2. INTRODUCTION
The word posology is derived from the Greek words ’posos’
meaning how muchand‘logos’ meaningscience.
So posology is a branch of medical science which deals with
dose or quantity of drugs which can be administered to a
patient toget the desired pharmacologicalactions.
Thedoseof adrug cannot be fixed rigidly becausethere areso
manyfactors which influence the doses.
Thesefactors are age, condition of the patient, severity of the
disease, tolerances both natural and acquired, idiosyncrasy,
route of administration, formulation used, drug interactions
andrateof elimination.
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3. Factors affecting drug dosage
1. Age
2. Sex
3. Body weight
4. Route of administration
5. Time of administration
6. Environmental factors
7. Emotional factors
8. Presence of disease
9. Accumulation
10. Additive effect
11. Synergism
12. Antagonism
13. Idiosyncrasy
14. Tolerance
15. Tachyphylaxis
16. Metabolic disturbances
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4. 1. AGE
Thepharmacokinetics of manydrugschangeswith age.Sowhile
determining the doseof adrug, the ageof anindividual isof great
significance.
Childrenandold peopleneedlesseramount of drug than the normal
adult dose,becausethey areunableto excrete drugsto that extent as
adults.
Childrencantolerate relatively larger amounts of belladonna,
digitalis andethanol whereas,elderly patients aremore sensitive to
somedrug effects.
e.g.hypnotics andtranquillizers which may produce confusionstates
inthem.
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5. 2.SEX
Women do not always respond to the action of drugs in the same manner as
it isdonein 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 aloes
shouldbeavoidedduring menstruation.
Similarly the drugs which may stimulate the uterine smooth muscle e.g.
drastic purgatives, antimalarial drugs and ergot alkaloids are contra
indicatedduring pregnancy.
There are certain drugs which on administration to the mother are capable
of crossing the placenta and affecting the foetus e.g. alcohol, barbiturates,
narcoticandnonnarcoticanalgesicsetc.
During lactation, the drugs like antihistamines, morphine and tetracycline
which are excreted in milk should be avoided or given very cautiously to the
motherswho arebreastfeedingto thebabies.
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6. 3.BODY WEIGHT
The average dose is mentioned either in terms of mg per Kgweight
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 case of
obese patients, children and malnourished patients. It should be
calculated accordingto bodyweight.
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 with
intravenous route and this might enhance the chancesof drug toxicity. The
effectiveness controlled by the route ofadministration.
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7. 5.TIME OF ADMINISTRATION
The presence of food in the stomach delays the absorption of
drugs.
Thedrugsaremore rapidy absorbed from the 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 e.g. iron, arsenic and cod-liver oil should always be given
aftermeals.
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8. 6. ENVIRONMENTAL FACTORS
Daylight is stimulant, enhancing the effect of stimulating drugs
anddiminishingthe effect of hypnotics.
Darknessissedative.Hypnoticsaremore effective at night.
The amount of barbiturate required to produce sleep during
daytime is much higher than the dose required to produce
sleep at night.
Alcohol is better tolerated in cold environments than in
summer.
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9. 7. EMOTIONAL FACTORS
Thepersonality andbehaviour of aphysicianmay
influencethe effect of drugespecially the drugs which are
intended for usein apsychosomaticdisorder.
Thefemalesare more emotional than malesandrequires
lessdoseof certain drugs.
Inert dosageforms called placeboswhich resemble the
actual medicamentin the physicalproperties are known
to producetherapeutic benefit in diseaseslike angina
pectorisandbronchialasthma.
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10. 8. PRESENCE OF DISEASE
Drugs like barbiturates and chlorpromazine may produce
unusually prolonged effect in patients havingliver cirrhosis.
Streptomycin is excreted mainly by the kidney may prove toxic
if the kidney of the patient isnot working properly.
During fever a patient can tolerate high doses of antipyretics
than anormalperson.
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11. 9. ACCUMULATION
The drugs which are slowly excreted may built up a sufficient
high concentration in the body and produce toxic symptoms if it
is repeatedly administered for a long time e.g. digitalis, emetine
and heavymetals.
