Bioavalibility of drug

Definition:(Bio-availability): It is drfined as the fraction of unchanged drug reaching the systemic circulation following administration by any route.                 (Ref:Katzung-11th,P-43)

e.g. Bio-availability of digoxin is 62%.It means if a patient orally takes 250mg of digoxin.Only 155mg (62%) of drug will reach to the systemic circulation.


 Significance of bio-availability:

  1. It indicates the route of administration of drug.
  2. To compare different formulation of same drug.
  3. To compare different formulation produced by different comparies.
  4. To asses drug serum concentration of individualization of doses.

 Factors modifying bio-availability:

  • Route of administration:
Routes Bioavailability
1.Intervenous 100%
2.Transdermal 80-100%
3.Subcutaneous 75-100%
4.I.M 75-100%
5.Rectal 30-<100%
6.Oral 5-<100%
7.Inhalation 5-<100%


  • Dosages from a drug:Liquid>solid.            (Ref:Katzung-11th,P-43)
  • Size of drug:Crystalliod>Colloid.
  • Lipid solubility:↑ absorption-↑ Bioavailability.
  • Character of excipient:It also effect bioavailability.
  • Pre systemic first pass metabolism→↑Meatbolism→↓Bioavailability.
  • Presence of other agents:Vit c-↑ absorption of Fe-↑ bioavailability.
  • Vascularity:Increase vascular- ↑ Bioavailability.

Importance of Bioavailability:

  1. To compare different formulation produced by different compaines.
  2. To compare different formulations of same drug.
  3. To assess drug serum conc & individualization of doses:
  • When intensity or effects is difficulty to judge cliically(Phenytoin for epilepsy).
  • Low safety magrin.(Lithium).

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