Definition, Advantages and Disadvantages

1. POSOLOGY
Ms. Chitralekha G. Therkar
Assistant Professor
Department of Pharmaceutics,
Siddhivinayak College of Pharmacy, Warora

2. INTRODUCTION
 Posology is a branch of medical science which deals with dose and
quantity of drugs which can be administered to a patient to get its
pharmacological effect.
 Posology is derived from two Greek words “posos” meaning how
much and “logos” meaning science.
 Posology is the study of how medicines are dosed as the dose of
drug cannot be fixed very easily, it depends upon various factors
including age, climate, weight, gender and time of administration.
 It includes dosage for each indication, each method, route of
administration for appropriate dose recommendations, Frequency
of dosing etc. Eg, mg, mg/kg, mg/m2 e.g. once or twice daily or
every 6 hours.

3. 1 Age
2 Sex
3 Body weight
4 Route of Administration
5 Time of administration
6 Environmental factors
7 Emotional factors
8 Presence of disease
9 Accumulation
10 Additive effects
11 Synergism
12 Antagonism
13 Idiosyncrasy
14 Tolerance
15 Tachyphylaxis
16 Metabolic disturbance's
17 Hypersensitivity
Factors affecting dose

4. 1.AGE
The pharmacokinetics of many drugs changes with age. Newborn
infants (pediatrics) 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. Whereas, elderly
patients are more sensitive to some drug effect e.g. hypnotics
which may produce confusion state in them.
2.Gender
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 and
lactation. Eg, Aloes should be avoided during menstruation. And
antimalarial drugs, ergot alkaloids are contraindicated during
pregnancy also alcohol, barbiturate, narcotic drugs acts on foetus
through placenta so it should be avoided. During lactation,
morphine, tetracycline avoided because of its affect on babies.

5. 3. BODYWEIGHT
The average dose is mentioned in terms of mg/kg body weight.
Or another technique is used as a total single for an adult weighing
between 50-100kg meaning it includes same dose dispensing for all
body weights. However, the dose expressed in this fashion may not
apply in case of obese patients, children and malnourished patients.
It should be calculated according to body weight.
4. ROUTE OFADMINISTRATION
The effectiveness of any drug formulation is generally controlled by
the route of administration. Intravenous doses of drug are usually
smaller than the oral doses because it directly goes to blood stream
without going through the first pass metabolism and various other
eliminations. Intravenous route might enhance the chances of drug
toxicity sometimes. Oral route is most commonly used route for
drug administration because of advantages like self medication,
easy administration and no pain generation during administration
and so on.

6. 5.TIME OFADMINISTARTION
The presence of food in the stomach region can delay the absorption
process of drug and the drug gets rapidly absorbed from the empty
stomach. But it does not mean that all drugs will show much
effective absorption when taken during or after meal.
Iron, arsenic and cod liver oil should be given after meal and antacid
drugs taken before meal.
6. ENVIROMENTALFACTORS
The amount of drug prescribed is also affected by the environment.
Sedatives are the drugs consumed at night to induce sleep and treat
insomnia but if taken in day time, the dose required will much
higher than taken at night.
Similarly, stimulant drugs are mostly prescribed to be consumed in
day time, but if it has to be consumed at night, then the amount of
dose have to elevate to higher amount. Other than day and night,
the weather also affects the dose amount.
Eg, Alcohol containing formulations are more tolerant in winter
than in summer.

7. 7. Emotional factors
The personality and behavior of a physician may influence the effect
of drug especially for the drugs which are intended for use in any
psychosomatic disorders.
The females are more emotional than male and required less dose of
certain drug. Inert dosage forms called placebos which resemble to
the actual medicaments in the physical properties which are known
to produce the therapeutic benefits in disease like angina pectoris
and bronchial asthma.
8. PRESENCEOF DISEASE
The drugs like barbiturates and chlorpromazine may produces
unusual prolonged effect in patient having liver cirrhosis. Such as,
streptomycin produce toxic effect on the patient their kidney do not
work properly because streptomycin excreted through kidney.

8. 9.ACCUMULATION
Some drugs produces the toxic effect if it is repeatedly administered
for long time. Eg, 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 then it is equivalent to
the addition of their individual pharmacological action and this
phenomenon of addition is called as additive effect. Eg, ephedrine
and aminophylline in the treatment of bronchial asthma.
11. SYNERGISM
When desired therapeutic result needed is difficult to achieve with
single drug at that time then two or more drugs are used in the
combination to increase their action this phenomenon is called as
synergism. Eg, Procaine and adrenaline combination, increase the
duration of action of procaine.

9. 12.ANTOGONISM
When the action of one drug is opposed by the presence of 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.
Eg, milk of magnesia is given in acid poisoning where alkaline effect of
milk of magnesia neutralize the effect of acid poisoning. 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.
13. Idiosyncrasy
Idiosyncrasy is also called as an allergy. Drug idiosyncrasy refers to
untoward reactions to drugs that occurs in a small fraction of patients
and have no obvious relationship to dose or duration of therapy and
generally liver is the frequent target for toxicity. An extraordinary
response to a drug which is different from its pharmacological action is
called idiosyncrasy. Eg, small quantity of aspirin may cause gastric
hemorrhage.

10. 14.TOLERANCE
When an unusually large dose of a drug is required to elicit an affect
ordinarily which is produced by the normal therapeutic dose of the
drug, the phenomenon is called as drug tolerance. Eg, smokers can
tolerate nicotine, alcoholic can tolerate large quantity of alcohol.
The drug tolerance is of two types,
True tolerance is produced to oral and parenteral administration
of the drug.
Pseudo tolerance is produce only to oral route of administration.
15.TACHYPHYLAXIS
When some drugs administered repeatedly at short intervals, the
cell receptors get blocked and pharmacological response to that
drug get decreased. The decreased response cannot be reversed by
increasing the dose and this phenomenon is called tachyphylaxis.
Eg, ephedrine given repeated dose at short intervals in the
treatment of bronchial asthma may produces very less response due
to tachyphylaxis.

11. 16. METABOLICDISTURANCE
Changes in water and electrolyte balance or acid base balance, body
temperature and other physiological factor may modify the effect of drug.
Eg, salicylates reduces body temperature only if an individual has rise in
the body temperature. They have no antipyretic effect if the body
temperature is normal.
17. Hypersensitivity
This type of behaviour of drugs can be due to frequent or known toxic
effect of the drugs like antibiotics, vitamins or some proteinous
substance. If any person is sensitized by the prior administration of the
same drug, a minute dose of drug will produce allergic reactions.

12. Calculationsof doses
Doses proportionate to age –
1. Youngs Formula
Dose for child –
Age in years
Age in years + 12
× Adult Dose
2. Dillings Formula
Dose for child –
Age in years
20 dl
× Adult Dose
Doses proportionate to body weight –
Clark’s formula
Dose for child –
Weight in Pound
150
× Adult Dose
Or
Dose for child –
Weight in Kg
70
× Adult Dose

13. Doses proportionate to surface area
% Adult dose –
Surface area of child x 100
Surface area of adult

14. veterinarydoses
Dose required for animals are higher in comparison to human beings because
of body weight and size. Therefore it’s important to supply correct dose.
Factors affecting on veterinary doses
1. Age
2. Sex
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
11. Species
12. Character of drug

15. Thank You