This document discusses posology, which is the science of calculating drug doses. It defines posology and explains that many factors can influence the appropriate dose of a drug for a patient, including age, sex, body weight, route of administration, and medical conditions. The document provides details on calculating doses for children, adjusting for body weight, and determining veterinary doses for animals based on surface area and weight. Overall, the document outlines the key principles of posology and the various considerations involved in accurately prescribing medication doses for both human and animal patients.

1. POSOLOGY
Prepared By:
Lecturer: Mr. S. S. Mistree
M-pharm (P’ceutics)
Sub: Pharmaceutics-II
Ishwar Deshmukh Institute of Pharmacy, Digras
Yavatmal-445302
2020-21

2. CONTENTS
Introduction
Definition Of Posology
Factors Influencing Dose
Calculations Of Doses
Veterinary Doses
Reference

3. INTRODUCTION:
 The word posology is came from the Greek words.
 The meaning of “Posos” is How Much and “Logos” means
Science.
 Posology is a branch of medical science.
 We can not fixed the dose of drug, because there are many types of
factors, which may influence the doses.
 The official dose of any medicines are given in pharmacopoeia,
which is show the actual average dose of medicines for adults to
be taken within 24 hrs.
 Hence it is a duty of doctors prescribe accurate dose and mention
frequency of medicines in prescription.

4.  It is a responsibility of pharmacist to check carefully the
prescribed doses and prevent to dispense overdose of medicines to
patient.

5. DEFINITION OF POSOLOGY:
Posology: It is a science which teach how to
calculate dose and quantity of drug , which is
need to prescribe to patient in order to get desire
pharmaceutical action.

6. FACTORS INFLUENCING DOSE:
 The ideal dose of medicines only produce desired
pharmacological action but may this action varies from person to
person.
 due to this reason, the dose of official medicines are given in the
form of range, which give the pharmacological action and the
given range of dose depend on the adult requirement.
 There are some factors, which may affect the dose of drugs:
1.Age
2.Sex
3.Body weight
4.Route of administration

7. 5.Time of administration
6. Environmental factor
7. Emotional factor
8. Presence of disease
9. Accumulation
10. Additive effect
11. Synergism
12. Antagonism
13. Idiosyncrasy
14.Tolerance
15.Tachyphylaxis
16. Metabolic disturbances

8. 1.Age:
 The pharmacokinetic of drugs to be altered with age.
 Hence age plays an important role when prescribing the
medicines.
 Adult need high amount of dose than children or old people ,
because they are not able to excrete the drugs as adult can.
Eg: paracetamol- 500 mg is adult dose not for children
Pharmacokinetic: It means ADME of drug
2.Sex:
 Women do not always respond to the action of drug in same
manner as it is done in men.
 In women, morphine produce excitement before sedation but
not happen same in men.

9. Eg: Antihistamine, morphine and tetracycline not given to the
lactation mother, because these drugs are excreted in milk.
3.Body weight:
 The dose is refer as dose for adults body weight between 50-
100 kg.
 But in case of children or obese patient, the dose of drug
calculated according to body weight.
 If adult dose prescribed to children or obese patient may
chance of Causing undesirable effect
4.Route of administration:
 In oral route need high dose than intravenous ,because IV
administration of drug directly goes to blood stream.
 Hence, the effectiveness can control by route of
administration.

10. 5.Time of administration:
 Absorption of drug may decrease in presence of food in
stomach.
 Therefore, effect of drug become more if taken before meal.
Eg: Antacid should be taken before 1 hr of meal for getting
better effect.
6.Environmental factor:
 In day time the effect of stimulant drugs are more.
 Whereas sedative drugs are more effective at night.
Eg: hypnotic has less effective at day and more effective at
night.
7.Emotional factor:
 Personality and behavior of doctor may affect the effect of
drug if he prescribe drug for psychosomatic disorder.

11.  Female some time need less dose, because female are more
emotional than men.
8.Presence of disease:
 If patient have liver cirrhosis, then chloropromazine produce
prolong effect
 In case of fever patient can better tolerated antipyretic drugs
than normal one.
9.Accumulation:
 If drugs are repeatedly taken at short time drug become
accumulate into body, it will slowly excrete out and may cause
toxic effect.
10.Additive effect:
 When two or more drugs taken together, the total action of
drugs become more than their sum of individual action.

12. 11.Synergism:
 When drug taken in combination, their action become
increase.
 Synergism help to get desire action not achieve in single dose.
Eg: Pacacetamol+Aceclofenac+Seratopeptidase
12.Antagonism:
 Action of one drugs opposed by another drug if administrated
together .
 This kind of effect to be useful in poisoning cases.
Eg: Milk of magnesia given in acid poisoning, because milk of
magnesia (alkaline) neutralize the effect of acid.
13.Idiosyncrasy:
 It is a extra effect of drug, which is different from its actual
effect.

