Fraction of the dose of a drug contained in any dosage form that reaches the systemic circulation in unchanged or active form administered through any route is known as bioavailability.
Drugs injected using intravenous route of administration have 100% bioavailability, while others have much less bioavailability, because:
- All of the drug may not be adsorbed
- Metabolism of the drug might occur before reaching the site of action
Drugs not absorbed by the oral route are highly polar drugs, thus have low bioavailability.
Bioavailability = AUC (oral)/ AUC (I/V) x 100
Where AUC is the area under the curve
X-axis represents time, while y-axis represents the plasma concentration.
Bioavailability is the ratio of the area calculated for oral route of administration to the intravenous route of administration. It is determined by comparing the plasma levels of a drug after administration with plasma drug level achieved by I/V injection.
Factors Affecting Bioavailability:
1. Route of administration
Drugs given by intravenous route have 100% bioavailability. Exception includes prostaglandins, which are inactivated/metabolized in the lungs, therefore, their bioavailability may be zero after I/V injection. Those given by intramuscular route have bioavailability less than I/V route but more than subcutaneous route, while subcutaneous route has bioavailability more than the oral route. Only 10% of the dose of digoxin reaches systemic circulation after oral administration because of lack of absorption and bacterial metabolism within intestines. Even some of the drugs given by oral route may have 100% bioavailability but this is rare.
By rectal route, half of the drug undergoes first pass metabolism.
Chloramphenicol, an antibiotic, administered by intravenous route has bioavailability less than oral route because it is present in pro form and has to be activated in the intestines.
| Route |
Bioavailability |
Characteristics |
| Intravenous |
100% |
Most rapid |
| Intramuscular |
75≤100% |
Large volume may be injected but painful method |
| Subcutaneous |
75≤100% |
Smaller volume than IM, may be painful |
| Oral |
5≤100% |
Convenient, first pass metabolism occurs |
| Rectal |
30 |
Bioavailability of Drugs – howMed
Fraction of the dose of a drug contained in any dosage form that reaches the systemic circulation in unchanged or active form administered through any route is known as bioavailability.
Drugs injected using intravenous route of administration have 100% bioavailability, while others have much less bioavailability, because:
Drugs not absorbed by the oral route are highly polar drugs, thus have low bioavailability.
Bioavailability = AUC (oral)/ AUC (I/V) x 100
Where AUC is the area under the curve
X-axis represents time, while y-axis represents the plasma concentration.
Bioavailability is the ratio of the area calculated for oral route of administration to the intravenous route of administration. It is determined by comparing the plasma levels of a drug after administration with plasma drug level achieved by I/V injection.
Factors Affecting Bioavailability:
1. Route of administration
Drugs given by intravenous route have 100% bioavailability. Exception includes prostaglandins, which are inactivated/metabolized in the lungs, therefore, their bioavailability may be zero after I/V injection. Those given by intramuscular route have bioavailability less than I/V route but more than subcutaneous route, while subcutaneous route has bioavailability more than the oral route. Only 10% of the dose of digoxin reaches systemic circulation after oral administration because of lack of absorption and bacterial metabolism within intestines. Even some of the drugs given by oral route may have 100% bioavailability but this is rare.
By rectal route, half of the drug undergoes first pass metabolism.
Chloramphenicol, an antibiotic, administered by intravenous route has bioavailability less than oral route because it is present in pro form and has to be activated in the intestines.