Half-life, steady state, and how often a compound is dosed
5 min read
A compound's half-life is the single number that most shapes how often it appears in a schedule. It is worth understanding what it does and does not tell you, because it explains why a weekly compound and a daily one can look so different on a chart.
What half-life means
The elimination half-life is the time it takes for the amount in the body to fall by half. After one half-life, roughly 50% remains; after two, 25%; after three, about 12.5%. It is a property of how the body clears the compound, measured in a pharmacokinetic study, not a schedule.
Steady state and accumulation
When a compound is given again before the previous amount has cleared, the levels stack. They keep rising until the amount cleared between doses equals the amount added, which is steady state — reached after roughly five half-lives. The ratio between the steady-state peak and a single dose's peak is the accumulation ratio: the closer the dosing interval is to the half-life, the more it builds up.
Why frequency follows from it
This is why the numbers differ so much across compounds. A long-acting GLP-1 agonist with a multi-day half-life reaches steady, once-weekly-friendly levels; a short peptide cleared in minutes to hours never accumulates and is described on daily or twice-daily schedules instead. The half-life, not the compound's category, is what sets the shape.