“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 administration, 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 determine 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/Age in years + 12 x Adult dose = child’s dose
The age in years of the child is the numerator and the age plus 12 is the denominator. This fraction is multiplied by the normal adult dose.
Example: The adult dose of aspirin is 650 mg. What is the dose for a 3-year-old child?
3/3+12 x 650mg/15 = 130mg
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)/150 pounds x Adult’s dose = Child’s dose
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/150 pounds x 650 mg = 260 mg
Other factors that influence dosage are:
1. Sex. Females usually require smaller doses than males.
2. Race. Blacks usually require larger doses and Asians smaller doses than Caucasians.
3. Genetic make-up. The genetic structure of the individual may cause peculiar reactions to medications in some patients.
4. Occupation. Persons working in strenuous jobs may require larger doses than those who sit at a desk all day.
5. 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.
6. Time of administration. Therapeutic effect may be altered depending upon time of administration, Example: Before or after meals.
7. Frequency of administration. A drug given frequently may need a smaller dose than if administered at longer intervals.