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Pharmacology
Half-life
Half-life - The time it takes for half of an active substance to be broken down or cleared from the body. It drives how long a peptide acts and how often it is used.
Also called half-lifet½elimination half-life
Half-life (often „t½") describes how quickly the body removes an active substance: after one half-life, half of the original amount is broken down or cleared; after two, a quarter; after three, an eighth - and so on.
Why it matters
Half-life drives how often a peptide is sensibly used and how steady its blood level stays. A short half-life (minutes to a few hours, e.g. many GHRP/GHRH peptides) means more frequent dosing; a long half-life (days, e.g. some GLP-1 analogues) allows weekly use. As a rule of thumb a substance is considered practically fully cleared after about four to five half-lives.
Common mistakes
- Equating half-life with duration of effect - the biological action can outlast the measurable blood level.
- Assuming a substance is „gone after one half-life" - only half of it is.
Wording mix-ups
„Half-life" originally comes from radioactivity but in pharmacology means clearance from the body (elimination half-life).