Thisoccursdue to accumulative effect of thedrug.
The cumulative effects are usually produced by slow excretion,
degradation andrapid absorption of drugs.
Sometimes, a cumulative effect is desired in drugs like
phenobarbitone in the treatment of epilepsy.
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12. 10.ADDITIVEEFFECT
When the total pharmacological action of two or more drugs
administered together is equivalent to sum of their individual
pharmacological action, the phenomena iscalled as anadditive effect.
For example, combination of ephedrine and aminophylline in the
treatment of bronchial asthma.
11.SYNERGISM
When two or more drugs are used in the combination form, their
action isincreased. The phenomena iscalled synergism.
Synergism is very useful when desired therapeutic result needed is
difficult to achieve with asingledrug .
e.g. procaine and adrenaline combination, increases the duration of
action ofprocaine.
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13. 12. ANTAGONISM
When the action of one drug is opposed by the other drug on the
samephysiologicalsystemis known asdrug 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 alkaline effect of milk of magnesia neutralize the
effect of acidpoisoning.
When adrenaline and acetylcholine are given together, they
neutralise the effect of each other due to antagonism because
adrenaline is vasoconstrictor andacetylcholine isvasodilator.
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14. 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, small quantity of aspirin may cause gastric
haemorrhage and a small dose of quinine may produce
ringing in the ears.
Some persons are sensitive to penicillin and sulphonamide
because they produce severe toxic symptoms.
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15. 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 phenomena is termed as drug tolerence.
E.g. smokers can tolerate nicotine, alcoholic can tolerate
large quantity of alcohol.
Thedrugtolerance isof two types:-
(i) Truetolerance, whichisproducedby oral andparenteral
administrationof the drug.
(ii) Pseudotolerance, whichisproducedonly to the oral route of
administration.
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16. 15. TACHYPHYLAXIS
It hasbeen observed that when certain drugsareadministered
repeatedlyat short intervals,the cell receptorsget blocked up
and pharmacological responseto that particular drug is
decreased.
The decreasedresponsecannot bereversedby increasing the
dose.Thisphenomenon isknown as tachyphylaxis or acute
tolerance.
Forexample,ephedrinewhen givenin repeated dosesat short
intervals in the treatment of bronchialasthmamayproduce very
lessresponsedue to tachyphylaxis.
Similarly, drugslike amphetamine, cocaineandnitrates behave
in thisway.
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17. 16.METABOLICDISTURBANCES
Changesin water electrolyte balanceandacidbasebalance,body
temperature andother physiological factor maymodify the
effects of drugs.
Salicylatesreduce body temperature only in caseanindividual
has risein body temperature.
Theyhaveno antipyretic effect if the body temperature is
normal. The absorption of iron from G.I.T.ismaximum ifthe
individual hasaniron deficiency anaemia.
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18. VETERINARYDOSES
• Apracticing pharmacist(running chemist shop)isconsidered
to beresponsiblefor supplyingcorrect dosesin anytype of
prescription. Therefore it isrequired that he should have
complete knowledge of posology which pertains toanimal
medication.
• Thedosesrequiredfor animalsare moreor onhighersidein
comparisonto humanbeingsbecause the weight andsurfacearea
of animals isnormally more than that of human beings.Dosesfor
animals arementioned on body weights. Dosesarenormally
applicable to all speciesexceptin certain cases,where it is
specificallymentioned.
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19. FACTORSINFLUENCINGDOSEOFA DRUGINANIMALS
1. AGE
Young animals need less dose of a drug as compared to adults, because
younganimalsaremore susceptible thanadults.
2. SEX:
Sexpays an important role while prescribing the dose for an animal.
Thefemale may require less dose than the male animals. Certain drugs
are contra-indicated in pregnant animals because of hazards of
inducing abortion. Some drugs have tendency to pass into milk which
becomes unfit for human consumption. Such drugs must be used
carefully when administered in milk animals.
