The word posology is derived from the Greek words posos meaning how much and logos meaning science. So posology is a branch of medical science which deals with dose or quantity of drugs which can be administered to a patient to get the desired pharmacological actions.
The dose of a drug cannot be fixed rigidly because there are so many factors which influence the doses . these factors are age, condition of the patient, severity of the disease, tolerances both natural and acquired, idiosyncrasy, route of administration, formulation used, drug interactions and rate of eliminations.
FACTORS INFLUENCING DOSE.
The dose range is usually based on the average requirements of an adult patient.
The following are some of the factors which influence the dose-
- AGE– the pharmacokinetics of many drugs changes with age.
-children and old people need lesser amount of drug than the normal adult dose.
-children can tolerate relatively larger amounts of belladonna, digitalis and ethanol
– e,g. hypnotics and tranquilizers which may produce confusion state in them.
2) SEX- women do not always respond to the action of drugs in same manner as it is done in men.
– morphine and barbiturates may produce more excitement before sedation in women.
– special care should be taken when drugs are administered during menstruation, pregnancy and lactation.
-the strong purgatives such as aloe should be avoided during menstruation.
-there are certain drugs which on administration to the mother are capable of crossing the placenta and affecting the foetus. E,g. alcohol, barbiturates, narcotic and non narcotic analgesic etc.
– during lactation, the drugs like antihistamine morphine and tetracycline which are excreted in milk should be avoided or given very cautiously to the mothers who are breast feeding the babies.
3) BODY WEIGHTS– The average dose is mentioned either in terms of mg per kg body weight or as a total single dose for an adult weighing between 50-100 kg.
– it should be calculated according to body weight.
4) ROUTE OF ADMINISTRATION– intravenous doses of drugs are usually smaller than the oral doses, because the drugs administered intravenously enter the blood stream directly.
– due to this reason the onset of drug action is quick.
5) TIME OF ADMINISTRATION– the presence of food in stomach delays the absorption of drugs.
– the drugs are more rapidly absorbed from the empty stomach.
-the irritating drugs are better tolerated if administered after meals, e,g. iron, arsenic and cod-liver oil should always be given after meals.
6) ENVIRONMENTAL FACTORS– Daylights is stimulant.
– hypnotics are more effective at night.
-the amount of barbiturates required to produce sleep during day time is much higher than the dose required to produce sleep at night.
– alcohol is better tolerated in cold environments than in summer.
7) EMOTIONAL FACTORS– the personality and behavior of a physician may influence the effect of drug especially the drugs which are intended for use in a psychosomatic disorder.
-the females are more emotional than males and require less dose of certain drugs.
8) PRESENCE OF DISEASE– drugs like barbiturates and chlorpromazine may produce unusually prolonged effect in patients having liver cirrhosis
– during fever a patient can tolerate high doses of antipyretics than a normal person.
9) ACCUMULATION– the drugs which are slowly excreted may built up a sufficient high concentration in the body and produce toxic symptoms.
– sometimes, a cumulative effect is desired in drugs like phenobarbitone in the treatment of epilepsy.
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 phenomenon is called as an additive effect.
11) SYNERGISM– when two or more drugs are used in the combination form, their action is increased.
– e,g. procaine and adrenaline combination , increases the duration of action of procaine.
12) ANTAGONISM– When the action is opposed by the other drug on the same physiological system is known as drug antagonism.
– e.g. milk of magnesia is given in acid poisoning.
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.
-for example, small quantity of aspirin may cause gastric haemorrhage.
14) TOLERANCE– 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 termed as drug tolerance.
– e.g. smokes can tolerate nicotine , alcoholic can tolerate large quantity of alcohol.
-the drug tolerance is of two types- 1) true tolerance 2) pseudo tolerance
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 reversed by increasing the dose.
– this phenomenon is known as tachyphylaxis or acute tolerance.
16) METABOLIC DISTURBANCE– change in water electrolyte balance and acid base balance, body temperature and other physiological factor may modify the effects of drugs.
-salicylates reduce body temperature only in case an individual has rise in body temperature.
They have no antipyretics effect if the body temperature is normal.
-the absorption of iron from G.I.T is maximum if the individual has an iron deficiency anaemia.
CALCULATIONS OF DOSES
THE DOSES ARE ALSO CALCULATED IN PROPORTIONATE TO AGE, BODY WEIGHTS AND SURFACE AREA OF THE PATIENT.
A.DOSES PROPORTIONATE TO AGE- there are number of methods by which the dose for a child can be calculated from the adult dose.
1)YOUNGS FORMULA= age in years/ age in year+12 x adult dose
The formula is used for calculating the doses for children under 12 year of age.
Dose for the child= age in year/ 20 x adult dose
The formula is used for calculating the doses for children in between 4 to 20 years of age. This formula is considered better because it is easier and quick to calculate the dose.
- DOSES PROPORTIONATE TO THE BODY WEIGHT
Dose for the child= child weights in kg/70 x adult dose
- DOSES PROPORTINATE TO SURFACE AREA.
– the calculation of child dose according to surface area is more satisfactory and appropriate rather than the method based on age.
– the method is more complicated than the method based on age.
-the method is based on the following formula-
Percentage of adult dose= surface area of child/ surface area of adult x 100
FACTOR INFLUENCING DOSE OF A DRUG IN ANIMALS.
- Body weight and size
- Time of administration
- Route of administration
- Environmental condition
- Rate of elimination
- The effect of drugs
- Purpose of medication
- Character of the drug.
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