Posology means study of dose of drug. Various factors affecting on dose of drug in humans & animals. Dose calculation by using various formulae.

1. POSOLOGY
Prepared & presented by
Ms. Nisha S. Mhaske
Lecturer, PRES’s COPD, Chincholi.
Email : nisha.mhaske@pravara.in

2. LEARNING OUTCOMES
 Learner will be able to understand
 What do you mean by dose & dosage form?
 What is Posology?
 Various factors effect on dose of drug in human & animals
 Various formulae to calculate dose of drug.

3. CONTENT
 Posology
 Factors affecting on dose of drug in humans
 Calculation of child dose
 Factors affecting on dose of drug in animals

4. POSOLOGY
 Branch of science which deals with study of
Doses of drug.
 Posology is derived from Greek word;
Posos-How Much & Logus-Science or study.
 Dose- it is quantity of a drug administered into the
body to produce desired therapeutic effect.
 Dosage form- any suitable physical form of a drug
in which it is administered in a prescribed quantity is
referred as a dosage form of a drug.
 The dosage form may be solid- tablets, capsules,
powders
 Liquid- syrup, suspension, emulsion, mixture
 Semisolid- ointment, cream, paste
 Gaseous- aerosols.

5. VARIOUS FACTORS AFFECTING ON DOSE OF DRUG IN
HUMAN
Body weight Presence of disease
Age Metabolic disturbances
Sex Cummulative effect
Route of administration Additive effect
Time of administration Synergism (Potentiation)
Diet and environmental
factor
Antagonism
Genetic factor Drug Tolerance
Emotional factor Drug Dependence

6. BODY WEIGHT
 The dose of drug is related to body weight.
 The dose of drug is usually expressed as mg/kg of body weight.
 Body weight based formula
 Clarks formula
Child dose = Weight of child in Kg X Adult Dose
70
Clarks rule for infants
Dose for infant = Weight in pound X Adult Dose
150
1 kg=2.205 pounds (lbs)
Pound is a Latin word meaning Weight.
Lbs has been derived from a roman word Libra, it is represented by “lb”
or “lbs” international term.

7. AGE
 Infants and old patients need dose different from adults.
 In Infants, the metabolizing enzymes and excretory organs and
processes are not fully developed.
 Hence doses of children should be smaller than adults.
 Age based formula
1. Young’s formula- for children 2 to 12 year age.
Child dose = Age in years X Adult Dose
Age in Years +12
2. WJ Dilling formula
Child dose = Age in years X Adult Dose
20
3. Frieds rule for Infant
Infant dose = Age in months X Adult Dose
150

8. 4. Cowling’s rule
Child dose = Age at next Birthday (in Years) X Adult Dose
24

9. SEX
 Drug responses differs according to sex.
 Females requires smaller doses of drugs due to their lesser
body weight.
 Drugs must be administered with due care in females as they
have to pass through delicate periods of life like menstruation,
pregnancy, lactation.
 Examples
 Morphine crosses placental barrier and depress foetal respiration are
avoided in pregnant women.
 Many drugs are excreted in milk and either discourage breast feeding
or innocently appears in foetal circulation and produce toxicity.
 Drugs producing pelvic congestion like drastic purgatives should be
avoided during menstruation.

10. ROUTE OF ADMINISTRATION
 As the route of administration changes, the dose required
for same pharmacological action varies.
 Hence IV dose is smaller than SC dose which is also
smaller than oral dose.
IV Dose < SC Dose < Oral Dose

11. TIME OF ADMINISTRATION OF DRUGS
 When the drugs are taken before meals, the absorption is Greater, but there is
irritation to GIT due to direct contact of drug with walls of GIT.
 Some drugs are advised to be taken in an empty stomach:-
i. To get quick action
ii. To avoid interference of food
iii. To prevent destruction of drugs by digestive enzymes.
iv. Eg-Anti-motion sickness agent & Antibiotics-Penicillin, tetracycline
 When the drugs taken after meals, the absorption is poor but there is no
irritation to GIT. This is because drug is mixed with food material hence
direct contact of drugs with walls of GIT is avoided.
 So, most of drugs are advised to be taken after meal, so as to reduce risk of
gastric irritation, nausea, & vomiting.
 Eg- Salicylates & derivatives- Aspirin
And

12. DIET AND ENVIRONMENTAL FACTOR
 In the presence of diet, the absorption of drug is poor.
 Environmental factor also affects the variation in the doses of
drugs.
 Example
 The alcohol is well tolerated in cold environment than in
summer.
 During day time central nervous system is highly active.
Hence dose of barbiturate (Phenobarbitone) required to
produce a sleep during day time is larger than the dose
required during night.

