The First Pass Effect - Pharmacology

The most common way drugs are administered is orally, by swallowing a pill. The drug is then absorbed into the GI tract and enters the portal circulation system where drug particles are transported through the portal vein into the liver where the drug is metabolized. This is referred to as the first pass effect.

Not all drugs are metabolized in the liver. Some drugs bypass the first pass effect by sublingual administration (under the tongue) or buccal administration (between the gums and the cheek) where they are absorbed directly into the bloodstream from the mouth. These drugs do not enter the stomach where the hydrochloric acid might destroy drug particles. Other drugs go directly to the liver through the portal vein and also bypass the stomach. The drug is then metabolized in the liver and much of the drug may be eliminated and not available for a therapeutic effect. Sometimes this effect is so great that none of the drug is available for use if given by mouth. The drug must then be given in very high doses or parenterally (intramuscularly or intravenously) to bypass the liver.


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