A person should seek immediate medical attention if they experience any of the following while taking leukotriene modifiers:

• Rash or unexplained open sores or bruising
  
  
• Numbness, tingling or pain
  
  
• Worsening respiratory symptoms
  
  
• Symptoms of an allergic reaction (e.g., difficulty breathing, tightening of the throat, hives, swelling of the lips, tongue or face)

A physician should be informed if a person experiences less serious side effects of leukotriene modifiers, such as:

• Dizziness
  
  
• Headache
  
  
• Infection (e.g., ear infection, upper or lower respiratory infection)
  
  
• Joint pain (arthralgia)
  
  
• Abdominal pain or dyspepsia (painful digestion)
  
  
• Flatulence, constipation or diarrhea
  
  
• Severe itching (pruritus)
  
  
• Nausea or vomiting
  
  
• Back pain, chest pain, neck pain and rigidity or dental pain
  
  
• Muscle tenderness (myalgia) or weakness (asthenia)
  
  
• Cold-like symptoms (e.g., fever, nasal congestion, cough, sore throat)
  
  
• Fatigue or prolonged drowsiness (somnolence)
  
  
• Insomnia
  
  
• Nervousness
  
  
• Malaise (general ill feeling)
  
  
• Vaginal inflammation
  
  
• Frequent urination
  
  
• Swelling or redness around the eyes
  
  
• Heartburn
  
  
• Swollen lymph nodes (small, kidney-shaped organs of lymphoid tissue found in clusters in the neck, groin, armpit, abdominal organs and other regions of the body)

Patients experiencing these side effects should not stop taking the drug unless instructed to by a physician.

In rare cases, hepatic (liver) dysfunction may develop as a result of taking zafirlukast. Signs and symptoms of hepatic dysfunction include:

• Pain in the right, upper section of the abdomen
• Nausea
• Fatigue
• Lethargy
• Pruritus
• Jaundice
• Flu-like symptoms
• Loss of appetite

Patients who suspect hepatic dysfunction should contact their physician immediately.

There is also some evidence that montelukast sodium may cause rare cases of hepatitis and liver injury in some individuals. Patients at risk for these conditions may want to discuss this risk with their physician prior to using this drug.

A few years ago, there were concerns that use of leukotriene modifiers might lead to Churg-Strauss syndrome, which causes inflammation of the blood vessels, particularly in the lungs. However, a later review found no connection between the drugs and Churg-Strauss syndrome.