Which type of administration bypasses first-pass metabolism?

Intravenous administration is a method that delivers medication directly into the bloodstream. By doing so, it circumvents the liver where first-pass metabolism occurs. First-pass metabolism refers to the process where the concentration of a drug is significantly reduced before it reaches the systemic circulation, primarily when drugs are administered orally. Since intravenous administration allows for the direct entry of the drug into systemic circulation, the entire dose reaches the target area without any loss due to metabolic processes in the liver.

In contrast, oral administration requires the drug to be absorbed through the gastrointestinal tract and transported to the liver via the portal circulation before entering the systemic circulation, which often results in decreased bioavailability. Subcutaneous administration also does not directly enter the bloodstream but is absorbed into systemic circulation after passing through the layers of skin and subcutaneous tissue. Inhalation administration allows drugs to be absorbed through the lungs but may still have varying degrees of first-pass metabolism depending on the specific drug and its action, though it does minimize systemic exposure to the liver compared to oral routes.