This document discusses different types of drug interactions that can occur. It describes in vitro interactions that happen when drugs are mixed, such as thiopentone and suxamethonium chemically interacting. It also covers in vivo pharmacokinetic interactions that alter absorption, protein binding, metabolism, or excretion of drugs. This can increase or decrease a drug's effects. Specific examples are provided like antacids decreasing tetracycline absorption. Pharmacodynamic interactions are also discussed, such as synergism between aspirin and heparin, or antagonism between metoclopramide and anticholinergic drugs. Combined toxicity is described where drugs enhance each other's side effects.

1. DRUG
INTERACTIONS

2.  INTRODUCTION
 DEFINITION
 TYPES
 IN VITRO
 IN VIVO

3. In Vitro
These are also called incompatibility
reactions. Such interactions usually occur
with I / V fluids, whenever a drug is added to
a reservoir. The main factor is due to
change in pH.
Examples:
1.Thiopentone & Suxamethonium interact
chemically & should not be put in same
syringe.

4. 2.Heparin & Benzyl-penicillin also interact. In
presence of Dextrose, Hydrocortisone &
Sympathomimetics, activity of Heparin is
lost.
3.Sympathomimetics, Aminophylline,
compound vitamin preparations & vitamin
C are liable to interact with wide range of
drugs.

5. IN VIVO
 Pharmacokinetic Drug Interactions
 Pharmacodynamic Drug Interactions

6. PHARMACOKINETIC DRUG
INTERACTIONS
 Drug absorption
 Interactions due to changes in protein
binding of drugs
 Interactions affecting drug metabolism
 Interactions affecting renal excretion of drugs
When one drug alters these processes of the
other drug, there is increase or decrease in
the amount of the drug available for its
pharmacological actions.

7. Drug absorption (abs)
 Antacids interfere with abs of Tetracyclines
 Cholestyramine reduces abs of
Digitalis,Thyroxine
 Metoclopramide reduces abs of Cimitidine
 Liquid paraffin interferes with abs of Vit D
 Phenobarbitone reduces abs of Griseofulvin
 Phenytoin and OCPs reduce abs of Folic acid
 Rate of abs of Paracetamol is reduced by
Anticholinergics and increased by
Metoclopramide

8. Interactions due to changes in
protein binding of a drug
 Phenylbutazone,Indomethacin, Sulphonamide,
Tolbutamide and Clofibrate displace Warfarin
from its protein binding site and increase its
anticoagulant effect
 Dicoumarol,Sulphonamides and Salicylates
displace Tolbutamide from its protein binding
site leading to severe hypoglycemia

9. Interactions affecting drug metabolism
 Phenobarbitone increases metabolism of
Warfarin,Phenytoin,Griseofulvin and
Hydrocortisone due to enzyme
induction(MES)
 MAOIs interfere with metabolism of narcotic
analgesics,Barbiturates and TCAs to increase
their effects
 Isoniazid, PAS, Chloramphenicol, Disulfiram
and Dicoumarol increase the effect of
Phenytoin by inhibiting its metabolism
 Allopurinol increases the toxic effects of
Azathioprim and 6-Mercaptopurine

10. Interactions affecting renal
excretion of drugs
 Probenecid delays excretion of Penicillin,
Cephalosporin, Indomethacin, Dapsone
 Aspirin may increase toxicity of
Methotrexate
 NaHCO3 increases excretion of Aspirin
and Barbiturates
 NH4CL increases excretion of
Amphetamine

11. PHARMACODYNAMIC DRUG
INTERACTIONS
Synergism:
Aspirin, Dipyramidole increase effect of
Heparin and Phenindione
Antagonism:
Metoclopramide antagonise the effect of
Atropine, Propantheline, Benzhexol and
Narcotic analgesic on GIT

12. Combined Toxicity
 1.Alcohol,Antihistamines,Narcotic analgesics
increase
the effect of
sedative and hypnotics
2.Thiazides increase toxicity of Digoxin and
other Cardiac glycoscides

13. 3. Ethacrynic acid and Furosemide
increase ototoxicity of Aminoglycoscides
4. Antihistamines and Phenothiazine
derivatives increase side effects of
Anticholinergic drugs (Benzhexol