First pass metabolism is maximum with which route: September 2005
**Core Concept**
First pass metabolism refers to the process by which a drug is metabolized by the liver after its initial absorption, resulting in a reduced bioavailability. The liver's ability to metabolize a drug is influenced by the route of administration, with some routes allowing more extensive first pass metabolism than others.
**Why the Correct Answer is Right**
The route with maximum first pass metabolism is oral administration. When a drug is taken orally, it is absorbed into the bloodstream via the portal vein, which directly carries it to the liver. The liver then metabolizes a significant portion of the drug before it reaches the systemic circulation. This results in a lower bioavailability of the drug compared to other routes of administration. The liver's high concentration of enzymes, particularly cytochrome P450, plays a crucial role in this process.
**Why Each Wrong Option is Incorrect**
**Option A:** Intravenous (IV) administration bypasses the liver entirely, allowing the drug to directly enter the systemic circulation. This results in minimal first pass metabolism.
**Option B:** Sublingual administration allows the drug to be absorbed directly into the systemic circulation via the venous system, reducing the amount of first pass metabolism.
**Option C:** Topical administration, such as creams or ointments, is applied directly to the skin or mucous membranes, minimizing the amount of drug that enters the systemic circulation and undergoes first pass metabolism.
**Clinical Pearl / High-Yield Fact**
When choosing a route of administration, consider the drug's pharmacokinetics and the potential for first pass metabolism. Oral administration may not be the best choice for drugs with high first pass metabolism, as it can result in reduced efficacy and increased dosing requirements.
**Correct Answer: A. Intravenous (IV) administration has the least first pass metabolism.**