Bioavailability refers to the amount of drug absorbed relative to the quantity 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 as the area under the curve (AUC) of serum drug concentration over time. This is illustrated in the diagram on the right. The fraction of oral drug that makes it to the circulatory system is the bioavailability of 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 then 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 an IV drug and the drug takes longer to enter the circulation, clearing the drug will also most likely take a longer time.