How much of one dose is left
| Time elapsed | Half-lives | Left |
|---|---|---|
| 0 days | 0 | 100% |
| 5 days | 1 | 50% |
| 10 days | 2 | 25% |
| 15 days | 3 | 12.5% |
| 20 days | 4 | 6.3% |
| 25 days | 5 | 3.1% |
Plain arithmetic: left = 0.5^(number of half-lives).
Zepbound® half-life calculator
Zepbound® contains tirzepatide, and its US label gives a half-life of about five days, or about 5 to 6 days in people with overweight or obesity. Five days is filled in below and you can change it. Add the dose and schedule from your prescription to see an estimated curve.
Updated
Simple exponential decay from the numbers you enter. It is not a blood level and not medical advice. The dose and schedule come only from your prescription; PepLabo never suggests a dose.
The formula this calculator uses
One dose
remaining = dose × 0.5^(time elapsed ÷ 5 days). A half-life entered in hours is divided by 24 to work in days.
Repeated doses
Each dose fades on its own; the curve is the sum of every dose given up to that moment (up to 60 doses).
Three outputs
Highest point = the largest total at the dose moments · left before the last dose = the total just before the last dose (blank for a single dose) · left a week after = the total 7 days after the last dose.
Levelling off
With a fixed dose and interval, peak ≈ one dose × 1 ÷ (1 − 0.5^(interval ÷ half-life)); it settles after about 4 to 5 half-lives.
Worked example
Half-life 5 days: whatever its size, a single dose has 50% left after one half-life, 25% after two and about 3% after five. Repeated at the label’s 7-day interval, the peak levels off at about 1.61 times one dose: 1 ÷ (1 − 0.5^(7 ÷ 5)) = 1.61.
Worked examples
The dose field was set to 1: not a dose, just a way to read results as multiples of one dose. Each row was entered into this calculator and the result copied as shown.
| Entry | Highest point | Left before the last dose | Left a week after the last dose |
|---|---|---|---|
| 1 dose | 1 | – | 0.38 |
| 10 doses, once weekly | 1.61 | 0.61 | 0.61 |
What this calculator does not do
- It does not suggest a dose, a schedule, starting, stopping or switching medication.
- It does not show a blood level; it does not model absorption, time to peak or personal differences.
- It does not decide anything about a missed dose; it quotes the label’s rule below, and the decision is your prescriber’s.
- It does not tell you when a medicine is "gone" or when to switch to another one.
About this estimate and your privacy
The half-life comes from the label [1]; the dose, interval and number of doses are only what you enter. The model counts each dose as fully present the moment it is given, while a real injection is absorbed over time, which is why the curve steps straight up at each dose.
Everything is worked out by a script inside this page. The script makes no network requests, the page’s security policy (Content-Security-Policy with default-src none and no connect-src) would block them anyway, and nothing you type is stored, not even in your browser’s storage. Reload the page and the fields are empty again.
Next to each source below is what it supports on this page.
Zepbound® is a trademark of Eli Lilly and Company. PepLabo is not affiliated with or endorsed by Eli Lilly and Company.
Questions
Should I enter 5 or 6 days?
Both come from the label [1]. Five days is filled in; enter 6 to see the upper end the label gives for people with overweight or obesity. The difference shows up mostly in how slowly the curve tails off.
How long does Zepbound® stay in your system?
The label gives the half-life rather than a clearance time [1]. By the five-half-lives rule of thumb, about 97% is gone roughly 25 to 30 days after the last dose.
What does the label say about a missed dose?
Zepbound®: take it as soon as possible within 4 days (96 hours); if more than 4 days have passed, skip it and take the next dose on the usual day. [1] Your own prescriber’s instructions always come first; this calculator does not decide anything about a missed dose.
Can I change my injection day?
According to the label, Zepbound®: the injection day can be changed if at least 3 days (72 hours) separate two doses. [1] Check with your prescriber before changing it.
What does the label say about birth control pills?
Zepbound®: because it slows stomach emptying, it may make birth control pills work less well. The label flags two windows, the first 4 weeks of treatment and the 4 weeks after every dose increase, and for those weeks points to a method not taken by mouth or to a barrier method used alongside the pill. [1] Discuss it with your prescriber or pharmacist.
Is there anything to know before a planned surgery?
The labels say these medicines can slow stomach emptying, that rare cases of pulmonary aspiration have been reported during procedures under general anaesthesia or deep sedation, and that you should tell your healthcare providers you take it before any planned surgery or procedure [1].
Why do vomiting or diarrhoea matter?
The labels note that nausea, vomiting or diarrhoea can lead to dehydration, which has been linked to acute kidney injury [1]. If you cannot keep fluids down, contact your prescriber.
Why is the dose field empty?
PepLabo never suggests a dose. Enter the dose and schedule from your prescription; the calculator only draws how they add up and fade over time. Any unit works: enter mg and the results are in mg.
Is this curve my blood level?
No. It is a simple model based only on the half-life; it ignores absorption, time to peak, body weight, kidney function and other personal factors.
How is this different from the curve in the PepLabo app?
The app’s Today screen uses a simplified one-compartment model (Bateman) with the half-life and time to peak, scaled to the highest point in the window, so it shows a relative trend rather than an amount. This page skips absorption and shows absolute totals in the unit you enter.
Sources
Sources checked on
- Zepbound® (tirzepatide) injection: US prescribing information, Eli Lilly and Company (DailyMed) Supports: half-life of about 5 days, and about 5 to 6 days in people with overweight or obesity; missed-dose rule; changing the injection day; oral contraceptive advice; surgery and dehydration warnings.
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