There are a few threads on this here, so you might want to do a search for asthma in this forum.
I'm a flight instructor and I have a 1st class medical with no restrictions (other than for glasses) and I'm on Advare twice daily, 10mg of Singulair and 30mg of Prevacid plus Provental as needed (both neb and inhaler). I think they call me "moderate to severe" and I've had no problems holding down my first class medical. Worse yet, I even told the FAA my whole medical history and they still gave me my medical (though it took a few months to get the first one).