This ppt covers definition of Posology, Therapeutic dose, Official doses, factors deciding dose calculation, and formulae used for child dose calculation. It is useful for medical and pharmacy students

1. Posology
By
Pournima S Shrikhande
Lecturer
Gurunanak technical institute diploma in pharmacy, Nagpur
Maharashtra, India

2. Posos- how much
Logos – science
Posology:
It is the branch of medical science which deals with dosage or quantity of
drugs which can be administered to a patient to get the desired
pharmacological action.

3. The Therapeutic dose:
It is the quantity of drug which maintains the minimum effective
concentration of drug in the system.
Official Dose:
It is the quantity of drug/doses to be taken which is mentioned in the
official pharmacopeia like IP, BP, USP etc.

4. Factors influencing Dose
1) Age
2) Sex
3) Body weight
4) Route of administration
5) Time of administration
6) Environmental factors
7) Emotional factors

5. 8) Presence of disease
9) Synergism
10) Antagonism
11) Idiosyncrasy
12) Tolerance
13) Tachyphylaxis
14) Metabolic disturbance
15) Additive effect
16) accumulation

6. Age:
Age of an individual is one of the important factor affecting the dose and
pharmacological action of drugs.
Age is concerned with the capacity of metabolizing and excreting the drugs from
body
Children and old people need lesser amount of drug than the normal adult dose,
because they are unable to excrete drugs to the extent as adults.
Children can tolerate relatively large amounts of belladonna, digitalis and
ethanol on the basis of high rate of metabolism.
infants do not tolerate large quantity of drug and are highly sensitive due to
their immature physiological functions of liver and kidney.
whereas, elderly patients are more sensitive to some drug e.g. hypnotics and
tranquilizers which may produce confusion states in them.

7. Sex:
Sex, either males or females affect the dose of the drugs because of the
difference in the weight, physique and certain physiological phenomenon
Woman require less doses than that of the male.
Morphine and barbiturates may produce more excitement before sedation in
woman.
The strong purgatives such as aloes should be avoided during menstruation
and pregnancy.
During lactation drug like antihistamine, morphine and tetracycline should be
avoided.

8. Body weight:
Dose of the drug can be calculating according to body weight.
children and malnourished patients require less doses.
Obese patients require larger dose than normal patient.

9. Time of administration:
Presence of food in the stomach delays the absorption of drug. The drug more
rapidly absorbed from empty stomach. So the amount of drug is very effective
when taken before meal.
Appetizer given before meal, digestant given after meal, laxative given at night.
Iron, aspirin, chloroquine phosphate given after meal to avoid gastric irritation.

10. Route of Administration:
Route is selected on the basis of severity of the diseases.
Intravenous doses of drugs are usually smaller than oral doses, because the
drug administered intravenously enters the blood stream directly.
Rate of drug absorption increases
Oral S.C. I.M. Rectal I.V.
Dose of drug increases

11. Environmental Factor:
Environmental Factor:
Environmental Factor:
:
Affect the dose of drug due to some physiological
changes in the body functioning.
Daylight is stimulant enhance the effect of
stimulating drugs and diminish the effect of hypnotics.
Darkness is sedative. Hypnotic are more effective at
night. The amount of barbiturates required to produce
sleep during day time is much higher than the dose
require in the night.
Alcohol is better tolerated in cold environments than in
summer.

12. Presence of disease:
Drug like barbiturates chlorpromazine may produce prolongs effect in
patients having liver cirrhosis. Streptomycin is excreted mainly by
kidney may prove toxic if kidney is not functioning properly
Emotional factors:
The personality and behavior of a physician may influence the effect of
drugs especially the drug which are intended for use in a
psychosomatic disorder.
The female are more emotional than male require less doses.
Placebo medicine

13. Accumulation:
Drug excreted slowly may get accumulated in the body and
produce toxic effects. Repeated administration of drug like
digitalis, emetine and heavy metal may produce toxic effects.
In such condition only maintenance dose of such drug should be
continued to avoid further accumulation.
Hence to avoid the accumulation of the drug one must check liver and
kidney function

14. Idiosyncrasy:
An extra ordinary response to a drug which is different from its characteristic
pharmacological action is called idiosyncrasy. The word idiosyncrasy has now
been replaced by the term drug allergy.
lack of enzyme glucose 6 phosphate dehydrogenase in the body of person
causes hemolysis with primaquine.
Penicillin & sulphonamide
Small quantity of aspirin may cause gastric hemorrhage
and a small dose of quinine may produce ringing in the ears.

15. Antagonism:
When the action of one drug is opposed by the other drug on the
same physiological system is known as drug antagonism.
When adrenaline and acetylcholine are given together, they neutralize the
effect of each other.
Milk of magnesia with acid poisoning drug

16. Synergism:
When two or more drugs are used in the combination form, their action is
increased. The phenomena is called synergism. Synergism is very useful
when desired therapeutic result needed is difficult to achieve with a single
drug
e.g. procaine and adrenaline combination, increases the duration of action
of procaine.
Combination of aspirin and paracetamol increase analgesic effect,
combination of penicillin and streptomycin increase antibacterial effect of
antibiotics.

