I saw a 12 year-old boy today with a 1 day history of impressive acute urticaria. A few weeks ago, he started Singulair for seasonal allergies; the night before the hives appeared he enjoyed shrimp (which he has never had any difficulty with before). He has been followed by an allergist, and interestingly, had previously tested negative for shellfish. I have prescribed montelukast (in combination with antihistamines) a innumerable times for urticaria and have never seen a hypersensitivity reaction to the drug. Litt says that urticaria may be seen in up to 2% of patients on montelukast. My guess is that shellfish was the culprit in my patient. My instructions were to stop the medication, eat kosher, use standard antihistamines, and make a beeline back to his allergist (hopefully no hymenoptera allergy!). Does anyone have experience with montelukast hypersensitivity?