13. GENETIC FACTOR
 The patients with hereditary metabolic disorders
rarely show a disturbance in the metabolism of
drugs.
 This is because of microsomal enzyme systems
involved in the metabolism of drugs.
 Thus the genetic factor gives individual
variations in response to drugs.
 Some drugs pass through genes and modify drug
response from generation to generation.
 Eg-Diabetes mellitus.

14. EMOTIONAL FACTOR
 The personality of the physician may influence the drug effect particularly
in psychic patients. Hence nervous patients require smaller dose of drugs
as compared to normal patients.
 The inert dosage forms, which resemble the actual medicament only in
physical properties like color, size, shape, smell is called as Placebo.
 Placebos are effective in conditions like angina pectoris, bronchial asthma
and psychic disorder to avoid aggravation of condition due to emotional
factors.

15. PRESENCE OF DISEASE
 Drug responses are altered in presence of diseases.
 Eg- in Liver cirrhosis, drugs like barbiturate and
chlorpromazine may produce prolonged effects.
 Drugs which are excreted mainly by the kidney may prove
toxic if administered during kidney impairment.

16. METABOLIC DISTURBANCES
 The changes in water balance, electrolyte balance, acid-
base balance, body temperature and other physiological
factors may affect drug response.
 Eg- salicylate reduce body temperature only in case of
pyrexia.
 Absorption of Iron from GIT is more in iron deficient
anaemia.

17. CUMULATIVE EFFECT
 When excretion of drug is slow, repeated administration of drug
accumulates in the body and may lead to toxicity. This is known as
cumulation & the phenomenon is known as cumulative effect.
 Examples
 Digitalis, emetine, heavy metals, chloramphenicol show
cumulation.
 Cumulation can be avoided by following ways:-
 Stop the administration of drug at the appearance of first warning
symptoms.
 It must be known that the drug is eliminated slowly or rapidly.
 Carefully select the dosage form in which the drug is to be
administered.
 Check the liver and kidney function before and during drug
administration.

18. ADDITIVE EFFECT
 When two or more drugs administered, act on same
physiological system. The total pharmacological action is
equivalent to sum of their individual pharmacological actions.
This phenomenon is termed as “Additive effect.”
 Eg- Combination of ephedrine and aminophylline shows
additive effect in the treatment of bronchial asthma.
1+1 = 2

19. SYNERGISM (POTENTIATION)
 When two or more drugs are used in combination form, their
action is increased, the phenomenon is called as synergism.
 Examples
 Aspirin + codeine as analgesic
Where Syn means With & Ergo means Work.
1+1 = 3

20. Sulphamethoxazole + trimethoprim as antibacterial

21. ANTAGONISM
 When the action of one drug is opposed by the other drugs on the
same physiological system is known as drug antagonism.
 Opposite actions of two drugs on the same physiological system.
 Eg- when adrenaline and acetylcholine are given together, they
neutralize the effect of each other due to antagonism because
adrenaline is vasoconstrictor and acetylcholine is vasodilator.

22. IMPORTANCE OF ANTAGONISM
 Antagonism is useful in poisoning cases to block
the actions of poison.
 Antagonism is useful to control the adverse effects
of the drugs.
 Antagonism is useful to adjust the doses of the
drugs combined.

23. DRUG TOLERANCE
 When larger dose of drug is required to get an effect, which is
produced by the normal therapeutic dose of a drug, the
phenomenon is known as drug tolerance.

