What it means
After oral dosing, drug concentration in plasma rises, peaks, then declines. The declining phase follows first-order kinetics for most small-molecule drugs: the rate of elimination is proportional to the current concentration. This produces an exponential decay, and half-life is the constant time interval over which concentration halves — regardless of the starting concentration.
Half-life determines how long a drug stays in the body, how often a dose must be repeated to maintain therapeutic levels, and how long it takes to reach steady state during repeated dosing (approximately 4–5 × t½).
Formula
| Equation | Variables | Notes |
|---|---|---|
| t½ = ln(2) / ke | ke = elimination rate constant (h⁻¹) | Most common form |
| t½ = 0.693 / ke | ln(2) ≈ 0.693 | Numeric shorthand |
| t½ = (Vd × ln 2) / CL | Vd = volume of distribution; CL = clearance | Relates t½ to physiology |
| C(t) = C₀ · e−ke·t | C₀ = initial concentration; t = time | Concentration at any time t |
Half-life is only constant for linear (first-order) kinetics. Some drugs, like aspirin at high doses, show saturation (zero-order) kinetics ↗ where t½ changes with concentration.
Example: ibuprofen
Ibuprofen ↗ has a plasma half-life of approximately 2 hours after oral dosing (DailyMed label ↗). Starting from a peak concentration of 30 µg/mL, here is how concentration falls over time:
| Time (h) | Half-lives elapsed | Fraction remaining | Approx. conc. (µg/mL) |
|---|---|---|---|
| 0 | 0 | 1.000 | 30.0 |
| 2 | 1 | 0.500 | 15.0 |
| 4 | 2 | 0.250 | 7.5 |
| 6 | 3 | 0.125 | 3.8 |
| 8 | 4 | 0.063 | 1.9 |
| 10 | 5 | 0.031 | 0.9 |
See the ibuprofen drug page for the full concentration–time chart with repeat-dosing simulation, or the StatPearls NSAID pharmacology entry ↗ for a clinical overview.
Common mix-ups
How long a drug produces a therapeutic effect. Duration depends on both the half-life and the minimum effective concentration ↗ — not the same thing.
Half-life describes elimination only. It tells you nothing about how quickly a drug is absorbed or when it peaks. See tmax ↗ for the absorption side.
Renal or hepatic impairment, age, and drug–drug interactions can significantly alter clearance and therefore half-life. The FDA Drug Approval Package ↗ for each drug lists population PK studies covering these variables.
For some drugs (e.g. biologics or radiopharmaceuticals), "biological half-life" refers to the reduction of effect, not plasma concentration. The ICH E3 guideline ↗ distinguishes these definitions in clinical study reporting.
FAQ
How is half-life calculated from a blood test?
Collect at least two blood samples during the elimination phase, plot ln(concentration) vs time — the slope is −ke. Then t½ = ln(2) / ke ≈ 0.693 / ke. In clinical pharmacokinetics software, non-compartmental analysis (NCA) automates this fit.
Does a longer half-life always mean a drug stays in the body longer?
Yes, in terms of plasma concentration. A drug with t½ = 20 h takes ~4–5 days to clear, vs ~10 h for t½ = 2 h. However, duration of clinical effect can be shorter than plasma half-life (if drug must exceed a minimum effective concentration) or longer (irreversible binding, e.g. aspirin on platelets).
Why does ibuprofen need dosing every 4–8 hours if its half-life is only 2 hours?
After ~4 h (2 × t½) the plasma concentration has fallen to ~25% of peak — often below the analgesic threshold for full effect. Dosing intervals are set to keep concentration above the minimum effective concentration (MEC), not to wait for complete elimination. See the ibuprofen label on DailyMed ↗ for approved dosing schedules.
What is the difference between half-life and steady-state?
Steady state is reached when drug input equals drug output on average. With regular dosing, this takes 4–5 half-lives to achieve. The half-life governs both how fast steady state is reached and how long washout takes after stopping.