I used to have bad asthma, but now, since starting the fast (I'm on day 14) my asthma is almost gone...don't need the puffer except occasionally on my runs (I can run!) and even that is a lot less than before.
I don't think you should suffer if your asthma is bad during your fast. I have no problems using mine when absolutely needed. But now its just a faded memory in my mind, most of the time.