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Taking EPO or Testosterone is increasing the levels of naturally occurring substances in the body. My understanding of Asthma medication is that they reduce the levels of substances which cause contraction in the airways, or reduce the response to those substances. Ergo, if you're not experiencing contraction of the airways, Asthma meds do nothing.
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Can a placebo still be a marginal gain?
http://www.runnersworld.com/sweat-science/do-asthma-meds-make-you-faster
If you take it without having an asthmatic requirements then there're arguments that they enhance performance. EPO, cortisone, testosterone etc all have genuine medical uses much like asthma medication but are all abused.