Bioavailability refers to the amount of drug that actually reaches the circulation. It is calculated by comparing (in the same subjects) the area under the serum concentration - time curve (AUC) of an equivalent dose of the intravenous form and oral form. This is illustrated in the diagram on the right.

For IV drugs, the bioavailability is 100%

For oral medications, the bioavailability will be less than 100%, due in part to any of these reasons:

* Oral drugs take longer to enter the circulation.
* Oral drugs have slower absorption and distribution than IV drugs.
* The amount of drug that is absorbed can depend on the status of the GI tract (stomach pH, presence of food, integrity/health of the intestines, speed of the GI tract, etc.)

For oral drugs to be effective, bioavailability typically should be greater than 70%.

Not all of a drug taken orally is able to have a pharmacologic effect; the dose would need to be higher than an IV dose.

Since the absorption of an oral drug is slower than an IV drug and the drug takes longer to enter the circulation, clearing the drug will also most likely take a longer time.