POSOLOGY PART 1

 POSOLOGY

Hey readers, this is again Sarang More. this is another blog on pharmaceutics 'posology'. please have a look and do share with yur friends and colleagues. 

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The posology is derived from two Greek words ‘poso’ meaning how much and ‘logos’ meaning science.

Posology is the branch of medical science which deals with dose and quantity of drugs which can be administered to the patients to get desired pharmacological actions. The dose of the drugs cannot be fixed rigidly because there are so many factors which influence the dose.

These factors are age, condition of patient, severity of the disease, tolerance both natural and acquired, idiosyncrasy, route of administration, formulation needed drug interaction and rate of elimination.

The official doses represents the average range of quantities suitable for adults which administered within 24 hours orally. When other route of administration are followed the relevant appropriate dose is given.

It is responsibility of the prescriber regarding the amount of drug tom be prescribed or the frequency to be administered.

But before dispensing any prescription, it becomes duty of pharmacist to satisfy himself that overdose has been prescribed. This can be confirmed either by consulting the prescriber or the pharmacopoeia.

 

§  FACTORS INFLUENCING DOSES:-

The optimum dose of the drug which produces the desired therapeutic effect varies from person to person, because every individual varies both in the degree and character of the response produced by the drug.

The following are some factors which influence the dose :-

1.  Age :-

The pharmacokinetics of many drugs changes with age. So while determining the dose of the drug, the age of an individual is of great significance.

Children and old age people need lesser amount of drugs than normal adult dose, because they are unable to excrete drugs to that extent of adults.

Children can tolerate larger amount of belladonna, digitalis and ethanol, whereas, elderly patients are more sensitive to some drug effects e.g. hypnotics and tranquilizers which may produce confusion state in them.

 

2.  Sex :-

Women does not response to action of drugs in the same manner as it is done in men. Morphine and barbiturates may produce more excitement before sedation.

Special care should be taken while administering the drugs during menstruation, pregnancy and lactation. The strong purgatives such as aloes much be avoided during menstruation.

Similarly the drugs which stimulate the smooth uterine smooth muscles like anti-malarial, ergot alkaloids and drastic purgatives should e avoided in pregnancy.

The drugs which crosses the placenta of the mother much be avoided which may consequence the adverse effect to the foetus like alcohols, barbiturates and narcotics.

The drugs such as tetracyclines, morphine and antihistaminic should be avoided by the lactating mothers as it will pass to the baby who is consuming the milk.

 

3.  Body weight:-

The average dose is mentioned in mg per kg body weight or as a single dose for an adult weighing between 50-100kg.

However, the dose expressed in this fashion may not apply in obese patients, children and malnourished patients.

It should be calculated according to body weight.

 

4.  Route of administration:-

Intravenous dose of drugs are usually smaller than oral doses, because the drug administered through intravenously get entered directly in blood streamline.

Due to this reason the onset of action of the drugs is quick with intravenous route and this might changes the enhances of drug toxicity. The effectiveness of drug action generally controlled during route of administration.

 

5.  Time of administration:-

The presence of food in stomach delays the absorption of drugs. The drugs are more rapidly absorbed from empty stomach. So the amount of drug which is taken before meal very effective than the drug when taken after meal, the irritating drugs which are better tolerated should be taken after meals e.g, arsenic, iron and cod-liver oil should be taken after meal.

 

6.  Environmental factors:-

Daylight is stimulant, enhancing the effect of stimulating drugs and diminishing the effect of hypnotics.

Darkness is sedative, hypnotics are more effective in night. The amount of barbiturates required to produce the sleep during day time is much more higher than the dose required to produce the sleep during night.

Alcohol is better tolerated in cold environments than in summer.

 

7.  Emotional factors:-

The personality and behaviour of a physician may influence the effect of drug especially the drugs which are intended for use in psychosomatic drugs.

The female are more emotional as compare to men and requires less amount of drugs. Inert dosage forms called as placebos which resemble the actual medicament in physical properties are known to produce the therapeutic effects in diseases like angina pectoris and bronchial asthma.

 

8.  Presence of disease :-

Drugs like barbiturates and chlorpromazine may produce unusually prolonged effect in patients having liver cirrhosis. Streptomycin is excreted mainly by kidney may prove toxic if kidney of the patient is not working properly.

During fever a patient can tolerate high dose of antipyretic than a normal one.

 

9.  Accumulation :-

The drugs which are slowly excreted may built upa a sufficient high concentration in the body any may produce toxic symptoms if administered repeatedly for long time.

This occurs due to accumulative effect of drugs. The cumulative effect are usually produce by slow excretion, degradation and rapid absorption of drugs.

Cumulative effect is seen n phenobarbitone during epilepsy.

 

10.      Additive effect:-

When the total pharmacological action of two or more drugs administered together is equal to sum of their of their individual effects, this phenomenon is called as additive effect.

e.g. aminophylline and ephedrine given together gives instant treatment in asthma.

 

11.      Synergism:-

When two or more are administered in combination their action is increased. The phenomenon is called synergism.

Synergism is very useful when desired pharmacological effect is needed and it is difficult to achieve with single dose.

e.g. procaine and adrenaline combination, increase the duration of action of procaine.

 

12.      Antagonism :-

When the action of one drug is oppose by the effect of another drug on the same physiological system is known as a drug antagonism.

The use of antagonistic drugs is valuable in poisoning e.g. milk of magnesia is given in acid poisoning where alkaline effect of milk of magnesia neutralizes the effect of drug.

 

13.      Idiosyncrasy :-

An extraordinary response to the drug which is different from its characteristic pharmacological action is known as idiosyncrasy.

Idiosyncrasy means ‘allergy’. E.g. small amount of aspirin  may cause gastric haemorrhage and small amount of quinine may produce ringing in ears.

 

14.      Tolerance :-

When an usually large dose of drug is required to elicit an effect ordinarily produce by normal therapeutic dose of the drug, the phenomenon is known as drug tolerance.

e.g. smokers can tolerate more nicotine as compare to normal person.

True tolerance may be produce by the oral and parenteral route of administration.

Pseudo tolerance may be produced by only oral route f administration.

 

15.      Tachyphylaxis:-

It has been observed that when certain drugs are administered repeatedly at short intervals, the cell receptors get blocked up and pharmacological response to that particular drug is decreased.

The decreased response cannot be reverse by increasing the dose of the drug. It is known as acute tolerance .

e.g. amphetamine, ephedrine, etc.

 

16.      metabolic disturbances :-

changes in water electrolyte balance, acid base balance, body temperature and other physiological factors may modify the effect of drugs.

Salicylates can decrease the body temperature only if there is hike in body temperature.

The absorption of iron is higher in patient suffering from anaemia.

 

§  CALCULATIONS OF DOSES :-

1.  Doses proportionate to age:-

There are number of methods by which the dose for a child can be calculated as follows:

 

i)              Young’s formula :-

 

Dose for a child = age in years /age in years +12 X adult dose.

This formula is used for calculating the dose for children under 12 years of age.

 

ii)            Dilling’s formla :-

 

Dose for a child = age in years /20 X adult dose.

 

The formula is used for calculating the dose for children in between 4 to 20 years of age.

 

2.  Doses proportionate to body weight:-

 

i)              Clark’s formula :-

Dose for a child = child’s weight in kg /70 X adult dose

 

3.  Dose proportionate to surface area :-

Percentage of adult dose = surface area of child / surface area of adult X 100

 

The body surface area is calculated from the height and weight of the child.