What is half-life?
Half-life is the time required for the measured amount or concentration of a compound to fall by about 50% under the conditions being studied.
A simple example
| Time | Approximate amount remaining |
|---|---|
| Start | 100% |
| 1 half-life | 50% |
| 2 half-lives | 25% |
| 3 half-lives | 12.5% |
| 5 half-lives | About 3% |
Why it matters
- Spacing: Longer half-lives often support wider dosing intervals in formal studies.
- Accumulation: Repeated exposure before full elimination can increase average concentrations.
- Steady state: With repeated administration, concentrations often approach a repeating range after roughly four to five half-lives.
- Washout: Researchers often allow multiple half-lives before assuming most of the compound has cleared.
What half-life does not tell you
- It does not equal duration of benefit.
- It does not define a safe or effective protocol.
- It can differ between animals and humans.
- Route, formulation, organ function, and assay method can change the estimate.
Morning or night?
Time of day should come from pharmacology, study design, food effects, adverse effects, and practical consistency. A compound with a long half-life often depends more on consistent intervals than the exact hour. Short-acting growth-hormone secretagogues are often studied around fasting and sleep-related hormone pulses. Metabolic compounds are sometimes studied near activity periods. These are study-design choices, not universal rules.