Pharmacodynamics is the study of drug ef-
fects (biochemical and physiological) and
their mechanism of action. When the drug
reaches its site of action it has a pharmaco-
logical effect which may be responsible for
an eventual therapeutic effect and also re-
sponsible for the adverse effects as well as
some other effects which may be of no clini-
cal importance.

SITE AND MECHANISM OF DRUG ACTION

The site of drug action means where a drug
acts and mechanism means how the drug
acts. Drug which act only at the site of
application (i.e. localized region) are termed
as local or topical action for example,
ointments, paste, creams and certain other
local preparations used externally produce
only local effect. The local anaesthetics like
lignocaine, procaine produce anaesthesia
(local) in a localized region only.

The second type of action is systemic or
general action which produce their action
after absorption, for example, general
anaesthetics act centrally after absorption,

(Mode of Action of Drugs)

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1.4

Chapter

1.5

and often drugs like antibiotics, antacids,

and so many other drugs acts systemically.

The proper localization of the site of

drug action can be determined pharmaco-

logically. When a new drug is introduced

its screening gives an idea about the site and

mechanism of drug action, for example, if

any drug is said to be antihypertensive in

nature and by blocking this action by prior

administration of an antihistaminic, it will

gives an idea that the drug may act in the

same place and same mechanism as hista-

mine. The use of certain blocking agents also

help in suggesting the probable site of ac-

tion of drugs.

TYPES OF DRUG ACTION

The drug may produce their effects by:

i. Stimulation.

ii. Depression.

iii. Irritation.

iv. Replacement.

v. Bactericidal and cytotoxic action.

Stimulation

(Mode of Action of Drugs)(Mode of Action of Drugs)(Mode of Action of Drugs)(Mode of Action of Drugs)(Mode of Action of Drugs)

PharmacodynamicsPharmacodynamics