My allergist suggested Singulair when Benadryl, Allegra, and avoiding my suspected food triggers weren't cutting it. It seemed to work decently until day #2, when my face and truck broke out in a horrible rash. I looked like I had smallpox.
I googled Singulair's ingredients, and lo and behold, it contains lactose which is one thing I'm fairly certain that I'm allergic to.
So that being said, I didn't really get an extended chance to try it out, but any one sensitive to lactose should steer clear of it (or at least the 10 mg pills; I don't think the 5 mg pills do; just research the ingredients!).