Great question.  You will find the answer by clicking here.  I write about theophyllin in this post, but the same rings true for all asthma medicines.  While most asthma meds lose potency after so much time on the shelf, they are not harmful if used.  However, if you do use expired meds, you should know that they may be less potent. 

On rare occasion I've found expired Ventoiln inhalers stuffed inside the couch cushions or under the bed, and I have still used these even though I knew they were expired.  However, Ventolin has a nasty taste too it when it's expired, kind of like rotten mints. 

Bottom line, you are better off tossing expired meds in the trash and getting new ones.