Half-life in plain words: an interactive explainer

"Half-life" turns up on every GLP-1 label and in almost every online discussion about weekly injections. It's a simple idea that often gets wrapped in confusing charts and confident claims. This guide sets out what it means, with arithmetic you can play with, and is careful about what it can't tell you.

One thing first: nothing here is a measurement of anyone's blood level, and none of it is a reason to change when or how you take your medicine. It's an explanation of an idea, so the numbers on your label make more sense.

What half-life means

A medicine's half-life is the time it takes for the amount in the body to fall by half. After one half-life, half is left; after two, a quarter; after three, an eighth; and so on. The amount never falls off a cliff. It eases down, halving again with every half-life that passes.

The US labels for the weekly injections give these half-lives:

• semaglutide (Ozempic®, Wegovy®): about 7 days
• tirzepatide (Mounjaro®, Zepbound®): about 5 days

Those figures come straight from the prescribing information, which is also where Hushweek takes them from.

Why weekly doses build up

Because a weekly dose doesn't fully clear before the next one arrives, each new dose lands on top of what's left of the last. At first that leftover is small, so the total climbs from week to week. After a while, the amount that clears in a week matches the amount a dose adds, and the pattern settles into the same rise and ease every week. That settled pattern is often called steady state.

Try it below. Slide the half-life and watch how much of each week's amount is still there a week later, and how many weeks the pattern takes to settle.

What to notice

• A longer half-life means more carries over from week to week, so the weekly rise and ease is gentler.
• A longer half-life also means a longer climb before the pattern settles.
• A shorter half-life means bigger swings within each week, and a quicker settle.

What this simple model leaves out

Real medicines don't arrive all at once the moment you inject. They're absorbed over hours or days, and the labels describe when levels peak. People also differ from each other. That's why any chart like this is an illustration of an idea, not a picture of what's happening inside you.

A half-life explains the shape of a week. It doesn't tell you what to do with it.

It also doesn't answer practical questions, like what to do after a late or missed dose, or how to restart after a break. Labels cover those separately, with their own instructions, and they aren't something to work out from a chart. Our guide to finding your label's missed-dose rule shows where to look.

How Hushweek shows it

Hushweek Pro includes an estimate chart that uses the half-life and time to peak on your label and the doses in your log; without Pro you see a softened preview of your own curve. It's labelled Estimate everywhere it appears, it's shown as a share of where your current dose settles (never as a concentration), and every assumption it makes is written out on its "How the estimate works" sheet. A dose written in mL or units has no estimate, because Hushweek never converts doses. It never drives reminders, alerts or advice. If you change dose, it simply redraws from your log; your prescriber decides the dose.

Questions people ask