The document discusses different types of interactions that can occur when two or more drugs are taken together. It describes additive or summation effects when the effects of the drugs combine in an additive fashion. It also explains drug synergism, where the combined effect is greater than the sum of the individual drug effects. Additionally, it covers drug antagonism, where the combined effect is less than additive due to one drug opposing or decreasing the effects of another drug. The types of antagonism discussed include competitive, non-competitive, chemical, physiological, and physical antagonism.

1. COMBINED EFFECTS OF DRUGS
By:
Dr. Dhruva Kumar Sharma
Department of Pharmacology
Moderator:
Dr. Supratim Datta
Associate Professor
Department of Pharmacology
SMIMS

2. Protocol:
Summation
Additive effects
Synergism
Drug antagonism
2

3. EFFECT OF COMBINATION OF DRUGS
Combinations of two/ more drugs, simultaneously or
in quick succession
1. No interference with each other’s effects.
2. May oppose each other’s actions (antagonism)
3. May produce similar actions on the same organ
(synergism)

4. Interaction
Pharmacokinetic Pharmacodynamic

5. Drug Synergism
Syn- together ; ergon- work

6. Drug Synergism:
This is facilitation of the effects of one drug
by another when given together
Types:
a. Additive (summation)
b. Supra-additive (Potentiation)

7. Summation /Addition
Effect of drugs A + B = Effect of drug A + Effect of drug B
• Final effect is same as the algebraic sum of the
magnitude of individuals drugs
• Side effects do not add up
Examples of Summation: Different MOA
Aspirin : (-) PG synthesis  analgesia +
Codeine : Opioid agonist  analgesia +
Examples of Addition: Same MOA
Ibuprofen: (-) PG synth  analgesia +
Paracetamol: (-) PG synth  analgesia+
Analgesia
++
Analgesia
++

8. Other Additive Drug Combinations
Drug
Combination
Effect
Amlodipine +
Atenolol
Antihypertensive
Glibenclamide +
Metformin
Hypoglycemic

9. Supraadditive ( Potentiation)
Effect of drug A + B > Effect of drug A + Effect of drug B
When two drugs are given together the final effect is much
more than the simple algebraic sum of the magnitude of
individuals drugs.
Examples:
Sulphamethoxazole & Trimethoprim--- sequential blockade
of two steps in synthesis of folic acid in micro-organisms.

11. Synergism by altering Pharmacokinetics
of the other:
• Levodopa + Carbidopa

12. Other supraadditive drug
combinations
DRUG PAIR BASIS OF POTENTIATION
Ach + Physostigmine Inhibition of break
down
Adrenaline + Cocaine Inhibition of neuronal
uptake
Tyramine + MAO
inhibitors
Increasing
releaseable CAT
store

13. Drug Antagonism

14. Drug Antagonism
Definition:
Combined effect of two drugs is less than the sum of
the effects of the individual drugs
Effect of drugs A + B < Effect of drug
A + Effect of drug B
One drug decreases / opposes / reverses / counters
the effect of other drug by different mechanisms

15. Types:
a. Pharmacological Antagonism :
i. Competitive (Reversible)
ii. Non-competitive (Irreversible)
b. Chemical Antagonism
c. Physiological Antagonism
d. Physical antagonism

16. Pharmacological Antagonism:
Competitive Non Competitive
Irreversibly
competitive
Reversibly
competitive
Pseudo-
reversibly
competitive
Interfere “Down-stream events” Act on
“allosteric site”
PHARMACODYNAMIC ANTAGONISM

17. Competitive
Antagonism
18

18. 19
D-R interactions
RBB
D
B

19. = Agonist = Antagonist
0
20
40
60
80
100
120
-10.5 -10 -9.5 -9 -8.5 -8 -7.5 -7 -6.5 -6

20. = Agonist = Antagonist
0
20
40
60
80
100
120
-11 -10 -9 -8 -7 -6

21. = Agonist = Antagonist
0
20
40
60
80
100
120
-11 -10 -9 -8 -7 -6

22. = Agonist = Antagonist
0
20
40
60
80
100
120
-11 -10 -9 -8 -7 -6

23. = Agonist = Antagonist
0
20
40
60
80
100
120
-11 -10 -9 -8 -7 -6

24. = Agonist = Antagonist
0
20
40
60
80
100
120
-11 -10 -9 -8 -7 -6

25. = Agonist = Antagonist
0
20
40
60
80
100
120
-11 -10 -9 -8 -7 -6

26. 27
Reversible-Competitive
B
D
R
• Weak bond
• Same agonist
site
• Short duration

27. 28
LDRC shift to R
B
R
D
D
D
D
D
Reversible-Competitive
Conc dependant  Dynamic Equilibrium

28. Competitive (Reversible) Antagonism /Competitive
(Equilibrium ) Antagonism
1. Same receptor by forming Weak bonds
2. Maximal response depends on concentration of
both agonist and antagonist
3. The effect of antagonist can be overcome by
increasing the concentration of agonist. The same
maximal response can be attained by increasing
dose of agonist---It is “surmountable antagonism”.
4. Parallel rightward shift of DRC

29. = Agonist = Antagonist
0
20
40
60
80
100
120
-11 -10 -9 -8 -7 -6

30. = Agonist = Antagonist

31. Examples: Atropine and Acetylcholine at Muscarinic -R
Naloxone and Morphine at opioid-R
Propranolol and NE at β2 - R
%Response
50
ED 50 ED 50 ED 50

