Detailed definition
Drug half-life (t½) is a fundamental pharmacokinetic property determined by the drug's volume of distribution and clearance rate. After 4–5 half-lives of repeated dosing, plasma concentration reaches steady state. Estradiol has a plasma half-life of approximately 1–3 hours when given orally, which would require multiple daily doses for stable levels — so transdermal formulations (releasing continuously over 24 hours to 7 days) are pharmacokinetically more sensible for stable systemic levels. Progesterone has a similarly short half-life (~5 hours when oral micronized), which is why it is generally dosed once daily at bedtime. Long-acting depot formulations (LNG-IUD releasing for years, vaginal rings releasing for 90 days) achieve sustained levels through controlled release rather than long molecular half-life.
Why it matters in menopause
Half-life explains why a missed estradiol patch produces noticeable symptom return within a day or two, while a missed bisphosphonate dose makes essentially no difference. It also explains why "blood level" testing of HRT mid-day vs. near patch change can give very different numbers — and why interpreting hormone levels requires knowing the timing.
Related terms
Sources
External references: Wikipedia.