2. POSOLOGY DEFINITION
• Is a branch of pharmacology dealing with doses with the idea of
producing an optimum therapeutic effect in a particular patient
with the lowest dose possible.
• The word "posology" is derived from two greek words , that is,:-
a. posos meaning how much and
b. logos meaning science.
Hence in general posology deals with calculations, weighing and
administration of amounts and quantities of drug to be administered
in a particular individual to get therapeutic effects.
3. FACTORS AFFECTING POSOLOGY
• Age
• Sex
• Body weight
• Route of administration
• Antagonism
• Idiosyncrasy
• Tachyphylaxis
• Tolerance
• Additive effect
• Synergism
• Time of administration
• Accumulation
4. AGE
• The amount of drug given to children is less than that given to adults due
to the fact that small children have an immature and not strong renal
system hence they can't detoxify and eliminate large quantities of drugs.
• very old people are also given less amounts of drugs because most of their
systems are slowering down hence they cant effectively allow proper
absorption of drugs due to lower sensitivity of drug receptors on cells.
• Certain drugs to old people leads to confusion in them if administered in
large quantities but they achieve their therapeutic effects well in infants ,
for example, ethanol.
• Lots of drugs in infants leads to accumulation in tissues upto toxic levels
resulting in many defects. Hence it is recommended to calculate the
amount or quantity of drug to infants and children inorder to give accurate
doses.
5. CALCULATION OF DOSE OF INFANTS AND
CHILDREN BASED ON AGE.
• Young's rule
States that:-
Adult dose multiplied by [age of child / age of child + 12 years]
Fried's rule
States that:-
Adult dose multiplied by [age in months/ 150]
Dilling's rule
states that:-
Adult dose multiplied by [ age in years/ 20]
6. SEX
• Certain precautions are taken when giving drugs to pregnant women
since drugs can be easily transported from the mother to the fetus.
Drugs to pregnant women should be administered with only the
guidance of a physician.
• Women usually respond differently to certain drugs than men do.
Sedatives usually brings more excitement to women than in men.
• Precautions should be taken when taking drugs during menstruation
for it might the flow of blood.
• Certain drugs should be avoided during lactation for they may pass
from mother to baby through breast milk
7. ANTAGONISM
• Use of two different drugs that have two opposite and different
therapeutic effects. This normally works in poisoning.
• Use of milk magnesia helps in acid poisoning because both drugs have
actions that oppose each other
• Antagonism is when action of one drug is opposed by another drug
8. ADDITIVE EFFECT
• When two or more drugs are administered together, their total
pharmacological action is equal to their individual pharmacological
actions.
9. SYNERGISM
• When two or more drugs are administered together, their
pharmacological action and the therapeutic effects are effectively
increased. This is mainly useful to those drugs whose action cannot
be accomplished by only one drug
10. IDIOSYNCRASY
• This is when administration of a particular drug leads to the opposite
pharmacological action and therapeutic effect
• For example, a small quantity of aspirin can lead to gastric
hemorrhage or administration of quinine can lead to massive ringing
in ears.
11. Tachyphylaxis
• This is called acute tolerance
• It is when the drug is administered repeatedly and on short intervals
of time. This results in blockage of drug receptors leading to decrease
in pharmacological action.
• This cant be corrected by increase in drug since the drug receptors on
cells are blocked.
12. ROUTE OF ADMINISTRATION
• Drugs administered intravenously get straight to the blood steam
hence there should be administered in small quantities. Drugs taken
orally should be administered in large doses since they are absorbed
after time and they meet lots of barriers during absorption.
