Bioavailability
Quick Definition
Bioavailability is the fraction of an administered dose of unchanged drug that reaches systemic circulation. By definition, intravenous administration has 100% bioavailability; other routes (oral, transdermal, intramuscular) have varying bioavailability based on absorption, first-pass metabolism, and other factors.
In Depth
Bioavailability quantifies how much of a given drug dose actually reaches the systemic circulation in active form. It is influenced by:
- Absorption from the route of administration. - First-pass hepatic metabolism (orally absorbed drugs pass through the liver before reaching systemic circulation, where they may be metabolized). - The chemical stability of the drug in the gastrointestinal tract. - Drug interactions with food or other medications.
By convention, intravenous administration has 100% bioavailability since the drug is delivered directly to the systemic circulation.
Examples relevant to this site:
- Oral semaglutide (Rybelsus) has very low bioavailability (~1%) and requires a special absorption-enhancing formulation. Subcutaneous injection has dramatically higher bioavailability. - Oral testosterone has historically had near-zero bioavailability due to extensive first-pass metabolism. The newer lipid-based oral testosterone preparations (Jatenzo, Tlando) bypass first-pass metabolism via the lymphatic system, making oral testosterone clinically viable. - Topical finasteride has lower systemic bioavailability than oral, while still achieving meaningful scalp DHT suppression. - Sildenafil has high oral bioavailability but is meaningfully reduced by high-fat meals due to delayed and reduced absorption.
Bioavailability is a key consideration in:
- Choosing a route of administration. - Comparing branded vs generic vs compounded formulations. - Understanding food-drug interactions. - Selecting alternatives within a drug class.
Compounded medications typically use the same active pharmaceutical ingredient as branded products. Bioavailability of compounded preparations is generally similar to the branded equivalent, though the formulation differences can produce variation. Bioequivalence studies are not required for compounded preparations.
Related Terms
Half-life
Half-life (t½) is the time required for the plasma concentration of a drug to decrease by 50%. Half-life determines dosing frequency, time to steady state, and time to drug clearance after discontinuation.
Area Under the Curve
Area Under the Curve (AUC) is a measure of total drug exposure over time, calculated as the integral of the plasma concentration vs time curve. AUC is used in pharmacokinetics to compare drug formulations, doses, and bioequivalence.