PD

1. COMBINED EFFECT
OF DRUGS
Dr S P Dhanya

3. Combined effect of drugs
Synergism Antagonism

5. + =Effect of
drug A
Effect of
drug B
Effect of drug A+B

6. Additive synergism
+ =
1 1
2

7. Aspirin+
Paracetamol

8. + <Effect of
drug A
Effect of
drug B
Effect of drug A+B

9. Supradditive synergism
+ =1 1
3

11. Cotrimoxazole

12. Only very small % reaches the
brain
Dopa
decarboxylase
Dopa
decarboxylase
Potentiation

13. + >Effect of
drug A
Effect of
drug B
Effect of drug A+B

14. Antagonism
+ =Effect of
drug A
Effect of
drug B
No effect

15. D
r
u
g
a
n
t
a
g
o
n
i
s
m
Physical
Chemical
Physiological
Dispositional
Receptor

16. Physical antagonism
Activated charcoal and alkaloids

17. Chemical antagonism
Anatcids neutralize HCl

19. Physiological antagonism
Adrenaline 0.5ml of 1:1000-
******Anaphylaxis*****

21. Dispositional antagonisn
Affecting A D M E 
↓concentration at
target site

22. Digoxin(inotropic agent)
Decreased efficacy of digoxin

23. Rifampicin
Oral
contraceptive
Pills

24. For an acidic drug
(salicylate) poisoning
what should we do?

25. Receptor antagonism
Competitive Non-competitive

26. Receptor Interactions
Agonist Receptor
Agonist -Receptor
Complex
EFFECT

27. Receptor Interactions
Antagonist Receptor
Antagonist-Receptor
Complex
DENIED!
Atropine
NO
EFFECT

28. i) Competitive Antagonism
Competition between agonist &
antagonist for specific site or
receptor
Antagonism can be overcome
by  concentration of agonist
Surmontable antagonism

29. COMPETITIVE ANTAGONISM
Agonist - maximal response in higher concentration
Shifts to Log DRC to?
Which means?

30. Receptor Interactions
Antagonist Receptor
Antagonist-Receptor
Complex
ACh
Atropine
ACh
ACh
ACh
ACh
ACh
EFFECT
Competitive
antagonism

31. Morphine

32. Agonist Receptor
Antagonist
‘Inhibited’-Receptor
DENIED!
Non-competitive
Inhibition

33. iii) Non-competitive
(non-surmountable) Antagonism
Antagonist binds to allosteric
site of receptor
Unrelated to agonist

34. Bicuclline
Diazepam(agonist)
Flumazenil
Competitive
anatgonist
Non-Competitive
anatgonist

35. Non-competitive Antagonist
What is decreased?
DRC is flattened and maximum response is 

36. II) Irreversible Antagonist
Antagonist binds to receptor with
covalent bond-irreversible
Slow dissociation-pseudo-irreversibe
 max. response

38. COMPETITIVE NON COMPETITIVE
Surmountable Unsurmountable
Antagonist resembles
chemically with the agonist
Does not resemble
Bind with the same site Binds with another site
Maximum response
obtained by increasing the
dose
Maximum response
suppressed
Parallel rightward shift of
DRC
Flattening of DRC
Eg. Atropine on muscarinic
receptors
Eg. Bicuculine on
benzodiazepine receptors