DosageThe amount of medication to be administered isreferred to as the dose. The study of dosage and thecriteria that influence it is called posology. The dosesgiven in the United States Pharmacopeia and NationalFormulary (USP-NF) are average therapeutic dosesand are known as “usual adult doses.” The followingterms are used in connection with doses.THERAPEUTIC DOSE.—Therapeutic dose isalso referred to as the normal adult dose, the usual doseor average dose. It is the amount needed to produce thedesired therapeutic effect. This therapeutic dose iscalculated on an average adult of 24 years who weighsapproximately 150 pounds.DOSAGE RANGE.—Dosage range is a term thatapplies to the range between the minimum andmaximum amounts of a given drug required to producethe desired effect. Many drugs (such as penicillin)require large initial doses that are later reduced tosmaller amounts. Closely associated with “dosagerange” are the terms minimum dose (the least amountof drug required to produce a therapeutic effect),maximum dose (the largest amount of drug that can begiven without reaching the toxic effect), and toxic dose(the least amount of drug that will produce symptomsof poisoning).MINIMUM LETHAL DOSE.—Minimum lethaldose is the least amount of drug that can produce death.Factors Affecting DosageThe two primary factors that determine orinfluence the dosage of a medication are the age andweight of the patient.AGE.—Age is the most common factor thatinfluences the amount of drug to be given. An infantrequires a lower dose than an adult. Elderly patientsmay require a higher or lower dose than the averagedose, depending upon the action of the drug and thecondition 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, andthe age plus 12 is the denominator. This fraction ismultiplied by the normal adult dose.WEIGHT.—In the calculation of dosages, weighthas a more direct bearing on the dose than any otherfactor, especially in the calculation of pediatric doses.The rule governing calculation of pediatric dosesbased on weight is Clark’s Rule, expressed as follows:The child’s weight in pounds is the numerator, and theaverage adult weight (150 pounds) is the denominator.This fraction is multiplied by the adult dose.OTHER FACTORS THAT INFLUENCEDOSAGE.—Other factors that influence dosageinclude the following:Sex—Females usually require smaller dosesthan males.Race—Black individuals usually require largerdoses, and Asians require smaller doses thanCaucasians.Occupation—Persons working in strenuousjobs may require larger doses than those who sitat a desk all day.Habitual use—Some patients must takemedications continuously, causing their bodiesto build up tolerance to the drug. This tolerancemay require larger doses than their initial dosesto obtain the same therapeutic effect.Time of administration—Therapeutic effectmay be altered depending upon time ofadministration (e.g., before or after meals).6-2age in yearsage in years +12adult dose = child's doseExample: The adult dose of aspirin is 650 mg.What is the dose for a 3-year-old child?33 + 12mg15= 130 mg650weight in pounds150adult dose = child's dose60 lbs150 lbs650 mg = 260 mgExample: The adult dose of aspirin is 650 mg.What is the dose for a child weighing 60 pounds?