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 montelukast have had new or worsening mental problems. Stop taking  montelukast 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.
Common side effects may include:
stomach pain, diarrhea;
fever or other flu symptoms;
ear pain or full feeling, trouble hearing;
headache; or
cold symptoms such as runny or stuffy nose, sinus pain, cough, 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.
 Montelukast side effects (more detail)