17. Additive effect:
When the total pharmacological action of two or more drugs administered
together is equivalent to sum of their individual pharmacological action, the
phenomena is called as an additive effect.
For example, combination of ephedrine and aminophylline in the treatment
of bronchial asthma.

18. Tolerance:
When an unusually large dose of drug is required to elicit an
affect ordinarily produced by the normal therapeutic dose of the drug,
the phenomenon is termed as tolerance.
eg. Smokers can tolerate Nicotine
Alcoholic can tolerate large quantity of alcohol

19. Tachyphylaxis:
When certain drugs are administered repeatedly at short intervals, the
cell receptors get blocked up (depletion of NT takes place) &
pharmacological response to that particular drug is decreased. The
decreased response cannot be reversed by increasing the dose .This
phenomenon is known as tachyphylaxis.
Eg. ephedrine when given in repeated doses at short intervals in the
treatment of bronchial asthma may produce very less response due to
tachyphylaxis.

20. Metabolic disturbances:
Changes in water electrolyte balance and acid base balance, body
temperature and other physiological factor may modify the effect
of drugs. The absorption of iron from GIT is maximum if the
individual has iron deficiency anemia.

21. Calculation of doses

22. • Dose proportionate to age:
Young’s formula:
Child dose = Age in years X adult dose
Age in years +12
• Used for calculating the doses for children under 12 years of age.

23. Calculate dose of Paracetamol for 4 years child and whose Adult dose is
500mg.
Solution:
Given: Adult dose = 500 mg, age of child= 4 years
According to Young’s formula :
Dose for child = Age in years X Adult dose
Age + 12
= 4 X 500 mg
4+12
= 125 mg

24. What will be dose for child of 3 yr if the adult dose of drug is 500 mg?

25. If the usual adult dose of drug is 60 mg, what is the dose for
a) A child of 6 years old
b) A child of 8 years old

26. Dose proportionate to age:
Dilling’s formula :
Child dose = Age in years X adult dose
20
• Used for calculating the doses for children in between 4 to 20 years of age.

27. What is the dose for 13 years old children if the average adult dose of drug
is 600 mg?
Solution:
Given: Adult dose = 600 mg, Age of child= 13 years
According to Dilling’s formula :
Child dose = Age in years X adult dose
20
= 13 X 600
20
= 7800
20
= 390 mg

28. If the usual adult dose of drug is 200 mg, what is the dose for a child of 12
years old and a child of 16 years ?

29. • Dose proportionate to age:
Fried’s Formula:
Child Dose = age in month X Adult Dose.
150
• Used for calculating the doses for children under 2 years of age.

30. What is the dose for 6 months old infant if the average adult dose of drug is 500
mg?
Solution:
Given: Adult dose = 500 mg, Age of child= 6 months
According to Fried’s Formula:
Child Dose = age in month X Adult Dose.
150
= 6 X 500
150
= 3000
150
= 20 mg

31. What is the dose for 24 months old infant if the average adult dose of drug is 50
mg?
Solution:
Given: Adult dose = 50 mg, Age of child= 24 months
According to Fried’s Formula:
Child Dose = age in month X Adult Dose.
150
= 24 X 50
150
= 1200
150
= 8 mg

32. • Dose proportionate to body weight:
Clark’s formula :
Child Dose = Weight in pound X Adult dose
150
Child dose = child’s weight in kg X adult dose
70

33. • Calculate the dose for 12 pounds if the average adult dose of drug is 100 mg?
Solution:
Given: Adult dose = 100 mg, weight of child= 12 pounds
According to Clark’s formula :
Child Dose = Weight in pound X Adult dose
150
= 12 X 100
150
= 1200
150
= 8 mg

34. What is the dose of medicament for a child that weight is 15 pound if the
adult dose is 200mg?

35. Dose proportionate to body surface area :
Child Dose = body surface area of child m2 X Adult Dose.
average body surface area of adult
(consider 1.73m2 as adult body surface area)

36. Calculate the dose for child that has body surface area of 0.57 m2 when the average
adult dose of drug is 500 mg?
Solution:
Given: Adult dose = 500 mg, body surface area of child= 0.57 m2
According to formula :
Child Dose = body surface area of child m2 X Adult Dose.
average body surface area of adult
= 0.57 X 500
1.73
= 2.87
1.73
= 164.74 mg

37. What is the dose for 9 months old infant if the average adult dose of drug is
25 mg?

38. What is the dose for child of 5 years age when the adult dose of drug is 100
mg?
The maximum daily dose of drug is 120 mg, how much of it should be given
to a child of 11 years and 15 years?

39. Thank you