Bioavailability

Definition

Bioavailability is the percentage of an administered drug that enters systemic circulation in an active form. Intravenous drugs have 100% bioavailability by definition, while oral and other routes are typically lower due to first-pass metabolism.

Factors Affecting Bioavailability

• Route of administration (IV > IM > SubQ > PO)
• First-pass hepatic metabolism
• Food interactions (some drugs require empty stomach, others with food)
• GI pH and transit time
• Drug formulation (tablet, capsule, suspension, enteric-coated)
• Patient factors: age, liver function, GI disease

Clinical Implications

Oral doses must often be higher than IV doses to achieve equivalent effect. Converting routes requires bioavailability-aware dose adjustment (e.g., oral to IV morphine reduces dose).

Nursing Considerations

Teach patients to take medications per instructions, such as before meals, with food, with water, or at bedtime. To optimize absorption. Avoid grapefruit juice with certain drugs (statins, calcium channel blockers) due to CYP3A4 inhibition.

NCLEX Relevance

Explains why IV doses are smaller than oral. Understand why tablets cannot be crushed or altered.