stomach acid. Additionally, oral medications are subject to metabolism by the gut and liver before reaching circulation (first-pass effect). Sublingual medications are directly absorbed into systemic venous circulation, avoiding first-pass effect. The absorption can vary from fast to slow, according to the composition of the drug. Inhalation drugs are commonly used for respiratory diseases, as this method brings the drug into close contact with the target organ (the lungs). Suppositories are also directly absorbed into systemic venous circulation. This method allows for the absorption of larger doses of a drug and may be useful for patients who cannot use sublingual or orally administered medication. Some drugs are administered parenterally, meaning they enter the body through a route other than the alimentary (gastrointestinal) tract. Parenteral administration includes injection or infusion of a sterile preparation into the tissue, and includes the use of subcutaneous, intramuscular, intravenous, intrathecal and intra-articular routes. Intravenous administration introduces the drug into systemic circulation immediately and provides complete absorption. Both intramuscular and subcutaneous administrations allow faster and more complete absorption than oral administration, although subcutaneous administration is slower than intramuscular methods. Intramuscular injections tend to be more painful than subcutaneous injections, but facilitate larger dosages of the drug, which are not possible in subcutaneous administration. Drug-body interaction It is sometimes said that pharmacokinetics is the study of what the body does to a drug, while pharmacodynamics is the study of what a drug does to the body. Therapeutic drug administration attempts to achieve the desired beneficial effect of the drug with minimum negative, or adverse, effects. The dose-effect relationship can be separated into pharmacokinetic (dose-concentration) and pharmacodynamic (concentration-effect) portions. Concentration is the link between pharmacokinetics and pharmacodynamics. The three main processes of pharmacokinetics are absorption, distribution and elimination. See the following figure: Processes of Pharmacokinetics
In order to interact chemically with a receptor, the drug molecule must match the specific size and shape of the receptor, as well as the appropriate electrical charge and atomic composition. Additionally, as most drugs are administered far from the target action site (a pill is taken by mouth and must travel to its needed location in the body), the drug must have the ability to travel from the site of administration to the site of action during the time period in which it is effective, avoiding excretion or inactivation before its mission is complete. The vast majority of drugs range from a molecular weight of 100 to 1,000. Drugs within this range are large enough to allow selectivity of action (sufficiently unique in fit and charge to prevent binding to other receptors), and small enough to allow movement throughout the different compartments of the body. Very large drugs that will not diffuse through body compartments must be administered directly into the target area. Drug administration Drugs are commonly administered through the following routes. Each method has different pharmacokinetic implications: ● Oral (swallowed). ● Intravenous. ● Sublingual (under the tongue). ● Rectal (suppository). ● Intramuscular. ● Transdermal. ● Subcutaneous. ● Inhalation. ● Topical. ● Transdermal. Drugs typically enter the body through areas that are some distance from the targeted tissue. Before a drug can enter the bloodstream, it must be absorbed from the site of administration. The completeness or efficiency of absorption, as well as the rate, varies according to the type of drug and route of administration. Drugs actions may be local, limited to a specific area, or systemic, meaning the drug enters the body tissues through the vascular and lymphatic systems. Most drugs intended for local action are applied topically to the target area, while oral or subcutaneous administration is typically used for systemic action. Topical applications, used in adequately large or frequent doses, may also be absorbed into systemic circulation over time. Topical agents are applied to the skin or mucous membrane, and may be intended for absorption through the cheek, throat, nose, cornea, ear, urethra, rectum or vagina. Topical preparations take many forms, including creams, gels, plasters and patches, as well as sprays, powders and suppositories. Transdermal medications are applied to the skin for systemic effect. They typically take the form of patches that stick to the skin, and may be worn for a period of hours or days. Orally administered medications, which are taken by mouth, may be in the form of a solid pill, tablet, capsule or lozenge, but may also be powder or granular in form. Orally administered preparations also take the form of liquids and may be referred to as solutions, emulsions, syrups or tinctures, among other names. Oral medication is convenient, but may be slower and less complete than dosage through parenteral (non-oral) channels. Solid medication must be dissolved and withstand exposure to Pharmacokinetics Pharmacokinetics refers to the movement of drugs within the body and its effects, often in relation to a specific time-frame. Once a drug is administered though one of the routes discussed above, it is absorbed, distributed, metabolized and excreted by the body. Many factors,
Drug
Pharmacokinetics Absorption into the blood
Distribution to the tissues
Pharmacodynamics
Elimination
Receptor(s)
Therapeutic Toxic Effects Effects
Note . Masters & Trevor, 2012.
including the drug’s composition, the dose and the health or condition of the client, determine the therapeutic value of the drug and manner and the timing in which it undergoes absorption, distribution, metabolism and excretion.
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Book Code: MIL1224
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