Certain drugs cannot be taken by mouth because the entire do

Certain drugs cannot be taken by mouth because the entire dose of the drug is destroyed in the liver before it ever reaches the general circulation. Explain why these same drugs can be given by injection, either intravenously or intramuscularly.

Solution

The drugs taken orally enters the hepatic portal system where it enters the liver and undergoes biotransformation where the drug is transformed into another substance before it reaches the target, so efficiency and concentration of the drug eventually decreases. The same drugs when administered either intravenously or intramuscularly bypasses the hepatic portal system and thus give a rapid effect than orally taken drugs.