Absorption (pharmacology) in the context of "Nasogastric intubation"

Pharmacology is the science of drugs and medications, including a substance's origin, composition, pharmacokinetics, pharmacodynamics, therapeutic use, and toxicology. More specifically, it is the study of the interactions that occur between a living organism and chemicals that affect normal or abnormal biochemical function. If substances have medicinal properties, they are considered pharmaceuticals.

The field encompasses drug composition and properties, functions, sources, synthesis and drug design, molecular and cellular mechanisms, organ/systems mechanisms, signal transduction/cellular communication, molecular diagnostics, interactions, chemical biology, therapy, medical applications, and antipathogenic capabilities. The two main areas of pharmacology are pharmacodynamics and pharmacokinetics. Pharmacodynamics studies the effects of a drug on biological systems, and pharmacokinetics studies the effects of biological systems on a drug. In broad terms, pharmacodynamics discusses the chemicals with biological receptors, and pharmacokinetics discusses the liberation, absorption, distribution, metabolism, and excretion (LADME) of chemicals from the biological systems.

The first pass effect (FPE), also known as first-pass metabolism (FPM) or presystemic metabolism, is a phenomenon of drug metabolism at a specific location in the body which leads to a reduction in the concentration of the active drug before it reaches the site of action or systemic circulation. The effect is most associated with orally administered medications, but some drugs still undergo first-pass metabolism even when delivered via an alternate route (e.g., IV, IM, etc.). During this metabolism, drug is lost during the process of absorption which is generally related to the liver and gut wall. The liver is the major site of first pass effect; however, it can also occur in the lungs, vasculature or other metabolically active tissues in the body.

Notable drugs that experience a significant first pass effect are buprenorphine, chlorpromazine, cimetidine, diazepam, ethanol (drinking alcohol), imipramine, insulin, lidocaine, midazolam, morphine, pethidine, propranolol, and tetrahydrocannabinol (THC).

In pharmacology, bioavailability is a subcategory of absorption and is the fraction (%) of an administered drug that reaches the systemic circulation.

By definition, when a medication is administered intravenously, its bioavailability is 100%. However, when a medication is administered via routes other than intravenous, its bioavailability is lower due to intestinal epithelium absorption and first-pass metabolism. Thereby, mathematically, bioavailability equals the ratio of comparing the area under the plasma drug concentration curve versus time (AUC) for the extravascular formulation to the AUC for the intravascular formulation. AUC is used because AUC is proportional to the dose that has entered the systemic circulation.

An excipient is a substance formulated alongside the active ingredient of a medication. They enhance the active ingredient's therapeutic properties; to facilitate drug absorption, to modify viscosity, to enhance solubility, to improve long-term stabilization (preventing denaturation and aggregation during the expected shelf life). During the manufacturing process, excipients can improve the handling of active substances and facilitate powder flow. The choice of excipients depends on factors such as the intended route of administration, the dosage form, and compatibility with the active ingredient.

Virtually all marketed drugs contain excipients, and final drug formulations commonly contain more excipient than active ingredient. Pharmaceutical regulations and standards mandate the identification and safety assessment of all ingredients in drugs, including their chemical decomposition products.