My daughter doesn't have asthma like I do, but she has pretty bad allergies. Her allergist (the same one I see) added Singulair to her Zyrtec D and that seems to work well for her. I used to take Singulair also. It doesn't seem to have any ill side effects and it is used a lot for allergies also, not just asthma.
I really worry that this daughter will end up with asthma because she seems to be following in my footsteps with allergies and respiratory problems. She takes good care of herself and she doesn't smoke either, so there's not much else to do, well there is. It's in God's hands.