Question

First pass effect is relatively high when drugs are administered by
(A) Subcutaneous route
(B) Oral route
(C) Intramuscular route
(D) Intravenous route

• A. Subcutaneous route
• B. Oral route
• C. Intramuscular route
• D. Intravenous route

Hint:

To bypass a high hepatic first-pass effect, veterinarians administer drugs via parenteral routes (such as IV, IM, or SC) or sublingual/rectal mucosal routes.
This allows the drug to enter the systemic circulation directly before reaching the liver.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The first-pass effect (first-pass metabolism) is a pharmacological phenomenon where the concentration of a drug is significantly reduced before it reaches the systemic circulation.
This pre-systemic clearance is primarily mediated by enzymes in the gastrointestinal tract and the liver.

Step 2: Detailed Explanation:
Let us analyze how the route of administration affects drug absorption and metabolism:
- Oral Route (B): When a drug is administered orally, it is absorbed through the mucosal lining of the stomach and intestines.
This venous drainage enters the portal venous system, transporting the absorbed drug directly to the liver via the portal vein.
The liver, rich in cytochrome P450 enzymes, metabolizes a significant portion of the active drug before it can enter the hepatic veins and the systemic circulation.
As a result, the oral route is associated with a high first-pass effect, significantly reducing the systemic bioavailability of many drugs.
- Parenteral Routes (A, C, D): Subcutaneous, intramuscular, and intravenous routes bypass the gastrointestinal tract and the portal circulation.
These routes deliver the drug directly into the systemic venous or capillary circulation, avoiding initial hepatic first-pass metabolism.

Step 3: Final Answer:
The first-pass effect is highest when drugs are administered via the oral route. This corresponds to option (B).