Detailed definition
Bioavailability is expressed as a fraction or percentage. By definition, intravenous administration is 100% bioavailable; other routes are compared to that. For oral drugs, bioavailability is reduced by incomplete absorption from the gut and by first-pass metabolism in gut wall and liver. Estradiol's oral bioavailability is approximately 5%, requiring much higher oral doses to achieve the same systemic levels as transdermal delivery. Vaginal estrogen has variable bioavailability — the local effect on tissue is the goal, and systemic absorption is typically minimal. Progesterone's oral bioavailability is also low (around 10%) but the first-pass metabolism produces the active sleep-promoting metabolite allopregnanolone.
Why it matters in menopause
Knowing oral vs. transdermal bioavailability differences explains why a 50 mcg/day estradiol patch is roughly equivalent in symptom-control terms to a 1 mg/day oral estradiol tablet despite the dose appearing 20× different. It is the systemic level that matters, not the input dose.
Related terms
Sources
External references: Wikipedia.