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Concept of Biopharmaceutics

Many factors have been found to influence the rate and extent of absorption, and hence the time course of a drug in the plasma, and therefore at its site(s) of action. These include the foods eaten by the patient, the effect of the disease state on drug absorption, the age of the patient, the site(s) of absorption of the administered drug, the co-administration of other drugs, the physical and chemical properties of the administered drug, the type of dosage form, the composition and method of manufacture of the dosage form, the size of the dose and the frequency of administration.

Thus, a given drug may exhibit differences in its bioavailability if it is administered:

  • in the same type of dosage form by different routes of administration, e.g. an aqueous solution of a given drug administered by the oral and intramuscular routes,
  • by the same routes of administration but different types of dosage form, e.g. a tablet, a hard gelatin capsule and an aqueous suspension administered by the peroral route,
  • in the same type of dosage form by the same route of administration but with different formulations of the dosage form, e.g. different formulations of an oral aqueous suspension.

Variability in the bioavailability exhibited by a given drug from different formulations of the same type of dosage form, or from different types of dosage forms, or by different routes of administration, can cause the plasma concentration of the drug to be too high, and therefore cause side-effects, or too low, and therefore the drug will be ineffective.

The above figure shows the plasma concentration time curve following a single oral dose of a drug, indicating the parameters associated with a therapeutic effect. The therapeutic window is the drug concentrations which are above the minimum effective concentration and below the maximum safe concentration.

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