POSOLOGY

Dr. M. ArunSundar
Assistant Professor
School of Pharmacy
Sathyabama Institute of Science and Technology
Chennai
Posology
This word is derived from Greek word posos – how
much and logos means science.
So posology is a branch of medical science which
deals with dose & quantity of drugs which can be
administered to a patient to get a desired action. In
this, there are many factors which influence the
doses.
Factors affecting dose
1. Age 9. Accumulation

2. Sex 10. Additive effects

3. Body Weight 11. Synergism

4. Route of Administration 12. Antagonism

5.Time of Administration 13. Idiosyncrasy

6. Environmental Factors 14. Tolerance

7. Emotional factors 15. Tachyphylaxis

8. Presence of disease 16. Metabolic disturbance

1. AGE
The pharmacokinetics of many drugs changes with age.

Newborn infants (pediatric) are abnormally sensitive to

certain drugs because of the immature state of their
hepatic and renal function by which drugs are inactivated
and eliminated from the body. Failure to detoxify and
eliminate drugs results in their accumulation in the tissues
to a toxic level.

Elderly patients are more sensitive to some drug effect

e.g. hypnotics which may produce confusion state in
them.
2. SEX
Women do not always respond to the action of drug in the
same manner as it done in men. Special care should be taken
when drugs are administered during menstruation, pregnancy
& lactation.
The strong purgative eg. Aloes should be avoided during
menstruation. Similarly the drugs which may stimulate the
uterine smooth muscles e.g. drastic purgative, antimalarial
drugs, ergot alkaloids are contra indicated during pregnancy. -
Alcohol, barbiturate, narcotic drugs acts on foetus through
placenta.
During lactation, morphine, tetracycline avoided because its
affect on babies.
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3. BODY WEIGHT
The average dose is mentioned either in terms of mg
per kg body weight.
Another technique used as a total single for an adult
weighing between 50-100kg.
However, the dose expressed in this fashion may not
apply in case of obese patients, children &
malnourished patients. It should be calculated
according to body weight
4. ROUTE OF ADMINISTRATION
I.V doses of drug are usually smaller than the oral
doses.
Intravenous route this might enhance the chances of
drug toxicity.
The effectiveness of drug formulation is generally
controlled by the route of administration.
5. TIME OF ADMINISTARTION
The presence of food in the stomach delay the
absorption of drug & rapidly absorbed from the
empty stomach.
But it does not mean that much effective when taken
during or after meal.
Iron, arsenic & cod-liver oil should be given after
meal & antacid drugs taken before meal.
6. ENVIROMENTAL FACTORS
Stimulant types of drug are taken at day time and
sedative types of drugs are taken at night. So the dose
of a sedative required in day time will be much higher
than at night.
Alcohol is better tolerated in winter than in summer
7. EMOTIONAL FACTORS
The personality & behavior of a physician may
influence the effect of drug especially the drugs
which are intended for use in a psychosomatic
disorders.
The females are more emotional than male &
required less dose of certain drugs. Inert dosage
forms called placebos which resemble the actual
medicament in the physical properties are known to
produce therapeutic benefit in disease like angina
pectoris & bronchial asthma.
8. PRESENCE OF DISEASE
Drugs like barbiturates & chlorpromazine may
produce unusually prolonged effect in patient having
liver cirrhosis. Such as, streptomycin produce toxic
effect on these patient their kidney function is not
working properly because streptomycin excreted
through kidney.
9. ACCUMULATION
Some drugs produces the toxic effect if it is repeatedly
administered for long time e.g. digitalis, emetine,
heavy metals because these drugs excreted slowly.
This occurs due to accumulative effect of the drug.
10. ADDITIVE EFFECT
When two or more drugs administered together is
equivalent to sum of their individual pharmacological
action, the phenomenon is called as additive effect.
E.g ephedrine & aminophylline in the treatment of
bronchial ashtma.
11. SYNERGISM
When desired therapeutic result needed is difficult to
achieve with single drug at that time two or more
drugs are used in the combination form for increasing
their action this phenomenon is called synergism. -
E.g. procaine & adrenaline combination, increase the
duration of action of procaine.
12. ANTOGONISM
When the action of one drug is opposed by the other
drug on the same physiological system is known as
drug antagonism. The use of antagonistic response to
drugs is valuable in the treatment of poisoning. E.g.
milk of magnesia is given in acid poisoning where
alkaline effect of milk of magnesia neutralise the
effect of acid poisoning. When adrenaline &
acetylcholine are given together, they neutralise the
effect of each other due to antagonism because
adrenaline is vasoconstrictor & acetylcholine is
vasodilator.
13. IDIOSYNCRASY
Idiosyncrasy is also called as allergy. An extraordinary
response to a drug which is different from its
characteristic pharmacological action is called
idiosyncrasy.
E.g. small quty. of aspirin may cause gastric hemorrhage.
E.g some persons are sensitive to penicillin &
sulphonamide because they produce severe toxic effect.
14. TOLERANCE
When an unusually large dose of a drug is required to
elicit an affect ordinarily produced by the normal
therapeutic dose of the drug, the phenomenon is
called as drug tolerance. E.g. smokers can tolerate
nicotine, alcoholic can tolerate large quantity of
alcohol. The drug tolerance is of two types:
True tolerance, which is produced by oral &
parenteral administration of the drug.
Pseudo tolerance, which is produced only to the oral
route of administration.
15. TACHYPHYLAXIS
Tachy = fast, phylaxis = protection
When some drugs administered repeatedly at short
intervals, the cell receptors get blocked up &
pharmacological response to that drug decreased. The
decreased response cannot be reversed by increasing
the dose this phenomenon is called tachyphylaxis or
acute tolerance. - E.g. ephedrine given repeated dose
at short intervals in the treatment of bronchial asthma
may produce very less response due to
tachyphylaxis.
16. METABOLIC DISTURANCE
Changes in water electrolyte balance & acid base
balance, body temperature & other physiological
factor may modify the effect of drug.
E.g. salicylates reduce body temperature in only in
case an individual has rise in body temperature. They
have no antipyretic effect if the body temperature is
normal.
Calculations of doses-
Doses proportionate to age:

Young’s formula: (For children under 12 years of age)

Dose for child = Age in years x Adult Dose
(Age in years + 12)

Dilling’s formula: (For children aged 4-20 years)

Dose for child = Age in years x Adult Dose
20
Doses proportionate to body weight:

Clark’s formula:

Dose for child = Child’s weight in Kg x Adult dose

70

*70 indicates average body weight of an adult

Doses proportionate to surface area:
Child dose = Child’s body surface area x adult dose
Avg. body surface area of an adult

Average body surface area an adult = 1.85m2

The nomogram chart is used to determine the

patient's body surface area based on the height and
weight.
Veterinary doses.
The dose required for animals are more or on higher in
comparison of human beings just because of body
weight, size etc.
Therefore it’s very important for supply the correct dose.
Many factors influencing the dose for humans are
applicable to animals.
Factors affecting on veterinary doses.
1. Age 11. Species
2. Sex 12. Character of drug
3. Body Weight
4. Route of Administration
5. Time of Administration
6. Environmental Factors
7. Habit
8. Rate of elimination
9. Effect of drug
10. Purpose of medication