Wiggins claims that he wasn’t using levels that someone would use if they were doping, just enough to put an asthmatic on a level playing field with a non-asthmatic in the Tour de France. Well if it was enough to do that, I would imagine it would be enough to give a non-asthmatic cyclist an advantage. In any case, I read that once a cyclist has Therapeutic Use Exemption (TUE) the authorities don’t check to see if the drug is within reasonable limits, so there is nothing to say someone couldn’t take more than prescribed.
On the other hand, a lot of pro cyclists do suffer bouts of asthma and are more prone due to the amount of air that they breathe in. One cycling mag claims that around 40% of our cyclists at the Olympics were using inhalers.