Corticosteroids are medications commonly used by people with asthma. They work by reducing airway inflammation. They're known as controller medications (also called preventive or maintenance medications) because they control the condition overall and prevent the symptoms from developing. Controller medications are slow acting, meaning they can take days or even weeks to begin working. Because they can't provide immediate relief of symptoms, corticosteroids shouldn't be used when an effect is needed quickly. This requires faster-acting medications (known as rescue medications) that can work on the spot.