POSOLOGY
POSOLOGY
• It is derived from greek words
POSOS- meaning “how much”
LOGOS-meaning “science”
POSOLOGY is defined as branch of science
which deals with dose or quantity of drugs
which can be administered to the patient to
get the desired pharmacological actions.
FACTORS AFFECTING DOSE SELECTION
• Optimum dose to achieve desired therapeutic
effect varies from person to person as each
individual varies in both DEGREE and
CHARACTER of response produced by the
drug.
• Doses of drugs of official preparation are
available in form of RANGE which is based on
average requirement of an individual patient.
Factors Influencing Dose of Drug
• Age • Presence of disease
• Sex • Accumulation
• Body weight • Additive effects
• Route of administration • Synergism
• Time of administration • Antagonism
• Environmental factors • Idiosyncrasy
• Emotional factors • Tolerance
• Change in metabolic • Tachyphylaxis
disturbances
SETA
PC of RBI
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AGE
• Pharmacokinetics(what body does to drug) of
drug changes with age.
• Children and old age need lesser amount of dose
than normal adults because they are unable to
excrete drugs to that extent as adults.
E.g. 1. Children tolerate relatively larger amounts of
digitalis and belladonna.
2.Elderly patients are more sensitive to some drug
effects like hypnotics which may produce
confusion states in them.
SEX
• Women does not respond to action of drugs in
same manner as in men.
• Special care to be given when administering
drugs either in pregnancy, mensturation or
lactation.
E.g. 1. In pregnancy uterine smooth muscle
stimulants like antimalarial drugs and also drugs
having capability to cross placenta like alcohol
and narcotics should be avoided.
2.During lactation drugs like tetracycline which are
excreted in milk should be avoided.
BODY WEIGHT
• Average dose is given as mg/kg of body wt.
OR
As total single dose for an adult weighing
between 50-100 kg.
EXCEPTION: Obese patients
Malnourished patients
Children
ROUTE OF ADMINISTRATION
• I/V (intravenous) dose of drugs are relatively
less than oral because in i/v drug reaches the
systemic circulation(blood) directly.
• Since absorption is fast there are chances of
toxicity in i/v.
TIME OF ADMINISTRATION
• Presence of food delays absorption of drugs.
• Drugs are rapidly absorbed from empty
stomach.
• Irritating drugs are better tolerated if
administered after meals.
E.g. Iron and arsenic.
ENVIORNMENTAL FACTORS
• Daylight- stimulant. Enhances the effects of
stimulants and diminishes the effect of
hypnotics (sleep inducing drugs)
• Darkness – sedative. Diminishes the effects of
stimulants and stimulates the effect of
hypnotics.
• Hypnotics are effective at night.
E.g. Less dose of barbiturates are required to
produce sleep during night in comparison to
daytime.
EMOTIONAL FACTORS
• Personality and behaviour of physician may
influence the effect of drugs specially those
which are used in pychosomatic disorder.
• Females are more emotional than males and
require less doses of drugs.
• Inert or blank dosage forms called placebos
which resembles to actual medicament are
also sometimes helpful.
PRESENCE OF DISEASE
• Streptomycin (antibiotic) which is basically
excreted by kidney may prove toxic if the
patient kidney is not working properly.
• In patients suffering from liver cirrhosis drugs
like chlorpromazine (anti-psychotic) may
produce unusually prolonged effect since it is
metabolised by liver.
ACCUMULATION
• Drugs excreted slowly may built up a
sufficiently high concentration in body and
can produce toxic symptoms if repeatedly
administered for a long time. E.g. Digitalis.
ADDITIVE EFFECT
• When total pharmacological action of two or
more drugs administered together is
equivalent to sum of their individual
pharmacological actions it is known as
Additive effect.
• E.g. Combination of ephedrine
(bronchodilator & decongestant) and
aminophylline (bronchodilator) in treatment
of bronchial asthma.
SYNERGISM
• Two or more drugs when used in combination
and increases the effect of each other the
process is known as Synergism.
• Useful when desired therapeutic effect is
unable to achieve by single drug.
• E.g. Procaine (anaesthetic) and adrenaline
(neurotransmitter) combination given
together increases the anaesthetic action of
Procaine by increasing the duration of action.
ANTAGONISM
• When action of one drug is opposed by the
simultaneous administration of another drug
the process is known as Antagonism.
• E.g. Adrenaline (vasoconstrictor) and
acetylcholine (vasodilator) when given
together neutralises the effect of each other.
IDIOSYNCRASY
• This in common known as Allergy.
• This is an extraordinary response to a drug
which is different from its characteristic
pharmacological action.
• E.g. Small dose of quinine (antimalarial drug)
may produce ringing in the ears.
TOLERANCE
• When an unusually large dose of a drug is
required to elicit (produce) an effect ordinarily
produced by the normal therapeutic dose of
the drug, the phenomenon is known as
Tolerance.
• E.g. Alcoholics can tolerate a large amount of
alcohol.
TACHYPHYLAXIS
• If certain drugs are administered repeatedly at
short intervals the cell receptors get blocked
up and pharmacological response to that
particular drug is decreased which cannot be
reversed by increasing the dose. E.g.
Antidepressants.
• Also known as Acute Tolerance.
METABOLIC DISTURBANCES
• Changes in acid base balance, body
temperature etc. may modify the effects of
drugs.
• E.g. Salicylates reduce body temperature only
in case an individual has a rise in body
temperature and has no antipyretic (fever
reliving) effect in individuals with normal body
temperature.
Calculation of Dose: ( For child)
• Dose proportionate to Age:
Young’s formula: (For children under 12 years of age)
Age in years
X Adult Dose
Dose for the child =
Age in years + 12
Dilling’s formula: (For children between 12-20 years of age)
Age in years
X Adult Dose
Dose for the child =
20
Fried’s formula: Age in months
X Adult Dose
Dose for the child = 150
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• Dose proportionate to Body Weight:
Clark’s formula:
Child’s weight in Kg.
Dose for the child = X Adult dose
70
• Dose proportionate to Surface Area:
Surface area of child
Percentage of adult dose= Surface area of X 100
adult(1.73m2)
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THANK YOU
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