Define Posology. Discuss the various factors which affects the dose of drug

 

The word "posology" is derived from the Greek words "posos", meaning 'how much' and "logos", meaning 'science'.

It means it is the branch of medical science which dals with doses or quality of drugs which can be administered to produce the required pharmacological actions.

Factors affecting dose and action of drugs:

1.Age: In general, children require smaller doses than adults Either Yong's formula (based on age) or Clark's formula (based on weight) can be used for calculating the doses for children but the formula based o body surface area is more reliable.

2.Sex: This is particularly important in the case of treatment with sex hormones. Female adults generally require smaller doses than males due to the presence of more body fat.

3.Body Weight: 

• The usual doses for drugs are mentioned generally for 70kg adult. The drug concentration at site of action is based on the ratio between the amount of drug administered and size of the body.
• The dose calculations for abnormality thin or obese patients are required to calculate on the basis of body weight.

4.Severity of disease: It is common experience that dull headache may be relieved by a single tablet of aspirin whereas severe headache may necessitate administration of 2-3 tablets of the same drug. But this is no true in all cases. For example, in case of iron deficiency anemia, the dose of iron can be absorbed from the intestine daily and incorporated in hemoglobin.

5.Health and nutrition: 

• Debilitated and anemic patients are, in general, more sensitive to the toxic effects of drugs and hence they are given smaller doses.
• Persons with severe anemic associated with hookworm infestation are more susceptible to the toxic effects of tetrachloroethylene . Myxedematous patients are known to show less response to drugs like amphetamine because of low cellular metabolism.

6.pathological state:

• If the organs, through which biotransformation or execration takes place, are diseased then smaller dose is indicated. for example, in case of renal insufficiency, phenobarbitone(mainly excreted by the kidneys) should be given in smaller dose (morphine is mainly inactivated in liver).
• Aspirin has no effect on normal body temperature in fevered patients. Quinine precipitates black water fever more often with falciparum malaria than otherwise.

7. Tolerance: 

• Some children can tolerate relatively large doses of arsenic, belladonna and calomel.
• Tolerance can be acquired as a result of repeated administration of some drugs e.g., morphine, heron and cocaine.

8.Simultaneous administration of two or more drugs 

    a) Addition: When two or more drugs given together produce the same resulting effect is the algebraic sum of their individual effects e.g., carbachol and acetylcholine.

    b) Synergism: In synergism, the effect produce is greater than the algebraic sum of the effects due to individual drugs e.g., adrenaline and cocaine.

    c)Antagonism: When two drugs having opposite effects e.g. the use of amphetamine to correct partially the sedation caused by anticonvulsant doses of  phenobarbital and the administration of ephedrine to correct hypotension resulting from spinal anesthesia.

9.Route of administration: In general, the rapidity of absorption of a drug decreases with route of administration in the following order: Intravenous>Intramuscular>Subcutaneous>Oral

Thus, in general, intravenous dose of a drug is smaller than its intramuscular or subcutaneous or oral dose. Example: Doses of ergotamine for various routes are a follows : Oral: 2 to 5 mg; Intramuscular:1mg(about to1/2 of oral drug);  Intravenous:0.25 mg(about to 1/8 of oral dose and %of IM dose)

10.Time and frequency of drug administration: Biological half-life of a drug i.e. the time required for the blood level. is the main for governing frequency of peak level, is the main factor governing frequency of drug administration. For example, if the biological half-life of sulphadiazine is 4hours, 1g of the drug has to be given every 4hours after initial dose of 2g. But in certain instances e.g., reserpine as a tranquillizer, biological half-life  of a drug has no relation to frequency of administration.

11.Idiosyncracy: Morphine normally depress central nervous system but may produce excretion in some individuals, specially women.

12.Allergy: Penicillin may produce anaphylactic shock (sudden fall of blood pressure) in allergic patient! but not in normal patients.