I have had asthma and allergies since 1989 and been on so many different types of medications to deal with both. New trial drugs that never work and drugs that are too harsh on the rest of my body. There was even a medication that acts just like Singulair that came out a few years before hand but it quickly went out of writing of a doctor's prescription.
As with any medication there are possible side effects but for me I have not noticed a single one. What I notice is my seasonal allergies and every day allergies of animal dander, mold, dust mites, etc., are under control unless there is a huge allergen lurking. My asthma inhalor is never used. It use to be nightly!!! I only need it when I am excerising and the occasional (yearly) asthma attack turned bronchitis.
I can NOT wait until this goes generic. Every doctor I have been to has loved to prescribe this for their asthmatic and allergy-prone patients.