As stated in a previous post I have severe allergies to a variety of different allergens both indoors and out. I also have allergy induced asthma. I had been taking Zyrtec for years and was doing very well with it when I developed a cough that wouldn't go away so I went to see a specialist who ran all sorts of fun tests with the result being that I should never ever go off my zyrtec and that he thought adding Singulair would clear up my asthma.
He was very right! Before I started to take the two together I would still get sick every other month or so after a bad allergy attack because the sinus membranes would get all swollen and infected (according to the doc) and now I almost never have a severe attack and haven't had to use my inhaler in almost a year. These are all good things :)
As a side note all my doctors (OB, Pediatrician, and allergy specialist) have said Singulair was fine to take while I was pregnant and while I am breast feeding :)