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. (Hint: Consider the hepatic portal system.)

The drug given intravenously or intramuscularly reaches the tissues via capillary beds before it destroyed in the hepatic portal system

Explanation:

The hepatic portal system consists of a number of veins that are responsible for the transportation of blood from the capillaries located in the gastrointestinal tracts (stomach) and the spleen and these veins return the blood to the liver .

When drugs are taken orally ( by mouth) they travel from the oesophagus down to the gastrointestinal tracts where they are absorbed by the hepatic portal system and taken into the liver.

There certain drugs that when taken orally maybe be destroyed by the liver therefore before they can achieve the aim for taking those drugs.

In order to prevent this from happening the drugs can be taken intravenously or intramuscularly where they reach the target tissues that need them via the capillary beds and they avoid destruction by the hepatic portal system.