32. Irreversibly Competitive or Non
Equilibrium Competitive Antagonism:
1.Have affinity for the same receptor sites and
bind in an irreversible manner by covalent
bond
2.Effects cannot be overcome even by
increasing the concentration of the agonist
(unsurmountable)
3.LDR curves of agonist (in presence of
antagonist) would show reduced efficacy but
unaltered potency

33. 34
Irreversibly- Competitive
B
D
R
• Same agonist site
• Strong bond
• LDRC  efficacy
(flatten)
• Long duration

34. 35

35. 36
Irreversible antagonist
+ Agonist
+ Agonist

36. • DOA of irreversible antagonist is longer
• Equilibrium between Antagonist - Agonist
cannot be established even after increasing
the dose of agonist hence the term “Non-
equilibrium competitive antagonism”
• E.g. Dibenamine and NE at α1 adrenoceptors

37. Pseudo-reversible Antagonism:
• Lesser degree of receptor occupancy by the
antagonist & availability of spare receptors
• Increasing conc. of agonist- shift LDR to right
• Increasing conc. of antagonist- reduction in
maximal response.
• Hence the term “Pseudo-reversible
Antagonism”

38. 39
Pseudo-Reversible Competitive
B
D
R
R
R
R
D
D
• Strong bond
• Spare receptors
Agonist
overcomes
antagonist
• Same agonist
site
• LDRC

39. 40
Pseudo reversible competitivePseudo - Reversible Competitive
Inc. dose of agonist

40. E.g.
Phenoxybenzamine - at α1 adrenoceptor
Methysergide - at 5HT receptor
(5HT receptor blocker)
41
Pseudo-Reversible Competitive

41. Non Competitive
Antagonism
42

42. 43
Non Competitive Antagonism
• Via Allosteric Modulation
• Receptor-Effector pathway
modulation
(Down-stream regulation)
NO Competition
for Agonist site

43. 44
B
D
R
•Different
Receptor site
•DR interaction
ineffective
•No Reversal
•LDRCflatten
Antagonism through Allosteric
receptor site binding:

44. i. Binds to site other than the agonist site
ii. Prevent the receptor activation by the
agonist
E.g.
• Flumazenil by binding to BZD site
antagonises the effects of BZD by
preventing the binding of GABA to GABAA
receptor
• Bicuculline and BZD
Antagonism through Allosteric
receptor site binding:

45. 46
GABA
GABA
binding site
Channel
blocker
(Picrotoxin)
Channel
modulators
(barbiturates)
Inverse
agonists
(β-carbolines)
Flumazenil
(antagonists)
BenzodiazepinesModulatory Site
Cl-
Cl-
Antagonism through Allosteric
receptor site binding:

46. 47
Receptor-Effector pathway modulation
(Down-Stream Regulation)
RD   

47. Receptor-Effector pathway modulation
(Down-stream regulation)
48
AT1-R
NE
Ag II
Prazosin
Comp. Ant
Losartan
Comp. Ant
IP3,
 DAG
α1-R
Ca2+
channel Activation
Free Ca2+
entry
Ca2+
Channel blocker
(eg., Nifedipine,
non-competitive antagonist
Vasoconstriction

48. Effects on log DRC
• There is downward shift .The slope is reduced
and maximum response is diminished
• The parallelism is not maintained
• No shift of curve on dose axis

49. 50
• Competitive Antagonism
(equilibrium or reversible)
 Action of agonist is blocked if
conc. of antagonist is 
 Antagonism can be overcome
by  conc. of agonist
 Agonist can produce max.
response in higher conc.
 Competitive antagonist shifts
LDRC of agonist to right
 ED50 of agonist  in presence
of antagonist, e.g., Ach &
atropine; Adr & Prop.;
Morphine & Naloxone
• Non-competitive
(non-surmountable
Antagonist)
 Antagonist binds to another
site of receptor
 LDRC is flattened + max.
response is 
 e.g. Diazepam and bicuculline

50. Chemical Antagonism
A type of antagonism where a drug counters the effect
of another by simple chemical reaction / neutralization
(not binding to the receptor)
1. Protamine sulphate & Heparin
2. Calcium sodium edetate form insoluble complexes
with arsenic / lead
3. Neutralization of gastric acid by antacids like
Aluminium hydroxide, Magnesium hydroxide,
Sodium bicarbonate

51. Physiological Antagonism
Definition:
A type of antagonism in which one drug opposes
/ reverses the effect of another drug by binding
to a different receptor and producing opposite
physiological effects
Examples:
1. Histamine and adrenaline on bronchial
muscles and BP
2. Glucagon and insulin on blood sugar level

52. Physical antagonism
• Based on the physical property of drugs e.g.
Charcoal adsorbs alkaloids and can prevent
their absorption- used in alkaloidal poisonings
53

53. REFERENCES
• Goodman Gilman - The Pharmacological Basis of
Therapeutics, 12th
Edition
• Katzung – Basic & Clinical Pharmacology,
12th
Edition
• Sharma – Priciples of Pharmacology,
2nd
Edition
• K.D Tripathi – Essentials of Medical
Pharmacology, 7th
edition
• R.S Satoskar – Pharmacology and
Pharmacotherapeutics, 18th
Edition
• www.google .com

54. 55
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