WHAT HAPPENS AFTER A DRUG HAS BEEN ADMINISTERED CHAPTER 4
Introduction 
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Drugs do not affect all clients the same way. Whether a drug achieves or falls short of achieving a therapeutic response is an important concern to clients and healthcare professions.
Introduction
Within a population, a dose of medication may produce a dramatic response in one client while having no effect in another. Many situations may alter a drug’s response.
Introduction
Clients sometimes take medications under conditions that interfere with drug activity. This interference is called a drug interaction. Well-known examples of food-drug interactions may occur when clients take their medication with food or beverages.
Introduction 
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Clients often take more than one medication at the same time. After drugs have been absorbed, the effectiveness of drug therapy may be altered by drug-drug interactions in the bloodstream.
Introduction 
In order to understand the impact that drug interactions have on drug safety and effectiveness, one must understand concepts from two important areas: pharmacokinetics and pharmacodynamics.
Pharmacodynamics 
Pharmacodynamics is the study of how the body responds to drugs and natural substances.
Pharmacokinetics
Pharmacokinetics is the study of how drugs are handled by the body. How the drugs move within the body. Movement involves 4 processes:
Absorption Distribution Metabolism Excretion
Absorption
Absorption is the first step in how the body handles a drug It is the process involving the movement of a substance from its site of administration across one or more body membranes. Ultimately most drugs move across many membranes to reach their target cells.
Absorption
General factors affecting absorption. Food— Body temperature Temperature of drug when dispensing
Distribution 
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Distribution is the process by which drugs are transported after they have been absorbed or administered directly into the bloodstream. Represents how drugs are transported throughout the body.
Distribution
Factors affecting: Binding: binding that occurs between drugs and other substances already present in the bloodstream, such as plasma proteins. Bioavailability—is used to describe how much of a drug will be available to produce a biological effect after administration.
Distribution
3 important organs contain anatomical barriers that prevent some drugs from gaining access even though these organs have a larger blood supply compared to most other organs in the body, usually only lipidsoluble substances may pass across these cellular barriers The brain—blood-brain barrier The placenta—blood-placental barrier The testes—blood-testicular barrier
Metabolism (Biotransformation) 
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Metabolism is the next step, involves the biochemical pathways and reactions that affect drugs, nutrients, vitamins, and minerals. Metabolism is a process that occurs in almost every cell, although the liver is the primary site.
Metabolism (Biotransformation)
Metabolism is often described as the sum total of all chemical reactions in the body. Individual chemical reactions are often referred to as biotransformation reactions. Metabolism is important to drug therapy because metabolic reactions deactivate most drugs.
Metabolism (Biotransformation) 
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One important mechanisms affecting metabolism and drug action is the firstpass effect. Substances absorbed across the intestinal wall enter blood vessels known as the hepatic portal circulation, which carries blood directly to the liver.
Metabolism (Biotransformation)
Drugs administered by the oral route are absorbed directly into the hepatic portal circulation. This is different than other areas in the body. Most veins lead directly back to the heart. Veins draining the upper digestive tract, however, take the nutrient-rich blood to the liver for metabolism first, before continuing on to the heart.
Metabolism (Biotransformation) 
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Thus, drugs may be rendered inactive by metabolic reactions in the liver before they are distributed to the rest of the body and their target organs. In some cases, the firstpass effect can deactivate over 90% of an orallyadministered drug before it can reach the general circulation.
Metabolism (Biotransformation)
Many clients differ with how efficiently their metabolic enzymes function. Age, kidney and liver disease, genetics, and other factors may dramatically affect enzymatic activity. Some clients metabolize drugs very slowly; others metabolize drugs very quickly.
Metabolism (Biotransformation) 
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Enzyme activity is generally reduced in very young and in elderly clients; therefore, pediatric and geriatric clients are usually more sensitive to medications than middleaged clients. If a client has an impaired liver they need a reduced dose.
Excretion
The last step is excretion. Most substances that enter the body are removed by urination, exhalation, defecation, and/or sweating. Drugs are normally removed from the body by the kidneys, respiratory tract, bile, or glandular activity.
Excretion
The main organ is the kidney. The major role of the kidneys is to remove all nonnatural and harmful agents in the bloodstream while maintaining a balance of the other natural substances. Majority of drugs are excreted by the kidneys.
Rate of Elimination & Half-Life of Drug 
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Rate of elimination refers to the amount of drug removed per unit of time from the body by normal physiological processes. The rate of elimination is helpful in determining how long a particular drug will remain in the bloodstream and is thus and indicator of how long a drug will produce its effect.
Rate of Elimination & Half-Life of Drug
The half-life of a drug is another measurement used to ensure maximum therapeutic dosages. Half-life is defined as the length of time required for a drug to decrease concentration in the plasma by one-half. Another indicator of how long a drug will produce its effect in the body. The larger the half-life value, the longer it takes for a drug to be eliminated.
Rate of Elimination & Half-Life of Drug 
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Ex. A drug with a half-life of 10 hours will take longer to be eliminated from the body than a drug with a half-life of 5 hours. Drugs with longer half-lives may be given less frequently, for example, once per day.
Rate of Elimination & Half-Life of Drug 
Some drugs have a half-life of just a few minutes, while others have a half-life of several hours or days.
Pharmacodynamics 
Pharmacodynamics is the study of how the body responds to drugs and natural substances.
Drugs Activate Specific Receptors
Generally, the response of a drug begins when the agent encounters at the receptor of its target cell. The receptor theory states that most responses in the body are caused by interactions of drugs with specific receptors. Receptors may be located on the plasma cell membrane, or they may be found in the cytoplasm or nucleus.
Drug Therapy Success
Potency and efficacy are terms used to describe the success of drug therapy. Potency relates to the concentration or amount of drug required to produce a maximum response. Efficacy refers to the magnitude of maximal response comparable with another drug.