Pharmacokinetics of inhaled corticosteroids

Sixty to 90 percent of the corticosteroid dose from a metered dose or dry powder inhaler lands in the mouth or pharynx and is swallowed and absorbed from the gastrointestinal tract. This fraction is then largely metabolized by the liver ("first pass metabolism") so that only a small amount is absorbed into the systemic circulation. Approximately 10 to 40 percent of a dose of inhaled corticosteroid is delivered to the respiratory tract. This portion is absorbed from the lung periphery directly into the systemic circulation. Thus, although less of the dose is delivered to the respiratory tract, that is the amount that contributes proportionately more to systemic side effects.