First pass metabolism.
Pharmacology is the study of the effects drugs have and how they exert them. Drugs can be administered enteral or parenteral.  Enteral refers to any form of administration that involves the alimentary tract. The four enteral routes of administration include the mouth, sublingual, buccal and rectal. Parenteral refers to routes of administration outside of or beside of the alimentary tract. The parenteral route includes intravascular (IV), Intramuscular (IM) and subcutaneous (SC) routes of administration. The parenteral routes are used for drugs that are poorly absorbed or that are unstable in the GI tract. The parenteral routes are used in unconscious patients or when a fast response is needed as it provides the most control over actual dose of the drug delivered into the system.  When a drug is administered it goes through four stages: absorbed, distributed, metabolised and eliminated. The absorption of the drug depends on first pass metabolism and bioavailability. First pass metabolism refers to “the elimination that occurs when a drug is first absorbed from the intestine and passes through the liver via the portal circulation”. The liver is the primary drug metabolizing organ in the body. Drugs that are easily metabolized have a large first pass effect and low bioavailability.  Drugs that are orally administered are mostly affected by first pass metabolism as they have pass through the gastrointestinal tract and the stomach. Routes like parenteral avoid first pass metabolism as they are directly administered into the blood system and surpass the liver.  The amount of administered dose of unchanged drug that reaches the systematic circulation is known as bioavailability. Bioavailability is important in pharmacokinetics, as it must be considered when calculating dosage for non-intravenous routes of administration. Bioavailability is affected by first pass metabolism, solubility and stability of the drug. Due to the low...
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