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?