POSOLOGY

POSOLOGY

Posology is derived from the Greek word posos meaning how much and logos meaning science So posology is the branch of medicine dealing with doses or quantity of drug required to produce therapeutic response.

The optimum dose of a drug varies from patient to patient. The dose is usually expressed as a range. The minimum dose or the lower limit of the dose is essential for eliciting an intended therapeutic response whereas the maximum dose or the higher limit of the dose is the amount of the drug substance that can be tolerated by an average individual. 

The pharmacist is much concerned with the maximum limit of the doses which, if exceeded, may cause untoward effects in the patient. The actual dose of a drug is to be decided by the prescriber depending on patient’s age, sex, symptoms, his medication history and the factors like tolerance, idiosyncrasy, route of administration etc.

Factors affecting dose of the drugs

1. Age: Human beings can be categorized into the following age groups:

·         Neonates:       From birth up to 30days.

·         Infant:            1 month to 2 years of age

·         Children:       2 to 12 years

·         Adolescents:  12 to 16 years

·         Adults:          More than 16 years

·         Geriatric (elderly) patients: above  60 years

In children the enzyme systems in the liver and renal excretion remain less developed. So all the doses of drugs given to children should be based on their age and preferably by body weight. In elderly patients the renal functions decline. Metabolism rate in the liver also decreases. Drug absorption from the intestine becomes slower in elderly patients. So in geriatric patients the dose is less and should be judiciously administered.

2. Sex: Special care should be taken while administering any drug to women during menstruation, pregnancy and lactation. Strong purgatives should not be given in menstruation and pregnancy. Antimalarials, ergot alkaloids should not be taken during pregnancy to avoid deformation of foetus. Antihistaminic and sedative drugs are not taken during breast feeding because these drugs are secreted in the milk and the child may consume them

3. Body weight: Dose of the drug can be calculating according to body weight. Obese patients, children and malnourished patients require less doses.

4. Route of Administration: Intravenous doses of drugs are usually smaller than oral doses, because the drug administered intravenously enters the blood stream directly.

5. 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

6. Environmental Factor: 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.

7. Emotional factors: The personality and behaviour 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.

8. 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.

9. 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.

10. 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

11. 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 herapeutic result needed is difficult to achieve with a single drug e.g. procaine and adrenaline combination, increases the duration of action of procaine.

12. 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 neutralise the effect of each other.

13. 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. Penicillin & sulphonamide.

14. Tolerance: When an unusually large dose of drug is required to elicit an affect ordinarily produce by normal therapeutic dose of the drug, the phenomenon is termed as tolerance. eg. smokers can tolerate Nicotine

15. 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 in bronchial asthma.

16. 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.

	Formulae for the calculations of doses. 1) Proportionate to age-
	1. Young’s formula:
	Age in years
	Dose for a child =   ------------------------	x	Adult dose
	Age in years +12
	2. Dilling’s formula:
	Age in years
	Dose for a child =   ------------------------	x	Adult dose
	20
	3. Fried’s formula:
	Age in months
	Dose for a child =   ------------------------	x	Adult dose
	150
	2) Proportionate to body weight-
	Weight of the child lb
	Dose for a child =   ------------------------	x	Adult dose
	150
	3) Proportionate to body surface area-
	Surface area of child
	Dose for a child =   ------------------------------		x   Adult dose
	Surface area of Adult

The average body area for an adult is = 1.73m²