24. 1.True Tolerance
i. Natural Tolerance
a. Species Tolerance
b. Racial Tolerance
ii. Acquired tolerance
a. Tissue Tolerance
b. Cross Tolerance
3.Tachyphylaxis
or
Acute tolerance
2. Pseudo Tolerance
or
False tolerance
or
Apparent tolerance

25. 1. TRUE TOLERANCE
 True tolerance is seen on both oral and parenteral
administration.
a. Species Tolerance
 Some animal species can tolerate large amount of
particular drug which may prove to be lethal (Toxic) to
man.
 Eg- Rabbits can tolerate large amounts of belladonna
due to presence of atropine esterase in liver.
i. Natural Tolerance

26. b. Racial Tolerance- In humans
 When Ephedrine is instilled into the conjunctiva sac of the
Caucasians, it produces prompt dilation of pupils but in Negros
ephedrine does not produce any dilation at all as they are tolerant to
sympathomimetic amines.
 Eskimos can tolerate high fat diet and carbohydrates.
Caucasians
White skin, European
origin Negros
Black skin, Spanish,
Portuguese
Eskimos
Indigenous- Canada,
Greenland, Siberia

27. ii. Acquired tolerance
 Repeated administration or consumption of a drug results in slow
development of resistance to its effect. Due to this, to achieve same
therapeutic response, it is essential to increase the dose of drug. This
phenomenon is called as acquired tolerance.
 Eg- morphine, cocaine, heroin, alcohol, nicotine, barbiturates etc.
a. Tissue Tolerance b. Cross Tolerance
Tolerance is developed to the certain
tissue, organ or systems.
Tolerance is developed to the different
drugs of same group or category.
Morphine produces tolerance for its
euphoria effect but pupil and GIT
never become tolerated.
Tolerance to Vasodilator effect of
Glyceryl trinitrite and pentaerythriol
tetra nitrite.

28. 2. Pseudo Tolerance / False tolerance /Apparent tolerance
 It is observed only by Oral route.
 When small dose of poison is taken orally, tolerance is
developed to that poisons by the GIT. This is possible after
repeated administration of small quantity by oral route in the
individual immune to such poison.
 This tolerance to the oral administration of drug is probably due
to local changes developed by GIT which prevent poison from
getting absorbed into the systemic circulation.

29. 3.Tachyphylaxis or Acute tolerance
 The repeated administration of drug within a short interval of
time, decreases the pharmacological response progressively. This
phenomenon is known as tachyphylaxis or acute tolerance.
 Tachyphylaxis probably can occur if the drug dissociates only
slowly from its combination with receptor and thus continuing
receptor blockade, while loosing its intrinsic activity, i.e. looses
its pharmacological effects.
 Eg- Ephedrine in the treatment of bronchial asthma.

30. Tolerance Tachyphylaixs
Reduces responsiveness to drug. It is also called acute tolerance.
It develops slowly It develops quickly
In this phenomenon original
effect may be obtained by
increasing the dose
Original effect may not be
obtained by increasing the dose.
Eg- Barbiturates, vasodilators Eg- Ephedrine

31. DRUG DEPENDENCE
 It is a psychic or physical condition of the person due to interaction
between living organism and drug, which includes a compulsion to
take the drug and tendency to increase the dose at the cost of health.
 Example- morphine, heroin, alcohol, tobacco etc.
Physical dependence Psychic dependence
It is the condition in which the body
shows dependant stage on the drug.
If the drug is withdrawn, the intense
physical disturbances occur.
A condition in which a drug
produces a feeling of satisfaction
and a psychic drive that requires
periodic or continuous
administration of the drug to
produce pleasure and to avoid
discomfort is called as psychic
dependence.

