Bioavailability refers to the amount of drug absorbed relative to the amount given. This is calculated as the ratio of drug exposure after equivalent doses of oral and intravenous forms of a drug are administered (to the same subjects). Exposure is measured on a kinetic graph and is easy to calculate. It is the area under the curve (AUC) of the drug level (in serum) over time. This is illustrated in the accompanying figure. The fraction of oral drug that makes it to the circulatory system will always be less than the amount of drug achieved with IV infusion. The percentage of drug that reaches the serum from an oral dose would be the "oral bioavailability" for that drug.
For IV drugs, the bioavailability is 100% since all of the drug is available instantly.
For oral medications, the bioavailability will be less than 100% due in part to any of these reasons:
- 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.)
- Oral drugs may be broken down by enzymes that are present anywhere along the GI tract.
If bioavailability is low, the oral dose needed has to be increased so that a given dose achieves the appropriate serum concentration. Since the absorption of an oral drug is slower than that of an IV drug and the drug takes longer to enter the circulation, clearing the drug will also take a longer time.
