“usual adult doses.” The following terms are used
in connection with doses:
Therapeutic dose. —Also referred to as the
normal adult dose, the usual dose or average dose,
it is the amount needed to produce the desired
therapeutic effect. This is calculated on an average
adult about 24 years old, weighing approximately
150 pounds.
Dosage range. —A term that applies to the
range between the MINIMUM amount of drug
and the MAXIMUM amount of drug required to
produce the desired effect. Many drugs, such as
antibiotics, require large initial doses that are later
tapered to smaller amounts. Closely associated
with this term are MINIMUM dose, the least
amount of drug required to produce a therapeutic
effect; MAXIMUM dose, the largest amount of
drug that can be given without reaching the toxic
effect; and the TOXIC dose, the least amount of
drug that will produce symptoms of poisoning.
Minimum lethal dose. —The least amount of
drug than can produce death.
FACTORS AFFECTING DOSAGE
In the administration of medicines there are
many factors that affect the dose, method of ad
ministration, and frequency of the dose. Although
a physician prescribes the amount to be given, you
need to know how and why these quantities are
determined. The two primary factors that deter
mine or influence the dose are age and weight.
Age
Age is the most common factor that influences
the amount of drug to be given. An infant would
require much less than an adult. Elderly patients
may require more or less than the average dose,
depending upon the action of the drug and the
condition of the patient.
The rule governing calculation of pediatric
doses is Young’s Rule as shown below:
Age in years
x Adult dose = child’s dose
Age in years + 12
The age in years of the child is the numerator
and the age plus 12 is the denominator. This frac
tion is multiplied by the normal adult dose.
Example: The adult dose of aspirin is 650 mg.
What is the dose for a 3yearold child?
Weight
In the calculation of dosages, weight has a
more direct bearing on the dose than any other
factor, especially in the calculation of pediatric
doses. The rule governing calculation of pediatric
doses based on weight is Clark’s Rule shown
below:
Weight of child (pounds)
x Adult’s dose = Child’s dose
150 pounds
The weight in pounds is the numerator and
the average adult weight, 150 pounds, is the
denominator. This fraction is multiplied by the
adult dose.
Example: The adult dose of aspirin is 650 mg.
What is the dose for a child weighing 60 pounds?
60 pounds
x 650 mg = 260 mg
150 pounds
Other factors that influence dosage are:
1.
2.
3.
4.
5.
6.
7.
Sex. Females usually require smaller doses
than males.
Race. Blacks usually require larger doses
and Asians smaller doses than Caucasians.
Genetic makeup. The genetic structure of
the individual may cause peculiar reactions
to medications in some patients.
Occupation. Persons working in strenuous
jobs may require larger doses than those
who sit at a desk all day.
Habitual use. Some patients must take
medications chronically, causing their
bodies to build up tolerance to the drug.
This tolerance may require larger doses
than their initial doses to obtain the same
therapeutic effect.
Time of administration. Therapeutic effect
may be altered depending upon time of ad
ministration, Example: Before or after
meals.
Frequency of administration. A drug given
frequently may need a smaller dose than
if administered at longer intervals.
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