What a half-life
actually tells you.
A half-life is the time it takes for half of what is in you to be gone. It is why levels between injections look like a saw rather than a line, and why two compounds on the same schedule can behave completely differently — the schedule matters only relative to the half-life.
Half-life is the number most often quoted and least often used. It is not how long something works, and it is not how long it stays detectable — it is a rate, and the useful things follow from treating it as one.
It is a rate, not a duration
After one half-life, half remains. After two, a quarter. After three, an eighth. Nothing ever reaches zero on paper; it just gets small enough to stop mattering.
So "a 24-hour half-life" does not mean it is gone in a day. It means that in a day, half of it is.
Why the curve is a saw
Inject on a schedule shorter than the half-life and each dose lands on top of what is left of the last one. The level climbs, dose after dose, until what leaves between injections equals what goes in — the plateau.
Inject on a schedule much longer than the half-life and the opposite happens: each dose has almost entirely gone before the next arrives, and the level is a series of unconnected spikes.
Where the number comes from, and how good it is
Published half-lives are averages measured in a study population, usually with a wide spread around them. Two figures for the same compound often disagree because they were measured in different people by different methods.
For a great many compounds no human measurement exists at all. Drawing a confident curve from a figure nobody has checked is the mistake this app tries hardest not to make: where the number is an estimate, it says so on the screen.
What does half-life mean?
The time it takes for half of the amount present to be eliminated. After one half-life half remains, after two a quarter, after three an eighth. It is a rate of decline, not a point at which something stops.
Does a longer half-life mean fewer injections?
Usually, yes — a longer half-life means less is lost between doses, so a schedule can be spread further without the level collapsing. The half-life sets what a schedule can be, not what it should be.
How long until something is fully out of your system?
Conventionally about four to five half-lives, at which point roughly 3 to 6 percent remains. Mathematically it never reaches zero; practically it stops mattering somewhere around there.
Why do published half-lives disagree with each other?
Because they are averages from different study populations, measured by different methods. Some are drawn from a handful of people. For many compounds no human figure exists at all, and what circulates is an estimate.
For informational purposes only — not medical advice