The document discusses posology, which is the science of determining drug doses. It provides formulas to calculate child doses based on age, weight, and body surface area. Several factors can influence drug dosing, including age, sex, weight, route of administration, time of administration, presence of disease, and drug interactions. The document outlines how these factors can impact drug metabolism, excretion, accumulation and a person's response to medication.

1. POSOLOGY
By:
VENUGOPAL N

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CONTENTS
• Definition
• Formulas to calculate Child Dose based on
– Age
– Body Weight
– Body Surface Area
• Factors influencing dose

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Definition
Posology is derived from the Greek word
“Posos” meaning how much
“Logos” meaning science.
So posology is the branch of medicine dealing with doses.
i.e., quantity of drugs which can be administered to a patient to get
the desired pharmacological action.
The optimum dose of a drug varies from patient to patient.

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Formulas to calculate child dose
Based on age:
1. Young’s Formula
2. Dilling’s Formula
3. Fried’s Formula
4. Cowling’s Formula
Based on body weight:
1. Clark’s Formula
Based on Surface area:
1. Catzel Formula
2. Mosteller Formula

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AGE
• Neonate: From birth to till 30 days
• Infant: Upto 1 year
• Toddler: 1 - 3 years
• Child: 3 – 12 years
• Adolescent: 13 – 18 years
• Adult
• Geriatrics

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Young’s Formula

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Dilling’s Formula

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Fried’s Formula

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Cowling’s Formula

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Clark’s Formula

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Catzel Formula

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Mosteller Formula

13. Factors Influencing Dose
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. Hypersensitivity
17. Metabolic disturbances
18. Drug dependence

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AGE
• Age of an individual is one of the important factor affecting the
dose and pharmacological action of the drugs.
• Age is considered with the capacity of metabolizing or excreting
the drugs from the body.
• Newborn infants (pediatric) are abnormally sensitive to certain
drugs because of the immature state of their hepatic and renal
function by which drugs are inactivated and eliminated from
the body.
• Failure to detoxify and eliminate drugs results in their
accumulation in the tissues to a toxic level.

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• The decline in renal and hepatic function in the elderly
(geriatric) may slow drug clearance and increases the
possibility of drug accumulation in the body and subsequent
toxicity.
• Elderly individuals may also respond abnormally to the usual
amount of a drug because of changes in drug-receptor
sensitivity or because of age-related alterations in target tissues
and organs.
New Born:
• Chloramphenicol cause grey baby syndrome because of
inadequate metabolism resulting drug accumulation.
• Absorption of Amoxycillin is higher because of less gastric
acidity.

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Children (Paediatrics):
• Need lesser dose than the normal adult dose, because of their
pharmacokinetic profile (metabolism & excretion).
• Children can tolerate relatively larger amounts of belladonna,
digitalis and ethanol whereas, elderly patients are more
sensitive to hypnotics and tranquillizers which may produce
confusion states in them.
• The blood brain barrier (BBB) of children are not well
developed so more sensitive to CNS stimulants.
Adults: Age (18 yrs), weight (70 kg) and BSA (1.7-1.8 m2 )
Old people (Geriatrics, age > 60 yrs):
• Need lesser dose because of their pharmacokinetic profile
• More sensitive to diazepam and morphine

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SEX
• Sex, either male or females affect the dose of the drugs.
• Women do not always respond to the action of the drugs in
same manner as it is done in men.
• Morphine and barbiturates produce more excitement before
sedation in women. Special care must be taken when drugs are
administered during menstruation, pregnancy and lactation.
• The strong purgatives such as aloes should be avoided during
menstruation.
• Drugs which may stimulate the uterine smooth muscles e.g.
drastic purgatives, antimalarial drugs and ergot alkaloids are
contraindicated during pregnancy.

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• During lactation drugs like antihistamines, morphine and
tetracycline are excreted in milk, should be avoided or use
cautiously.
• Alcohol, barbiturate, narcotic drugs acts on fetus through
placenta. Because of the undeveloped drug detoxification and
excretion mechanisms present in the fetus, concentrations of
drugs may reach a higher level in the fetus than in the maternal
circulation.
• The transfer of drugs from the mother to the nursing infant
through human milk may occur with various drugs with the
drug effects becoming manifest in the infant.

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Body Weight
• Weight of the drug is considered as a key factor on the theory
that the concentration of a drug at its site of action will often be
influenced by the size of the patient.
• The average dose is mentioned either in terms of mg/kg body
weight or as total single dose for an adult weighing 70 kg.
• However, in cases of obese patients, children and malnourished
patients the dose differs. It should be calculated according to
the body weight.

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Route of Administration
• They are many routes of administration.
• Route of administration affects therapeutic efficacy of drug.
• Drugs administered intravenously enter the blood stream
directly and thus the full amount administered is present in the
blood.
• In contrast, drugs administered orally are rarely fully absorbed
due to the various physical, chemical and biologic barriers to
their absorption, including interactions with the gastric and
intestinal contents.
• Thus, a lesser parentral dose of a drug is required than the oral
dose to achieve the same blood levels of drug.

