, lips, tongue, or throat.
Tell your doctor right away if you have signs of blood vessel inflammation: flu-like symptoms, severe sinus pain, a skin rash, numbness or a "pins and needles" feeling in your arms or legs.
Some people using zafirlukast have had new or worsening mental problems. Stop taking  zafirlukast and call your doctor right away if you have unusual changes in mood or behavior, such as:
agitation, aggression, feeling restless or irritable;
anxiety, depression, confusion, problems with memory or attention;
stuttering, tremors, uncontrolled muscle movements;
suicidal thoughts or actions;
hallucinations, sleep problems, vivid dreams, sleep-walking; or
compulsive or repetitive behaviors.
Also call your doctor at once if you have:
worsening or no improvement in your asthma symptoms; or
liver problems--nausea, loss of appetite, stomach pain (upper right side), tiredness, itching, dark urine, jaundice (yellowing of the skin or eyes).
Common side effects may include:
nausea, diarrhea, stomach pain;
headache; or
cold symptoms such as stuffy nose, sneezing, sore throat.
This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.
 Zafirlukast side effects (more detail)
