Broadly speaking, drugs for asthma can be split into two types: relievers, and preventers. The purpose of each is fairly self-explanatory: relievers are used for the relief of asthma symptoms when they become apparent, whilst the preventers are used to help stop these symptoms appearing in the first place. Different chemical compounds are required depending on which of the two results we want to accomplish. ..Reliever drugs are by necessity fast-acting, reaching their peak effect between 5-20 minutes after dosing, and lasting for up to two hours. As such, they provide more or less immediate relief from asthma symptoms. …Unlike the relievers, preventers are no use for relief from asthma attacks; instead, regular dosing is required for their effects to be apparent.
Better living through chemistry indeed.