Inhaled corticosteroids are far from ideal topical agents. After inhalation and even with good technique, a large proportion of the inhaled dose, approximately 80%, is deposited on the oropharynx and then swallowed. This fraction is then available for absorption into the systemic circulation.
Once absorbed, beclomethasone and budesonide are inactivated by first-pass metabolism whereas fluticasone is minimally absorbed by the gut. The metabolism of budesonide appears less saturable than beclomethasone and so the former is preferred where the patient is on high dose therapy. System absorption becomes significant at daily doses of 2mg in adults or 500 micrograms per metre squared body-surface area in children.
The fraction that is deposited on the oropharynx is much reduced if the glucocorticoid is administered through a large-volume spacer attached to a metered-dose inhaler. If the patient rinses his/her mouth after using inhaled steroids then this will also serve to reduce the amount available for systemic absorption.
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