3. BODYWEIGHT&SIZE:
The dose of the drug in case of animals depend on their body weight
and size.Thesizeof animals varies according to its breeds. Forexample,
in caseof dogs, adose which may be harmless for a heavy breed of dog
maybedangerousto dogsof lighter breed.
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20. 4. TIMEOFADMINISTRATION:
A small dose of a drug may be more effective if given in an empty
stomach rather than the same when given after meals. Purgatives and
anthelmintics are more effective when given in empty stomach.
Similarly, the hypnotics are more effective when given after sunset than
givenearlyin the morning.
5. ROUTE OF ADMINISTRATION:
• The dose of the same drug varies with the use of different routes of
administration. The maximum dose is required when the drug is given
orally. Theminimum doseisneeded when the drug isadministeredby
intravenous route. The following order is observed while
administering the doseof samedrug bydifferent routes:-
oral> s/c> i/m >i/v
6. ENVIRONMENTAL CONDITIONS:
• The atmospheric moisture and temperature have a great influence on
the tissues of the animals. In rainy season, when the climate is quite
humid and hot, less dose is required than in winter when climate is dry
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21. 7. HABIT:
• An animal which is constantly under the influence of a drug may
develop tolerance for that drug. In such animals the normal dose
may fail to produce the desired effect. Hence, large doses are
required to produce the requiredeffect.
8. RATE OF ELIMINATION:
The rate of excretion of drug has marked effect on the dose of the
drug. The drugs which are excreted at a faster rate require large
dosesthan those drugswhich areexcreted at aslowrate.
9. EFFECT OF DRUG:
When two drugs having the similar type of pharmacological action
are given in combination, the effect will become more powerful
than the individual drug even given in equivalent dose. But many a
times, when two or more drugs are combined together, they oppose
the effect of eachother andultimate therapeutic effect isdecreased.
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22. 10. PURPOSE OF MEDICATION:
The dose of drug varies with the purpose for which it is used. For
example, magnesium sulphate act as purgative in large doses but in
smaller dosesit actsasantacidandlaxative.
11. EFFECT OF SPIECES:
• Thedoseof adrug variesfrom speciesto species.Thedoseof adrug for a
cowwill bedifferentfrom horse,sheep,pig,catetc. Opiumisanarcotic
eventhan it producesexcitementin horseandcattle.Butit shows
narcosisin dog.Thesedifferencesin action aredueto anatomical and
physiologicalpeculiarities.
12. CHARACTER OF THE DRUG:
Largerdosesof drugarerequired if it given in crudeform to the
animals.Forexample,nuxvomicapowder isrequired to begiven in large
dosesthan its active constituentstrychninealkaloidwhich isgiven in
smallerdoses.
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23. Calculation of dose: ( For child)
• Dose proportionate to age:
Young’s formula: (For children under 12 years of age)
Dose for the child =
Dilling’s formula: (For children between 12-20 years of age)
Dose for the child =
Fried’s formula:
Dose for the child =
Adult Dose
Age in years
Age in years + 12
X
Age in years
20
X Adult Dose
Age in months
150
X Adult Dose
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24. • Dose proportionate to body weight:
Clark’s formula:
Dose for the child =
Child’s weight in Kg.
70
Adult doseX
• Dose proportionate to surface area:
Percentage of adult dose= X 100
Surface area of child
Surface area of adult
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25. Determination of children’s dose from Adult
Dose on the basis of Body Surface Area
Sr no: Weight (kg) Approx surface area in
square meters
Approx percentage of
Adult dose
1 2 0.15 9
2 4 0.25 14
3 6 0.33 19
4 8 0.40 23
5 10 0.46 27
6 15 0.63 36
7 20 0.80 46
8 25 0.95 55
9 30 1.08 62
10 35 1.20 70
11 40 1.30 75
12 45 1.40 81
13 50 1.51 87
14 55 1.58 91
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26. Calculation of dose(Posology)
• Refer R M Mehta (Page no: 88-90)
• Refrer Cooper and Gunn’s (Page no 724-726)
• Refer Dr A K Seth (Page no: 83-85)
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