Extravascular administration means drug administration by any route other than intravenous or intra-arterial (i.e. any route that does not involve direct administration of the drug into a blood vessel).

After extravascular drug administration it is possible that only a fraction of the administered dose reaches the systemic circulation.Only intravenous and infra-arterial administration guarantee that the entire dose reaches the systemic circulation (100% bioavailable).

Extravascular administration may result in complete or partial absorption of the administered dose to the systemic circulation (0%-100% bioavailable).

Bioavallability is a measure of the fraction of the administered dose that reaches the systemic circulation.

Absolute bioavailability is the fraction of the total dose that reaches the systemic circulation. It can have values between zero when the drug is not absorbed at all to 1 when all the administered dose reaches the systemic circulation. The absolute bioavailability can be expressed in terms of percentage (0% - 100%).

The relative bioavailability is the bioavailability of a drug product relative to the bioavailability of another drug product (reference standard preparation). The relative bioavailability can have any value above zero. It can be more than one when the product under investigation has bioavailability higher than that of the reference standard.