When a chemical comes in contact with the mucous membrane beneath the tongue,
    it diffuses through it. Because the connective tissue beneath the epithelium contains
    a profusion of capillaries, the substance then diffuses into them and enters the
    venous circulation. In contrast, substances absorbed in the intestines are subject to
    "first pass metabolism" in the liver before entering the general circulation.

    Sublingual administration has certain advantages over oral administration. Being
    more direct, it is often faster, and it ensures that the substance will risk degradation
    only by salivary enzymes before entering the bloodstream, whereas orally
    administered drugs must survive passage through the hostile environment of the
    gastrointestinal tract, which risks degrading them, either by stomach acid or bile, or
    by the many enzymes therein, such as monoamine oxidase (MAO). Furthermore,
    after absorption from the gastrointestinal tract, such drugs must pass to the liver,
    where they may be extensively altered; this is known as the first pass effect of drug
    metabolism. Due to the digestive activity of the stomach and intestines and the
    solubility of the GI tract, the oral route is unsuitable for certain substances...
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