Patients should not take leukotriene modifiers if they have been diagnosed with any of the following conditions:

• Drug allergies. Patients who are allergic to a leukotriene modifier, or any of its ingredients, should not take the drug. Patients should also notify their physicians if they are allergic to any other drugs.
  
  
• Liver disease. Leukotriene modifiers are removed from the body through the liver. Certain liver diseases may result in too much of this medication building up in the body, increasing the risk of side effects and/or overdose. Liver failure has also occurred in patients taking leukotriene modifiers. Those with a history of medical problems related to the liver should be monitored by a physician to ensure there is no increase in liver enzyme levels in the blood.
  
  
• Phenylketonuria (PKU). A condition in which the body is not able to convert the amino acid phenylalanine into tyrosine due to a defective enzyme. One type of leukotriene modifier (montelukast sodium chewable tablets) contains phenylalanine, which patients with phenylketonuria should not ingest.
  
  
• Blood-clotting disorders. Patients with these conditions may take anticoagulants which, when combined with leukotriene modifiers, can cause an increase in bleeding.
  
  
• Pregnancy. Leukotriene modifier use during pregnancy may not be advised.