I was studying the functions of Smooth endoplasmic reticulum and encountered a function which says 'In the liver the SER contains large amount of Cyrochrome P450 family enzymes that catalyse detoxification of metabolic waste products,drugs and alcohol.'. My question is why does the body need to detoxify drugs when they are taken for medical purposes?
Drugs that are taken orally go through the gastrointestinal tract along with food. Mostly from the small intestine drugs as well as nutrients and other substances pass through intestinal epithelium into the blood stream. Portal venous system delivers blood from intestines to the liver where it is processes by the hepatocytes before reaching the systemic circulation. So most of the substances regardless whether this is a nutrient, a drug or a toxin firstly pass through liver and only after that they are transported to the tissues where they are required. In the liver, a part of the drug is neutralized, so it reduces bioavailability. When drug is administered, for example, intravenously, it fully reaches the systemic circulation. So it has 100% bioavailability. Therefore, the bioavailability of the drug depends on the route of administration.