1.9 Bioavailability

Define bioavailability (F).

The proportion of an administered drug that reaches systemic circulation in active form.

What is the bioavailability of IV drugs?

100%.

Name two main factors that lower oral drug bioavailability.

Incomplete absorption and first-pass metabolism.

How does lipid solubility affect absorption?

Lipid-soluble drugs cross membranes more easily, increasing absorption.

Give an example of a factor in the stomach that can affect absorption.

Antacids changing pH.

What does F = 0.21 mean?

21% of the administered dose reaches systemic circulation.

In Example 1, with 500 mg dose and F = 0.45, how much reaches systemic circulation?

225 mg.

In Example 1, how much drug was not absorbed?

178.6 mg.

In Example 2, with 110 mg dose, F = 0.21, and 37% excreted, how much was metabolised?

46.2 mg.

Why can’t you solve for absorbed, metabolised, and excreted amounts if two are unknown?

Because there’s insufficient data; one unknown is the limit for calculation.