Dosage The amount of medication to be administered is referred to as the dose. The study of dosage and the criteria that influence it is called posology. The doses given in the United States Pharmacopeia and National Formulary (USP-NF) are average therapeutic doses and are known as “usual adult doses.” The following terms are used in connection with doses. THERAPEUTIC DOSE.—Therapeutic dose is 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 therapeutic dose is calculated on an average adult of 24 years who weighs approximately 150 pounds. DOSAGE RANGE.—Dosage range is a term that applies to the range between the minimum and maximum amounts of a given drug required to produce the desired effect. Many drugs (such as penicillin) require large initial doses that are later reduced to smaller amounts. Closely associated with “dosage range” are the terms 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 toxic dose (the least amount of drug that will produce symptoms of poisoning). MINIMUM LETHAL DOSE.—Minimum lethal dose is the least amount of drug that can produce death. Factors Affecting Dosage The two primary factors that determine or influence the dosage of a medication are the age and weight of the patient. AGE.—Age is the most common factor that influences the amount of drug to be given. An infant requires a lower dose than an adult. Elderly patients may require a higher or lower dose than the average dose, depending upon the action of the drug and the condition of the patient. The rule governing calculation of pediatric (child’s) doses, Young’s Rule, is expressed as follows: 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. 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, expressed as follows: The child’s weight in pounds is the numerator, and the average adult weight (150 pounds) is the denominator. This fraction is multiplied by the adult dose. OTHER FACTORS THAT INFLUENCE DOSAGE.—Other factors that influence dosage include the following:  Sex—Females usually require smaller doses than males.  Race—Black individuals usually require larger doses, and Asians require smaller doses than Caucasians.  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 continuously, 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 administration (e.g., before or after meals). 6-2 age in years age in years +12 adult dose = child's dose Example: The adult dose of aspirin is 650 mg. What is the dose for a 3-year-old child? 3 3 + 12 mg 15 = 130 mg 650 weight in pounds 150 adult dose = child's dose 60 lbs 150 lbs 650 mg = 260 mg Example: The adult dose of aspirin is 650 mg. What is the dose for a child weighing 60 pounds?