Which mode of administration below best avoids the first pass-effect by the liver and will have the highest bioavailability?
Select one: a. oral b. rectal c. intravenous
The correct answer and explanation is:
The correct answer is c. intravenous.
When a drug is administered intravenously (IV), it is directly introduced into the bloodstream, bypassing the digestive system and liver. This avoids the first-pass effect, which is the process by which a drug is metabolized in the liver before it reaches systemic circulation. Drugs that undergo significant first-pass metabolism in the liver lose a portion of their active ingredients before they can exert their effects on the body. This results in lower bioavailability for oral drugs.
In contrast, when a drug is taken orally, it first passes through the digestive tract and is absorbed into the bloodstream via the portal vein. The drug then reaches the liver, where it is metabolized, often reducing the amount of active drug available to the rest of the body. This is why oral administration typically has lower bioavailability compared to intravenous administration.
Rectal administration can also reduce the first-pass effect, although not entirely. The rectal veins lead to both the portal circulation (which passes through the liver) and the systemic circulation. Therefore, while some of the drug may bypass the liver, a portion will still be metabolized.
IV administration, being the most direct route into the bloodstream, ensures that the maximum amount of the drug reaches its target tissue without any loss due to metabolism in the liver. This means that IV drugs typically have the highest bioavailability, and the therapeutic effect can be more predictable and rapid.
In summary, intravenous administration avoids the first-pass metabolism, providing the highest bioavailability of a drug compared to oral and rectal routes.