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Time of Administration
• The time at which a drug is administered sometimes influences
dosage. This is specially true for oral therapy in relation to
meals.
• The drugs are more rapidly absorbed on empty stomach.
• Absorption proceeds more rapidly if the stomach and upper
portions of the intestinal tract are free of food, and an amount
of a drug that is effective when taken before a meal may be
ineffective if administered during or after eating.
• Irritating drugs are better tolerated by the patient if food is
present in the stomach to dilute the drug’s concentration. e.g.
Iron, Arsenic and cod liver oil should always be given after
meals
• Antacid drugs taken before meal.

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Environmental Factors
• These factors affect the dose of the drugs due to some
physiological changes in the body functioning.
• Daylight is 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
daytime is much higher than the dose required to produce sleep
at night.
• Consumption of alcohol enhances the effect of CNS depressants,
alcohol induces hepatic enzymes and causes rapid metabolism.

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Emotional Factors
• Sometimes emotional factors govern the dose of the drugs.
• Females are more emotional and responsive than males and
require less dose of certain drugs.
• The personality of the doctor may influence the patient for his
early recover; such faith on the mind of the patient may affect
the efficiency of the dose of the drug.
• Inert dosage forms called placebos which resemble the actual
medicament in the physical properties are known to produce
therapeutic benefit in diseases like angina pectoris and
bronchial asthma.

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Presence of Disease
• Higher doses of the drugs can be tolerated in diseased condition
of the patients than in normal patients.
• When there is the condition of hepatic and renal disturbances
the drugs are not metabolized properly and accumulation of
drug can be occurred even if normal dose is given.
• Drugs like barbiturates and chlorpromazine may produce
unusually prolonged effect in patients having liver cirrhosis.
• Streptomycin which is excreted mainly by kidney may prove
toxic for patients having kidney failure.

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Accumulation
• Accumulation of drug in the body is the result of slow excretion,
defective degradation or unexpected rapid absorption of the
drugs.
• The drugs which are slowly excreted may build up a sufficient
high concentration in the body and produce toxic symptoms if it
is repeatedly administered for prolonged time.
• E.g. digitalis, emetine and heavy metals.

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Additive effect
• When the total pharmacological action of two or more drugs
administered together is equivalent to the sum of their
individual pharmacological action. This phenomenon is called
as additive effect
• Ephedrine & aminophylline in the treatment of bronchial
asthma.
• 1+1=2

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Synergism
• When two or more drugs are used in combination form, their
action is increased. This Phenomenon is called synergism.
• E.g. Procaine and adrenaline combination increases the
duration of action of procaine.
• 1+1=3

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Antagonism
• When the action of one drug is opposed by the action of other
drug on the same pharmacological system is known as drug
antagonism.
• The use of antagonistic response 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 neutralizes
the effect of acid poisoning.

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a) Competitive/Reversible antagonism: Both agonist and
antagonist have same binding site.
Acetylcholine and atropine
b) Noncompetitive/Irreversible antagonism: Antagonist
inactivate receptor so that effector complex with agonist cant
be formed.
Phenoxybenzamine and adrenaline at α-receptor.
c) Physiological antagonism: Binding of agonist and antagonist
to two different receptors but their action is opposite.
Adrenaline (bronchodilatation) and histamine
(bronchoconstriction).

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Idiosyncrasy
• Extraordinary pharmacological response to a drug, which is
different from its characteristic pharmacological action is called
idiosyncrasy.
• The word idiosyncrasy has been replaced by allergy.
• This is the condition when persons do not respond alike to the
same drug due to individual factors.
• Small dose of quinine may cause ringing in ear and small dose
of aspirin may cause gastric hemorrhage.

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Tolerance
• When unusually large dose of drug is required to produce
pharmacological action which could have been otherwise
produced by normal dose, is termed as tolerance.
• e.g., Smokers can tolerate nicotine etc.
• The drug tolerance is of two types:
1. True tolerance is produced by oral and parenteral
administration of drug.
2. Pseudo is produced only by oral route of administration.

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Tachyphylaxis
• It is observed that when certain drugs are administered
repeatedly at short intervals, the cell receptors are 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.
• e.g., Ephedrine when given in repeated dose at short interval in
the treatment of bronchial asthma may produce very less
response due to tachyphylaxis.

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Hypersensitivity
• It is also called ‘allergic’ reaction of the drug. It is different from
the desired pharmacological action or known toxic effect of the
drug.
• This type of behavior of the drug can be due to frequent or
indiscriminate use of the drugs like antibiotics, vitamins or
some proteinous substances.
• If a person is sensitized by prior administration of the same
drug, a minute dose of the drug will produce allergic reaction.

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Metabolic Distrubances
• Changes in water electrolyte balance and acid base balance,
body temperature and other physiological factors may modify
the effects of drugs.
• Salicylates reduce the body temperature only in case an
individual has rise in body temperature. They have no
antipyretic action

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Drug dependence/ Addiction:
Dependence/Habituation
a. Physical Dependence: Tea, Nicotine
– Depend on drug to function normally
– Occurrence of withdrawal syndrome
– when stop taking drug abruptly
– Vary from one class of drug to another
– Compensating mechanisms produce imbalance
b. Psychological Dependence: LSD, Marijuana, Opiates
– Behavioral dependence
– High rate of drug use, craving for the drug & tendency to relapse
after stopping use
– Related to drug reinforcing properties

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