Definition of Drug Half-Life
The half-life (t½) of a drug is defined as:
- Tmax
- Half-life (plasma) — Correct Answer
- Steady-state time
- 75% elimination time
Explanation:
Correct Answer: Time for plasma concentration to be reduced to half its original value
Half-life (t½) — specifically the plasma half-life — is the time taken for the plasma concentration to be reduced to half its original value. It is calculated as: t½β = 0.693 / β (where β = elimination rate constant).
Drug Elimination Over Half-Lives
- 1 t½ → 50% eliminated
- 2 t½ → 75% eliminated
- 3 t½ → 87.5% eliminated
- 4 t½ → 93.75% eliminated
- 4–5 t½ → Plateau (steady state) achieved
Clinical Significance
Longer the half-life, longer the duration of action. Steady state concentration (Css) is used as target for long-term therapy. Loading dose = (target Cp × Vd)/F when immediate therapeutic levels are needed.
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