Quiz 2: Biological Factors Affecting the Action of Drugs

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A drug, after the administration undergoes absorption, metabolism excretion and reabsorption. After the absorption from the gastrointestinal tract, the drug appears in the blood. The time required by the concentration of the drug, in plasma or blood, to become half of the original quantity is called the half-life. The importance of half-life is to determine the frequency of the drug administration. This helps to maintain the plasma concentration above the minimal effective concentration. The rate of the drug metabolism by liver and the rate of the drug excretion by kidneys are the two important factors determining the half-life. If 100mg of a drug with a half-life of 4 hours is administered intravenously, it directly reaches the blood without any delay. After 4 hours, the blood concentration of the drug is 50%. Again after 4 hours, the drug becomes 25% in blood. Thus it will be as follows:- • 4hours- 50% • 8hours- 25% • 12hours- 12.5% • 16hours- 6.25% • 20hours- 3.125% • 24hours- 1.5625% • 28hours- 0.78125% • 32hours- 0.39% • 36hours- 0.195% • 40hours- 0.0976% • 44hours- 0.0488% • 48hours- 0.0244% • 52hours- 0.0122% • 56hours- 0.006% Hence, approximately it will take about 56hours or more, to be totally eliminated from the body. After each half-life the plasma level to provide the therapeutic effect decreases. Thus the time for the next administration is decided based on the effective plasma drug concentration.

Out of the whole amount of the drug administered, only a portion reaches the blood stream to be distributed and reached to the target region. Bioavailability is the percentage of the dose of the drug absorbed into the blood stream. Several factors like drug formulation, route of administration and the factors influencing the absorption from the gastro intestinal regions affect the process of bioavailability. Individual variations in age; weight; sex and percent of body fat; genetic variation, emotional state; placebo effect; presence of other diseases and patient compliance are the important factors affecting bioavailability. a. The patient has a history of stomach complaints which is a factor that could affect the absorption process influencing the bioavailability of the drug. In normal individuals the dosage is calculated for 150pounds. An important factor to be considered for the dosage is his weight which comes 324 pounds. Thus, he requires more dosage of the drug. b. The woman can be cured from muscle pain by providing a muscle relaxant and pain reliever. Here the factor involving in the cure of the disease is placebo effect. Even if, the woman doesn't take any medication, she will be relieved from the muscle pain after a while of taking rest. But a prescription can cause a placebo effect of positive attitude which will enhance her recovery from discomfort. So the dosage of the drug can be made minimal. c. The abnormality in the liver function tests indicates the failure in the liver function for drug metabolism. The factor involved to adversely influence the drug metabolism is disease or abnormal functioning of liver which can in turn result in the accumulation of the drug in the plasma. Hence the dosage is to be decreased in order to reduce the plasma drug levels. d. As the woman is emotionally distressed, the effects of drug in the body is different from that of a normal individual. A person in an emotionally altered state needs more dosage of the drug than that for an emotionally balanced individual. Hence the dosage is to be increased. e. Age is a factor to be considered for the effects of the drug. Infants, children and elderly people are more sensitive and the drug dosage is to be decreased to keep the plasma drug level in a therapeutic dose. Hence the adverse effects of the drug can be avoided.

The most common drug administration routes include parenteral and oral. Parenteral route of administration is any route where gastrointestinal tract is not involved and it includes topical application, inhalation and hypodermic injection. The most common parenteral route of administration includes intravenous injection, intramuscular injection, inhalation topical application. Inhalation is a route of administration in which the drug is administered through nose or mouth and then through respiratory inspiration into lungs. The approximate onset of action is within 1 minute. Hence, the option (A) is incorrect. Intramuscular route of administration is a route in which the drug is administered into the deltoid or gluteal muscles and the approximate onset of action is usually within several minutes. Hence, the option (C) is incorrect. Sublingual route of administration is a route in which the drug is administered by placing under the tongue and the approximate onset of action is several minutes. Hence, the option (D) is incorrect. Intravenous route of administration is a route in which the drug is administered through veins directly into the circulation. The approximate onset of action is almost immediate. Hence, the option (E) is incorrect. Transdermal route of administration is a route in which the patches of drug is placed on the skin for local effects or for systemic effects. The approximate onset of action is 30 to 60 minutes. Hence, the option img is correct.

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