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POSOLOGY - In Pharmacy

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POSOLOGY (Greek 'Posos-HOW MUCH and logos-SCIENCE') is the science of doses.Since the optimum dose of drug to produce the desired therapeutic effect varies from person to person,therefore the doses of drug are actually expressed in the form of range.The prescribed dose is that which exert the Therapeutic effect in the majority of subject.It should be minimum for eliciting the desired therapeutic response and maximum that can be tolerated by an average subject.


  • AGE :Children do not react in the same fashion as the adults and with a few exception,drugs are more active and toxic in children than in adults.This is due to many factors which are different from adults,for example,the newborn infants have relatively HIGH TOTAL WATER BODY CONTENT,LOW BODY FAT CONTENT,IMMATURE RENAL AND HEPATIC FUNCTIONS,ALTERED PROTEIN BINDING AND ALTERATION IN GASTRIC ACIDITY.But in some cases,activity of certain metabolic pathways such as, CONJUGATION and OXIDATION in children may actually exceed the adult values and necessitate the administration of high doses on mg/kg basis than those required to adult.e.g. In the case of Theophylline. In old people,since the metabolism of drug may diminish and many function decline with age they require different dose than adults.Thus ,with some exceptions drug tend to produce greater and more prolonged effect at the extremes of life.

  • BODY WEIGHT AND SURFACE AREA :The relative proportions of muscular and adipose tissue in the individual may be sufficient to alter the distribution and clearance of a drug from the body.Therefore the dose of children is usually calculated on body weight basis, in the terms of mg/kg/day.Many physical factors such as PLASMA VOLUME,OXYGEN CONSUMPTION,REQUIREMENT OF BODY FLUID,CALORIES and GLOMERULAR FILTRATION are proportional to surface area.The Surface Area is used to calculate dose;e.g.,anti-cancer drug METHOTREXATE is administered on mg/sq.m. of body surface.

  • OTHERS :The other factors which affect the dose of the drug are Sex,Disease Condition,Route Of Administration,Time of Administration,Frequency Of Administration,Tolerance, Drug Interactions etc.
According to the given factors the given above the doses are calculated using different formula's.The calculation's varies according to Age,Body Weight and Surface Area.

  1. YOUNG'S Formula : Child Dose = [Age (Years) / Age+12] ✖️ Adult Dose.
  2. DILLING'S Formula : Child Dose = [Age (Years) / 20] ✖️ Adult Dose.
YOUNG and DILLING's Formula are generally used for calculation of child dose.
The others formula include:
  1. COWLING'S Formula : [Age (Years) + 1 / 24] ✖️ Adult Dose.
  2. FRIED'S Formula : Child Dose = [Age(Months) / 150] ✖️ Adult Dose.
  3. BASTEDO'S Formula : Child Dose = [Age (Years) + 3 / 30] ✖️ Adult Dose.
  4. GAUBIN'S Formula : 
     Age

     Parts Of Adult Dose

     Age 

     Parts Of Adult Dose

     Under 1 Year   1/12
    		 14-20 Year   2/3
     From 1-2 Year  1/8 21-60 Year Full Adult Dose
     2-3 Year
    		 1/6 61-70 Year 4/5
     3-4 Year 1/4 70-80 Year 3/4
     4-7 Year 1/3 Over 90 Year   1/2
     7-14 Year 1/2  

CLARK'S Formula :  Child Dose = Weight(Pounds) / 150 ✖️ Adult Dose.
OR
Child Dose = Weight(Kg) / 70 ✖️ Adult Dose.

Here,the average weight of an adult is taken as 150 lb or 70 kg.

Recently, the calculation of according to surface area has gained wide acceptance. Number of body functions and requirements are proportional to the surface area of the body.The formula for dose calculation is-

log S = 0.425 log W + 0.725 log H + 1.8564.

Where,S is surface area,W is weight in kg and H is height in cm.

These are the formula's from which one (Pharmacists) can calculate the dose of the drug given to the patient.

PHARMACISTS THOUGHT
ITS EASY TO GET AND READ THOUSAND'S OF PRESCRIPTION,BUT HARD TO GET ONE SINGLE REMEDY.
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