32. METHODS OF CALCULATION
Methods of
calculation
Proportion to Age
1. Young’s rule
2. Diling’s rule
3. Fried’s rule
4. Cowling’s rule
Proportion to Body
weight
1. Clark’s rule for
infants
Proportion to body
Surface area
1. Catzel’s formula

33. Proportion to body Surface area
1. Catzel’s formula
Dose for patient= Surface are of patient (m² ) X Adult dose
Average adult surface area(1.73 m² )

34. CALCULATION OF DOSES
 Dose proportionate to surface area
Percentage of adult dose = Surface are of child X 100
Surface area of adult
 The body surface area is calculate from the height and weight of the child.
Determination of Child dose from adult dose on the basis of Body Surface area
Sr. No. Weight (Kg) Approx. surface area in square
meters
Approx. percentage of adult dose
1 2 0.15 9
2 4 0.25 14
3 6 0.33 19
4 8 0.40 23
5 10 0.46 27
6 15 0.63 36
7 20 0.80 46
8 25 0.95 55
9 30 1.08 62
10 35 1.20 70

35. VETERINARY DOSES
 Pharmacist should have complete knowledge of posology which pertains to
animal medication.
 The doses required for animals are more or in higher side in comparison to
human beings beacause the weights and surface area of animals is normally
more than that of human beings.
 Doses for animals are normally mentioned on body weights.

36. VARIOUS FACTORS AFFECTING ON DOSE OF DRUG IN
ANIMALS
Age Habit
Sex Rate of elimination
Body weight & size The effect of drug
Time of administration Purpose of medication
Route of administration Species
Environmental conditions Character of drug

37. Age
 Young animals need less dose of drug as compared to adults, because
young animals are more susceptible than adults.
Sex
 Female require less dose than the male animals.
 Certain drugs are contraindicated in pregnant animals because of hazards
of inducing abortion.
 Some drugs have tendency to pass into milk which becomes unfit for
human consumption.
 Such drugs must be used carefully when administered in milch animals.

38. Body weight & Size
 Dose of drug in animals depend on their body weight & size.
 The size of animals varies according to its breeds.
 Eg- In case of dogs, a dose which may be harmless for a heavy breed of
dog may be dangerous to dogs of lighter breed.
Time of administration
 A small dose of a drug may be more effective if given in an empty stomach
rather than the same when given after meals.
 Purgatives and anthelmintics are more effective when given in empty
stomach.
 Hypnotics are more effective when given after sunset than given early in
the morning.

39. Route of administration
 Maximum dose of drug is required when the drug is given orally.
 Minimum dose is required when drug is given by Intravenous route.
Environmental conditions
 In rainy season, when the climate is quite humid & hot, less dose is
required than in winter when climate is dry & cold.
 Atmospheric moisture and temperature have a great influence on dose of
drug in animals.
Habit
 In animals, drug tolerance can develop when animal constantly under the
influence of a drug.
 In that case, normal dose fail to produce the desired effect, hence larger
doses are required to produce the required effect.
Oral > SC > IM > IV

40. Rate of elimination
 Those drugs which excreted at a faster rate require larger dose of the drug.
 Those drugs which excreted at a slower rate require smaller dose of the drug.
The effect of drug
 Synergistic & antagonistic effect may occur in animal doses.
Purpose of medication
 Dose of drug varies with the purpose for which it is used.
 Eg- magnesium sulphate acts as purgatives in large doses but in smaller doses
it acts as antacid and laxative.

41. Species
 Dose of drug varies from species to species.
 The dose of drug for a cow will be different from horse, sheep, pig, cat etc.
 Eg- opium is a narcotic even than it produces excitement in horse and cattle
but it shows narcosis in dog.
 These differences in action are due to anatomical and physiological
peculiarities.
Character of the drug
 Larger doses of drug are required if it given in crude form to the animals.
 Eg- nux vomica powder is required in larger doses, but its active
constituent strychnine alkaloid is required in smaller dose.

42. ASSIGNMENT NO- 3
1. Define posology.
2. Define dose & dosage form.
3. Enlist and describe factors affecting on dose of drug in human.
4. Give the formulae for the calculation of pediatric doses.
5. Calculate the dose of phenacetin for 8 years child using WJ
Dilling’s formula. Adult dose-600 mg (Ans-240 mg)
6. Calculate dose of aspirin for a child of 6 yrs age. Adult dose is
1 gm. (Ans-0.5 gm)
7. Calculate the dose of ferrous fumerate for 3 months old infant.
Adult dose 0.6 gm (Ans- 0.012 gm)