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It covers all the aspects relating to “What a drug
does to the body”
It is the study of the mechanism by which the drug
produces a response.
Action: How and Where the effect is produced is
called as Action.
Effect: The type of response producing by drug.
TYPES OF DRUG ACTION
EFFECT (Type of responses):-
it is selective enhancement of the
level of activity of specialized cells.
Eg; adrenaline stimulate the heart.
it is selective diminution of activity
of specialized cells. Eg: quinidine
This refers to the use of natural
metabolites, hormones in deficiency
states. Eg; iron in anaemia.
This refers a nonselective, often
noxious effect and is particularly
applied to less specialized cells. Eg:
counterirritants increase blood flow
to the site.
For invading parasites or cancer cells. Eg: penicillin
Mechanism of Drug Action:
A drug may produce its effects through various
1- Physical action:
Drug retains water by osmosis as osmotic diuretics (Mannitol) and osmotic
Drug adsorbs diverse substances including toxins and fluid on its
surface thereby inactivating them (activated charcoal).
2- Chemical action:
- Antacids neutralize gastric acidity (e.g. NaHCO3)
- Chelating agents form biologically inactive complex
with other substances
3- Regulatory Proteins (Body Control systems):
Drugs may inhibit or activate certain enzymes.
Drugs may increase their synthesis or block their recognition site.
Drugs may open ion channels by acting on specific receptor which forms the
channel or drugs may physically close the channel
Responses to drug-receptor interaction can be as Agonist
(substance binds and activates the receptor) or
Antagonist (substance blocks the receptor.)
Receptors are macromolecules
Most are proteins
Present either on the cell surface, cytoplasm or in
Drug(D) +Receptor® Drug receptor complex
Drug receptor interaction:-
1. Selectivity:- Degree of complimentary co relation between
drug and receptor.
Ex:- Adrenaline Selectivity for α, ß Receptor
2. Affinity:- Ability of drug to get bound to the receptor.
3. Intrinsic activity (IA) or Efficacy:- Ability of drug to produce a
pharmacological response after making the drug receptor
Four types of receptors families
1. Receptors with intrinsic ion channels.
2.G-protien coupled receptor
3. Enzymatic receptors
4 intracellular receptor
Receptors with intrinsic ion channels
-located on the cell surface
-they enclose ion selective channels for
-when an agonist is bound to these receptors they
Eg: nicotinic cholinergic receptors
2.G-protien coupled receptor
located on the cell membrane
G protein is a connecting link between receptor and
effector systems ,like enzymes, carrier molecules
They are called so because of their link with GDP
Present either in cytoplasm or nucleus of the cell.
Vit A ,corticosteroids etc act through these
These are enzyme molecules themselves
Eg: tyrosine protein kinase.
SPECIAL TYPE OF RECEPTORS
Even when a receptor is blocked by irreversible
blocker ,an agonist is still capable of producing
undiminished maximum response.
Eg: if beta adrenergic receptors are blocked
irreversibly and then adrenaline is pushed , there is
still production of an undiminished maximum
A drug when bound to such receptors, exhibits no
Eg: silent tissue receptors.
Usually found in axonal terminals and on cell
bodies of neurons.
If these receptors are stimulated , usually there
occurs a inhibitory response.
It is due to inhibition of release of excitatory
REGULATION OF RECEPTORS
Receptor down regulation: or Desensitization
Prolonged use of agonist
Receptor number and sensitivity
Ex: Chronic use of salbutamol down regulates ß2
Receptor up regulation: or Super sensitivity
Prolonged use of antagonist
Receptor number and sensitivity
Eg:- if timolol is stopped after prolong use, produce
withdrawal symptoms. Rise iop.
DOSE RESPONSE CURVE
When a drug is administered, it produces a response and
this response shows alteration with change in dose.
The change can be plotted with dose as abscissa and
response as ordinate.
The resulting curve is known as DOSE RESPONSE
The dose response curve rises steeply at first, but after
that it become steady as the dose is increased.
Curve is popularly known as hyperbolic or exponential
Such a curve can be shown by plotting dose against
Initially no appreciable response until a particular
threshold is obtained.
Thereafter the curve rises steeply until a maximum
response is attained.
But after this there is no appreciable change in the curve
even with increased dose.
This is called sigmoid or s shaped curve (log dose is
plotted against percent data)
It is used to indicate the amount of drug required to
produce a specific response.
Efficacy is the maximal effect produced by a drug . This is
important to know when deciding between two drugs that have
similar action. For example, two antibiotics may effectively kill
the same organism, but one may take more doses than another,
making the other more effective.
Drug A is more potent than B
Drug A is less effective than B
Median effective dose(ED50 ).
Is a dose of the drug that gives a response equals to 50% of the
Median lethal dose(LD50 ).
Is the dose of a drug required to produce
toxicity in 50 % of patients.
It is a measure of the relative safety of a drug for a
Therapeutic index = LD50 / ED50
Large value - a wide margin of safety. Eg:Penicillin
Small value - a narrow margin of safety Eg:
COMBINED EFFECT OF DRUGS
When two or more drugs are given simultaneously or
directly after each other, they may be either indifferent to
each other or exhibit synergism or antagonism
Action of one drug is increased by the other.
combined effect of two drugs acting by same mechanism
Synergism (Supra additive):- (1+1=3)
The combined effect of two drug effect is higher than either
2. Levodopa + Carbidopa.
When one drug decreases or abolishes the action of another.
Effect of drug A+ B < Effect of drug A+ Effect of drug B
1) Physical Antagonism
Based on physical property.
Charcoal adsorbs alkaloids and can prevent their
absorption used in alkaloid poisoning.
2) Chemical Antagonism
Two drugs react chemically and form an inactive
KMnO4 oxidizes alkaloid used for gastric lavage in
3) Physiological Antagonism
Two drugs acting on different receptors by different
mechanism, have opposite effect on the same
Glucagon & insulin on blood sugar.
4) Pharmacological Antagonism
Two drugs compete for the same receptor.
The antagonist partially or completely
prevents the pharmacological agonist effect.
The antagonist dissociates rapidly from the receptor.
The antagonist effect can be overcome by increasing the
e.g. morphine & naloxone
The antagonist dissociates very slowly or not at all from
the receptor .
The action of antagonist cannot be overcome by
increasing the agonist concentration .
Appropriate amount of drug required to produce a
desired pharmacological action.
Dose required to achieve a target concentration rapidly.
used to retain the target level by balancing the
Loading dose of doxycycline is 200 mg and maintenance
dose is 100 mg/day.