Pharmacokinetic Drug Interactions

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Chapter: Biopharmaceutics and Pharmacokinetics : Pharmacokinetic Drug Interactions

Drug interactions are said to occur when the pharmacological activity of a drug is altered by the concomitant use of another drug or by the presence of some other substance.


Pharmacokinetic Drug Interactions

Drug interactions are said to occur when the pharmacological activity of a drug is altered by the concomitant use of another drug or by the presence of some other substance. The drug whose activity is affected by such an interaction is called as the object drug and the agent which precipitates such an interaction is referred to as the precipitant.

Drug interactions include –

1. Drug-drug interactions.

2. Food-drug interactions, for example, inhibition of metabolism of several drugs by grapefruit juice.

3. Chemical-drug interactions, for example, interaction of a drug with alcohol, tobacco or environmental chemicals.

4. Drug-laboratory test interaction, for example, alteration of diagnostic laboratory test results by the presence of drug.

5. Drug-disease interactions, for example, worsening of disease condition by the drug.

The net effect of a drug interaction is –

·           Generally quantitative i.e. increased or decreased effect.

·           Seldom qualitative i.e. rapid or slower effect.

·           Precipitation of newer or increased adverse-effects.

Most interactions are specific types of adverse reactions with altered efficacy of the drug, for example an enhanced pharmacological activity (e.g. haemorrhagic tendency of warfarin when phenylbutazone is given subsequently) or a decrease in the therapeutic activity resulting in loss of efficacy like that of tetracycline when concomitantly administered with food, antacids or mineral supplements containing heavy metal ions. Drug interactions are thus –

·           Mostly undesirable.

·           Rarely desirable (beneficial) – for e.g., enhancement of activity of penicillins when administered with probenecid.

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