“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 ad- ministration,  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 deter- mine  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 x  Adult  dose  =  child’s  dose Age in years + 12 The age in years of the child is the numerator and 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? 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) x  Adult’s dose = Child’s dose 150  pounds 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 x  650  mg  =  260  mg 150 pounds Other  factors  that  influence  dosage  are: 1. 2. 3. 4. 5. 6. 7. Sex.  Females  usually  require  smaller  doses than males. Race. Blacks usually require larger doses and Asians smaller doses than Caucasians. Genetic make-up. The genetic structure of the individual may cause peculiar reactions to  medications  in  some  patients. 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   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. Time  of  administration.  Therapeutic  effect may  be  altered  depending  upon  time  of  ad- ministration,   Example:   Before   or   after meals. Frequency  of  administration.  A  drug  given frequently may need a smaller dose than if administered at longer intervals. 7-2