Factors Which Influence Drug Dosage Effects

Many factors influence how a dose of a particular drug will affect a patient. Since not all patients are the same size, weight, age, and sex, it would be wise to consider how these factors might influence how much drug a person should receive and the effect(s) that drug might have on the patient. The usual recommended adult dose of medication, as found in standard references, is based on the assumption that the patient is a "normal" adult. Such a "normal" (or average) adult is said to be 5 feet 9 inches (173 centimeters) tall and weigh 154 pounds (70 kilograms). However, many people do not fit into this category. Therefore, the following factors should be considered when patients receive drugs:

a. Weight. Obese (overweight) patients may require more medication than thin patients may because the drug has more tissue to which it can go. The dosage of many drugs is calculated on a weight basis. For example, a person might be prescribed a drug that has a dosage of 5 milligrams of drug per pound of patient body weight.

b. Surface Area. A person's height and weight are related to the total surface area of his body. The "normal" (average) adult has a body surface area of approximately 1.73 square meters. A nomogram (see Subcourse MD08O2, Pharmaceutical Calculations) is used to determine the surface area of a patient. The dosage of certain drugs (for example, the anticancer drugs) is determined by the patient's body surface area.

c. Age. As a rule, the very young and the elderly require less than the normal adult dose of most medications. Part of this requirement for less medication is due to the altered metabolism of the drug. Since body enzyme systems greatly influence drug metabolism, considering the differences in these enzyme systems based upon age is important. In the infant, some enzyme systems are not yet fully developed. On the other hand, the enzyme systems of the elderly may not function as well as in the past. Although several formulas are available for calculating a child's dose of medication, the two most accepted methods are those based upon the patient's weight (that is, milligrams per kilogram of body weight) or body surface area (that is, milligrams per square meter of surface area).

d. Sex. Physiological differences between the sexes may influence the dose or the requirement for drugs. Since females have proportionately more fat tissue than males, drugs, which have a high affinity (likeness) for fat, may require larger doses in females. Moreover, estrogen and testosterone, two sex hormones, can affect the patient's rate of metabolism which can, in turn, influence the rate at which a drug is metabolized, absorbed, or excreted from the body. The requirement for iron is much higher in the female than in the male, because of the loss of blood in each menstrual cycle.

e. Genetic Factors. Various racial and ethnic groups have differences in some metabolic and enzyme systems which can affect the utilization of drugs.

f. Physical Condition of the Patient. The physical condition of the patient influences how a particular drug might act. Consequently, the weak or debilitated patient might require smaller doses of some medications. Patients who are in extreme pain may require larger doses of analgesic agents than those patients who are in less pain.

g. Psychological Condition of the Patient. The patient's attitude about his disease or treatment can influence the effectiveness of a drug. It has been shown that patients receiving placebo tablets (tablets that contain no active ingredient) sometimes have the same side effects as the patients who were taking tablets of the same appearance that did contain the drug. In some cases, both types of patients (those taking the placebo and those taking the drug) recovered at the same time.

h. Tolerance. The therapeutic effects of some drugs are lessened in individuals after the drugs have been used for long periods. Thus, an individual who has used such a drug for a long time needs larger doses of the drug than he did when he first began to take it in order to obtain the same effect. This effect is called tolerance. Persons who use opium, heroin, cocaine, amphetamines, and barbiturates develop a tolerance to these substances. Cross-tolerance occurs when the use of one drug causes a tolerance to another drug. Alcoholics, barbiturate addicts, and narcotic addicts develop a cross-tolerance to sedatives and anesthetics.

i. Time of Administration. The time when a drug is administered is important. Some orally administered medications should be taken before meals (that is, on an empty stomach) to increase the amount of drug absorbed into the system. Other oral medications (that is, those that cause irritation to the gastrointestinal tract) should be taken after meals on a full stomach.

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