BIOAVAILABILITY

Introduction:  This is the amount of drug that reaches systemic circulation following administration using any of the drug administration routes. Bioavailability gives the order whether the drug should be given intravenously, intramuscular, oral among others depending on the target tissue.  

It is normally expressed as a percentage of the drug reaching the systemic circulation in a given time t, to the total amount of the drug administered when time t=0.  The following should be noted as the three extremes when discussing bioavailability;  

1.  When a drug is administered through intravenous route, all of it reaches the systemic circulation and this means achieving 100% bioavailability, as long as it is not wasted by the the administrator.  Achieving 100% bioavailability is not only possible with IV route but also with Intramascular route and intradermal although the rate of drug deposition into blood may be slower.

Read more about intravenous drug administration 

2.  When a drug is taken topical, its bioavailability maybe less than 1% or even less, depending on the solubility of such a drug, and the drug administration techniques.  In fact some drugs are given that way because their access to circulation is not necessary.  Here we are looking at drugs such as ointments for topical use, creams for opthalamical applications and certain drugs applied on wounds.

Topical application of medications

3.  When a drug is administered through Gastrointestinal tract(GIT), its bioavailability will depend on various factors discussed earlier in drug distribution.

RELATED;

1.  PHARMACOKINETICS  

2.  DRUG DISTRIBUTION

3.  PHARMACODYNAMICS

4.  THE INTRAMASCULAR ROUTE OF DRUG ADMINISTRATION