Gaseous and volatile drugs may be inhaled and absorbed through the pulmonary epithelium and mucous membranes of the respiratory tract. Access to the circulation is rapid by this route because the lung's surface area is large (~140 m2) and first-pass metabolism is avoided. The principles governing absorption and excretion of anesthetic and other therapeutic gases are discussed in Chapters 13 and 15.
Drugs are applied to the mucous membranes of the conjunctiva, nasopharynx, oropharynx, vagina, colon, urethra, and urinary bladder primarily for their local effects.
Topically applied ophthalmic drugs are used for their local effects (see Chapter 63) requiring absorption of the drug through the cornea; corneal infection or trauma thus may result in more rapid absorption. Ophthalmic delivery systems that provide prolonged duration of action (e.g., suspensions and ointments) are useful, as are ocular inserts providing continuous delivery of drug.
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