13.  The idiosyncrasy word now replaced by allergy.
Eg: Small dose of quinine may cause ringing in ears.
14.Tolerance:
Large dose of drug is need to obtain a effect produced by normal
dose of drug are called as drug tolerance.
Types:
True tolerance: Produced by administration of oral route or
parental route of drug.
Pseudo tolerance: Produced by oral route of administration.
Eg: Alcoholic person may tolerate large quantity of alcohol.
15.Tachyphylaxis:
 If drug administered repeatedly at short interval then the action
of drug decrease due to blocking of cell receptor and this
decreased response can not reversed by increasing dose.

14. Eg: If ephedrine given in repeated dose in short interval may
produce very less effect for bronchial asthma.
16.Metabolic disturbances :
 In which the effect of drug changed due to change in body
temperature, electrolyte balance, acid-base balance
Eg: Salicylic acid work on patient having fever, not work on
normal patient.

15. CALCULATIONS OF DOSES:
 The administration of average dose for adult within 24 hrs by
oral route are represent in official book.
 Whenever other route is used then the dose is adjusted
accordingly.
 Actually , the doses are calculate in proportion to Age, Body
weight and Surface area of patient.
A. Doses proportion to age:
In which, child dose calculated from adult dose by using
following formula.
1. Young’s Formula: For under 12 years age child.
Age in year
Dose for child= ----------------------- X Adult dose
Age in year+12

16. 2. Dilling’s Formula: For children between 4 to 20 years old.
Age in year
Dose for child= ---------------------- X Adult dose
20
B. Dose proportionate to body weight:
Calculating dose for child by using body weight.
Child’s weight in Kg
Dose for child = ------------------------------ X Adult dose
70
C. Doses proportionate to Surface Area(S.A):
Child dose can calculate according to surface area, Its more
satisfactory and complicated than method based on age

17. S.A of child
Percentage of adult dose = -----------------------X 100
S.A of adult

19. VETERINARY DOSES:
 Pharmacist having a compounding or dispensing knowledge of
all medicines whether it is for human beings or animal.
 The dose required for animal are higher side than for human
beings, because of this the dose for animals are calculate on the
basis of their surface area or body weight.
Factors influencing dose of drug in animal:
1. Age
2. Sex
3. Body weight and size
4. Time of administration
5. Route of administration

20. 6.Environmental conditions
7. Habit
8. Rate of elimination
9. The effect of drug
10.Purpose of medication
11. Species
12. Character of drug
1. Age:
 Young animal required less dose than adult one, because
susceptibility reason of young animal.
Susceptibility: lack of ability to resist something.
2. Sex:
 Always female animal required less dose than male animal,
because many drugs are contra-indicated to pregnant animal
and may cause abortion.

21. 3. Body weight and size:
 The dose for animal is depend on their body weight and size.
 The size of animals are varies according to their breeds.
Eg: In case of dogs. Heavy breed can tolerate some drugs but
lighter breeds can not.
4. Time of administration:
 Little dose of drug is more effective if taken in an empty
stomach than food stomach.
 Hence, time of administration of drug consider to achieve
desire effect. (Purgative: remove unwanted waste from body)
Eg: Purgative is more effective in an empty stomach.
5. Route of administration:
 The dose of drugs changes with its route of administration.
Parenteral preparation given in small dose than oral preparation.

22. Oral >S/C>I/M>I/V
6. Environmental condition:
 Environmental moisture and temperature may affect to
animals tissue.
 In rainy season, when the climate little humid and hot, less
dose needed than in winter when climate is dry and cold.
7. Habit:
 Habit affect the dose and may develop tolerance for drug.
 Hence, normal dose fail to produce desire effect, then need to
increase the dose.
8. Rate of elimination:
 This factor affect the dose of drug.
 Those drug are excrete at faster rate , need large dose to
produce effect.

23. 9. The effect of drug:
 When two drugs has similar action , given in combination their
action will increases than their individual dose.
 Otherwise they oppose to another drug. Will not get actual
effect.
10.Purpose of medicine:
 The dose of drug changes with it use
Eg: Mg sulphate: act as purgative in large dose whereas act like
antacid or laxative in small dose.
Laxative: same as purgative but less effective
Antacid: remove gases from GIT or anti-flatulent
11. Species:
 The dose of drugs are not similar for all species.
 The dose for cow is different from horse, sheep, cat, etc.
Eg: opium produce excitement in horse but narcosis in dog.

24. Narcosis: keeps falling in a sleep.
12. Character of the drugs:
 Large dose of Crude drug need to produce effect in animal than
its active constituents.
Eg: Nux-vomica given in large dose than strychnine alkaloid
given in small dose.

25. REFERENCE:
R. M. Mehta, pharmaceutics-II, second adition-2003,reprint
2010,vallabh prakashan. Pg. No. 65 to 72