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 that deals with dose or quantity of drugs which can be administered to a patient to get the desired pharmacological actions.
Factors Affecting Posology
The following are some of the factors which influence
the dose.
1. Age: The
pharmacokinetics of many drugs changes with age. So while determining the dose
of a drug, the age of an individual is of great significance. Children and old
people need lesser amounts of drugs than the normal adult dose, because they
are unable to excrete drugs to that extent as adults. Children can tolerate
relatively larger amounts of belladonna, digitalis and ethanol, whereas elderly
patients are more sensitive to some drug effects. For example, hypnotics and
tranquillizers may produce confusion states in them.
2. Gender:
Women do not always respond to the action of drugs in the 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. 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 analgesics
etc.
3. Body weight: 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. However, the dose expressed in this fashion may not apply in cases of obese patients, children and malnourished patients. It should be calculated according to body weight.
4. Route of administration:
Intravenous doses of drugs are usually smaller than oral doses because the
drugs administered intravenously enter the blood stream directly. Due to this reason, the onset of drug action
is quick with the intravenous route and this might enhance the chances of drug
toxicity. The effectiveness of drug formulation is generally controlled by the
route of administration.
5. Time of administration:
The presence of food in the stomach delays the absorption of drugs. The drugs
are more rapidly absorbed from an empty stomach. So the amount of drug which is
very effective when taken before a meal may not be that much effective when
taken during or after meals. The irritating drugs are better tolerated if
administered after meals for example, iron, arsenic and cod liver oil should always
be given after meals.
6. Environmental factors:
Daylight is a stimulant, enhancing the effect of stimulating drugs and
diminishing the effect of hypnotics. Darkness is sedative. Hypnotics are more
effective at night. The amount of barbiturate 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 behaviour of a physician may influence the effect of drugs,
especially the drugs which are intended for use in a psychosomatic
disorder. Females are more emotional
than males and require fewer doses of certain drugs.
8. Presence of disease:
Drugs like barbiturates may produce unusually prolonged effects in patients
having liver cirrhosis. Streptomycin is excreted mainly by the kidney and may
prove toxic if the kidney of the patient is not working properly.
9. Accumulation:
The drugs which are slowly excreted may be built up a sufficiently high concentration
in the body and produce toxic symptoms if it is repeatedly administered for a long
time e.g. digitalis, emetine and heavy metals. This occurs due to the accumulative
effect of the drug.
10. Additive effect:
When the total pharmacological action of two or more drugs administered
together is equivalent to the sum of their pharmacological action, the phenomenon
is called an additive effect. For example, a combination of ephedrine and aminophylline
in the treatment of bronchial asthma.
11. Synergism:
When two or more drugs are used in the combination their action is increased.
The phenomenon is called synergism.
12. Antagonism:
When the action of one drug is opposed by the other drug in the same
physiological system is known as drug antagonism. The use of antagonistic
responses to drugs is valuable in the treatment of poisoning e.g. milk of
magnesia is given in acid poisoning where the alkaline effect of milk of magnesia
neutralises the effect of acid poisoning.
13. Idiosyncrasy:
An extraordinary 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, a small quantity of aspirin may
cause gastric haemorrhage and a small dose of quinine may produce ringing in
the ears.
14. Tolerance:
When an unusually large dose of a drug is required to elicit an effect ordinarily
produced by the normal therapeutic dose of the drug, the phenomenon is termed drug
tolerance. e.g., smokers can tolerate nicotine, alcoholics can tolerate large
quantities of alcohol.
15. Metabolic disturbances:
Changes in water-electrolyte balance and acid-base balance, body temperature
and other physiological factors may modify the effects of drugs. Salicylates reduce body temperature only in
case an individual has a rise in body temperature. They have no antipyretic effect if the body
temperature is normal.