I have asthma and ARDS. Taking meds prior to exercise helps, but it never stops the all the flair ups, at least not for me. Remember, asthma is inflammation that causes oxygen deprivation and when you add in exercise, your body think you are depriving more oxygen to your lungs, jumps your heart rate up to grab some up that O2 goodness, and then voila! Asthma attack.
Your best bet, is get with your doctor to work out a plan that works for you.
My pulmonary specialist and I have worked out a work out plan that helps, a lot. Hitting the pool is an awesome way to get my cardio up without sending me into attacks every day. When I am doing high intensity workouts, I drop it down a wee bit and never fully push myself, but add minutes on to make up the difference. If my heart rate gets to around 150 (number my doc gave me), I bring it down to a walk and get my heart rate back under control before jumping it back up. I always start slow, work up, and bring it back down slowly.
The key is to exercise. Ultimately, doing something is better than nothing, so even if you aren't able to fully push yourself to extremes like many other people do, you are still doing your body justice if you push to your actual limits, even if it is much lower than what you feel it can be.
And hey, with time, you can get more intense. I went from an asthma attack after every walk to only one in the past two months...and I now do a heck of a lot more than walking
(AND!!!! I might be able to decrease my medication...find out this afternoon...oh please oh please oh please oh please....)