Family Nurse Practitioner Exam

What is the main mechanism of first-pass metabolism in drug absorption?

1. Drugs that are administered by intramuscular injection undergo first-pass metabolism

2. Oral drugs are absorbed by the GI tract and metabolized by intestinal bacteria

3. Drugs administered through skin patches are metabolized by the dermis

4. Oral drugs are absorbed in the small intestines and metabolized in the liver before circulation

The correct answer is: Oral drugs are absorbed in the small intestines and metabolized in the liver before circulation

The main mechanism of first-pass metabolism involves the process by which orally administered drugs are absorbed in the small intestines and then transported to the liver for metabolism before entering the systemic circulation. When a drug is taken orally, it undergoes absorption through the gastrointestinal (GI) tract, particularly in the small intestines, where it enters the portal circulation. The portal vein carries the drug to the liver, where enzymatic activity can significantly alter or inactivate the medication before it reaches the general circulation. This phenomenon is critical because it can greatly affect the bioavailability of the drug and its overall efficacy. First-pass metabolism can lead to a reduction in the concentration of the active drug available to exert its therapeutic effects. For instance, certain drugs may require higher oral doses than parenteral doses to achieve comparable plasma concentrations due to this significant liver metabolism. In contrast, routes such as intramuscular injections or skin patches bypass the gastrointestinal system and the first-pass effect, which is why options referring to those methods do not accurately describe the mechanism of first-pass metabolism. Additionally, while intestinal bacteria can influence drug absorption and metabolism, this process does not constitute first-pass metabolism, which specifically refers to hepatic processing.