Inhaled Corticosteroids

It’s tempting to think of asthma as just "tight airways," but the tightness is the surface of a deeper problem: chronic airway inflammation. Allergic and immune triggers activate mast cells and recruit eosinophils and other immune cells into the airway wall, producing mucosal swelling, thick mucus, and airway hyperresponsiveness — a hair-trigger tendency to constrict. The bronchospasm you hear as a wheeze is driven by that inflammation underneath.

This is why a bronchodilator alone doesn’t fix asthma. A SABA relaxes the muscle and relieves the moment’s wheeze, but it does nothing to the swelling, mucus, or eosinophils — so the disease keeps smoldering, and relying on rescue inhalers alone actually raises the risk of severe attacks and death. To control asthma, you have to treat the inflammation — and that is the job of the inhaled corticosteroid.

A corticosteroid enters the airway cells and binds the glucocorticoid receptor; the complex moves to the nucleus and reprograms gene transcription, damping the pro-inflammatory signals (transcription factors like NF-κB and AP-1) that drive cytokine release. The result is fewer eosinophils, less mucosal swelling and mucus, and reduced airway hyperresponsiveness — the airway becomes calmer and less twitchy over time.

The genius of delivering it by inhalation is targeting: the drug lands directly on the airway lining and largely bypasses the bloodstream, so it achieves a strong local anti-inflammatory effect with far less systemic exposure than oral prednisone. That’s what makes an ICS safe enough for daily, long-term use. The trade-off is a controller, not a reliever: because it works by slowly turning down inflammation, it takes days to weeks to reach full effect and must be taken every day even when the patient feels well — it will not relieve a sudden attack.

The side effects map directly onto where the inhaled particles end up. A portion always deposits in the mouth and throat rather than the lungs, and local steroid there suppresses mucosal defenses and irritates tissue — causing oral thrush (candidiasis) and dysphonia (hoarseness). Both are prevented by using a spacer (so more drug reaches the lung, less the mouth) and by rinsing the mouth and spitting after each dose.

The small fraction that *is* absorbed systemically is what accounts for the dose-dependent effects seen mainly at high or prolonged doses: some adrenal suppression (much less than oral steroids, but real at high dose), a small reduction in growth velocity in children, and, over the long term, reduced bone density and cataracts/glaucoma (with a modest pneumonia signal when ICS is used in COPD). Reassuringly, inhaled corticosteroids used alone carry no boxed warning — the historical LABA boxed warning belonged to the LABA component and was removed from ICS/LABA combination inhalers in 2017.