This will be a double-blind, three-way crossover study comparing combination vilanterol+fluticasone with fluticasone monotherapy and a placebo (vilanterol is not available as a monotherapy). The order in which the treatments are given will be randomized and each treatment will be separated by a minimum one-week washout. Fifteen asthmatic participants will be recruited from the local community. Methacholine challenge testing (MCT) with the two-minute tidal breathing protocol will be used to quantify the bronchoprotective effects (if any) of each treatment against methacholine-induced bronchoconstriction.