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
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
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