Doctors generally consider inhaled corticosteroids the primary controller or maintenance therapy for patients with moderate and severe persistent asthma, because of the drugs’ anti-inflammatory properties. Doctors use these drugs because the inhaled forms directly deliver the medication to the airways with minimal side effects. Consistent and appropriate (as determined by your doctor) use of inhaled corticosteroids can also reduce the need for oral (or systemic ) corticosteroids, which may cause serious adverse side effects when used long-term. 

Inhaled corticosteroids usually start reducing airway inflammation after a week of regular administration, reaching their full effect within four weeks. However, although these drugs can work well as preventives, if you develop severe symptoms — from a sudden high exposure to potent asthma triggers, for example — you need to use a quick-relief medication (usually a bronchodilator that your doctor prescribes) as part of your asthma management plan.

Knowing your inhaled corticosteroid devices and formulations

Doctors can prescribe inhaled corticosteroids in metered-dose inhaler (MDI), dry-powder inhaler (DPI), or compressor-driven nebulizer (CDN) formulations. Because the pressurized canisters of some MDI medications are packaged in plastic sleeves of the same color, knowing which strength your doctor specifically prescribes for you is especially important.

Excerpted from "Asthma for Dummies" by William E. Berger, MD