“usual adult doses.” The following terms are usedin connection with doses:Therapeutic dose. —Also referred to as thenormal adult dose, the usual dose or average dose,it is the amount needed to produce the desiredtherapeutic effect. This is calculated on an averageadult about 24 years old, weighing approximately150 pounds.Dosage range. —A term that applies to therange between the MINIMUM amount of drugand the MAXIMUM amount of drug required toproduce the desired effect. Many drugs, such asantibiotics, require large initial doses that are latertapered to smaller amounts. Closely associatedwith this term are MINIMUM dose, the leastamount of drug required to produce a therapeuticeffect; MAXIMUM dose, the largest amount ofdrug that can be given without reaching the toxiceffect; and the TOXIC dose, the least amount ofdrug that will produce symptoms of poisoning.Minimum lethal dose. —The least amount ofdrug than can produce death.FACTORS AFFECTING DOSAGEIn the administration of medicines there aremany factors that affect the dose, method of ad-ministration, and frequency of the dose. Althougha physician prescribes the amount to be given, youneed to know how and why these quantities aredetermined. The two primary factors that deter-mine or influence the dose are age and weight.AgeAge is the most common factor that influencesthe amount of drug to be given. An infant wouldrequire much less than an adult. Elderly patientsmay require more or less than the average dose,depending upon the action of the drug and thecondition of the patient.The rule governing calculation of pediatricdoses is Young’s Rule as shown below:Age in yearsx Adult dose = child’s doseAge in years + 12The age in years of the child is the numeratorand 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 3-year-old child?WeightIn the calculation of dosages, weight has amore direct bearing on the dose than any otherfactor, especially in the calculation of pediatricdoses. The rule governing calculation of pediatricdoses based on weight is Clark’s Rule shownbelow:Weight of child (pounds)x Adult’s dose = Child’s dose150 poundsThe weight in pounds is the numerator andthe average adult weight, 150 pounds, is thedenominator. This fraction is multiplied by theadult dose.Example: The adult dose of aspirin is 650 mg.What is the dose for a child weighing 60 pounds?60 poundsx 650 mg = 260 mg150 poundsOther factors that influence dosage are:1.2.3.4.5.6.7.Sex. Females usually require smaller dosesthan males.Race. Blacks usually require larger dosesand Asians smaller doses than Caucasians.Genetic make-up. The genetic structure ofthe individual may cause peculiar reactionsto medications in some patients.Occupation. Persons working in strenuousjobs may require larger doses than thosewho sit at a desk all day.Habitual use. Some patients must takemedications chronically, causing theirbodies to build up tolerance to the drug.This tolerance may require larger dosesthan their initial doses to obtain the sametherapeutic effect.Time of administration. Therapeutic effectmay be altered depending upon time of ad-ministration, Example: Before or aftermeals.Frequency of administration. A drug givenfrequently may need a smaller dose thanif administered at longer intervals.7-2

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