Administration of different drug products that contain the same amount of the same active drug may not produce similar therapeutic effects. This can be explained by the difference in of the drug after administration of different drug products.
Causes of incomplete bioavailability
a- Route of administration
Generally parenteral administration produce higher bioavailability compared to oral or rectal administration.
Only iv administration guarantees 100% bioavailability.
Drug absorption is usually faster in areas that are rich in blood supply. For example IM administration results in faster drug absorption compared to subcutaneous administration because the skeletal muscles are supplied with more blood compared to subcutaneous tissues. Also, buccal absorption is very fast.
b- Factor related to the formulation.
Although it is not a general rule, the bioavailability of an oral solution is > oral suspension > capsule > tablet.
Excipients that are added to the formulations such as fillers, binders, coating material, etc. can also affect drug bioavailability.
Manufacturing conditions can also cause variation in drug bioavailability.
c- Factors related to the drug.
The physicochemical properties of the drug determine its solubility and dissolution rate which are major factors in determining the drug bioavailability.
The lipophilic/hydrophilic properties of the drug are very important in determining the ability of the drug to cross the biological membrane at the absorption site to reach the systemic circulation.
Stability of the drug at the site of administration is an important factor in determining the drug bioavailability.
d- Factors related to the patient.
Physiological factors such as gastric pH, gastric emptying rate, and gastric motility can significantly affect drug bioavailability.
Gastrointestinal diseases and aging usually affect the drug absorption process
Some drugs such as chemotherapeutic agents and phenytoin can decrease the absorption of other drugs after oral administration.
