Posology is the study of how medicines are dosed to produced  its therapeutic effect. 

This word is derived from Greek word Poses = How much, Logy = Science

It alsodependes upon various factors including age, sex, weight, geographical condition and time of administration.

Factors affecting dose

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. 
–  Whereas, 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.


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 


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. 


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. 


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. 


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. 


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. 


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. 


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. 


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. 


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.


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. 


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. 


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-

1. Doses proportionate to age:

Young’s formula-

Dose for child = Age in years .  Adult dose
                         Age in years + 12

Dilling’s formula-

Dose for child = Age in years .  Adult dose

2. Doses proportionate to body weight:

Clark’s formula-

Dose for child =  child’s weight in Kg .  Adult dose

3. Doses proportionate to surface area:

Percentage of adult dose= Surface area of child . 100
                                           Surface area of adult 

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. 

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