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The first-pass effect refers to the process by which a drug, after being absorbed from the gastrointestinal tract, is transported to the liver via the portal vein before it enters systemic circulation. During this initial passage through the liver, the drug can be metabolized, and its concentration can be significantly reduced before it reaches the systemic circulation.

This phenomenon primarily influences drug metabolism, as it essentially acts as a filter where some of the active ingredient may be converted into inactive metabolites. This process is crucial for oral medications because it can greatly affect their overall bioavailability, which is the proportion of the drug that reaches systemic circulation in an active form.

It is important to understand that the first-pass effect does not influence drug interactions in the bloodstream, nor does it play a role in patient adherence or bioavailability after intravenous administration. In cases of intravenous administration, the drug bypasses the gastrointestinal tract and liver initially, meaning the first-pass effect does not apply in this context.