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On administration of two or more drugs simultaneously to the patient, the effect of one Drug may be altered by another Drug.
Drug interactions can occur in-vitro (outside the body) or in-vivo (inside the body).
in-vitro Drug interactions may also called as Pharmaceutical interactions.
In-vivo Drug interactions may be pharmacokinetic or pharmacodynamic.
Drug Interactions can result in either beneficial or harmful effects.
A) Pharmaceutical Interactions:
These can occur as a result of incompatibility (physical or chemical) of a drug with a intravenous solution or when two or more drugs mixed in the same syringe/i.v. infusions. This may result in precipitation or inactivation of one or more drugs.
Examples of in-vitro drug interactions:
1. Phenytoin and Dextrose : Phenytoin should not be administered in dextrose solution as it gets precipitated.
2. Ampicillin and Dextrose Solution:
Dextrose solution is not suitable for i.v. infusion of ampicillin as it is unstable at acidic pH of dextrose.
3. Gentamicin and carbenicillin should not be given in the same infusion as it may result in loss of potency.
B) Pharmacokinetic interactions:
When absorption, distribution, metabolism or excretion (ADME) of one drug is altered by another drug. It is the type of in-vivo Interactions and called Pharmacokinetic interaction.
Examples of Pharmacokinetic interactions:
1. Drug interactions affecting absorption:
i) Antacids / iron and tetracyclines: antacids
(containing aluminium, magnesium and calcium), iron etc. interfere with the absorption of tetracyclines by forming un-absorbable complexes with it.
ii) metoclopramide and aspirin: (alters gastrointestinal motility)
Metoclopramide increases the rate of gastric emptying and promotes absorption of aspirin.
2. Drug interactions affecting distribution:
Plasma proteins and drug with higher affinity: (displacement interactions)
More than one drug can bind with the plasma proteins and in this case the drug with higher affinity will displace the drug with lower affinity. This increases the concentration of unbound drug. e.g. salicylate displace warfarin from binding sites resulting in increased free warfarin level in blood and enhance anticoagulant effect.
3. Drug interactions affecting metabolism:
Drug interactions can occur in-vitro (outside the body) or in-vivo (inside the body).
in-vitro Drug interactions may also called as Pharmaceutical interactions.
In-vivo Drug interactions may be pharmacokinetic or pharmacodynamic.
Drug Interactions can result in either beneficial or harmful effects.
A) Pharmaceutical Interactions:
These can occur as a result of incompatibility (physical or chemical) of a drug with a intravenous solution or when two or more drugs mixed in the same syringe/i.v. infusions. This may result in precipitation or inactivation of one or more drugs.
Examples of in-vitro drug interactions:
1. Phenytoin and Dextrose : Phenytoin should not be administered in dextrose solution as it gets precipitated.
2. Ampicillin and Dextrose Solution:
Dextrose solution is not suitable for i.v. infusion of ampicillin as it is unstable at acidic pH of dextrose.
3. Gentamicin and carbenicillin should not be given in the same infusion as it may result in loss of potency.
B) Pharmacokinetic interactions:
When absorption, distribution, metabolism or excretion (ADME) of one drug is altered by another drug. It is the type of in-vivo Interactions and called Pharmacokinetic interaction.
Examples of Pharmacokinetic interactions:
1. Drug interactions affecting absorption:
i) Antacids / iron and tetracyclines: antacids
(containing aluminium, magnesium and calcium), iron etc. interfere with the absorption of tetracyclines by forming un-absorbable complexes with it.
ii) metoclopramide and aspirin: (alters gastrointestinal motility)
Metoclopramide increases the rate of gastric emptying and promotes absorption of aspirin.
2. Drug interactions affecting distribution:
Plasma proteins and drug with higher affinity: (displacement interactions)
More than one drug can bind with the plasma proteins and in this case the drug with higher affinity will displace the drug with lower affinity. This increases the concentration of unbound drug. e.g. salicylate displace warfarin from binding sites resulting in increased free warfarin level in blood and enhance anticoagulant effect.
3. Drug interactions affecting metabolism:
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