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