How long the drug is still in you

The math, explained

These drugs leave the body by first-order kinetics, which means what disappears in any given hour is a fixed proportion of what is there rather than a fixed quantity. Decay of that sort is exponential: one half-life leaves 50% of the peak standing, two leave 25%, three leave 12.5%, and the pattern continues. Written out:

C(t) = C_peak × 0.5^(t / t_half)

The half-lives themselves are taken from FDA prescribing information and from the published clinical pharmacology [1][2][3][4][5]:

• Semaglutide — 168 hours, or 7 days. From the Wegovy and Ozempic labels, and Hall’s 2018 review of the pharmacokinetics[4].
• Tirzepatide — 120 hours, or 5 days. From the Zepbound and Mounjaro labels, and Urva 2021[5].
• Orforglipron — around 36 hours. Being a non-peptide taken by mouth, it clears far faster than any of the injected peptides.
• Retatrutide — around 144 hours, or 6 days, as reported in its published phase 2 obesity trial.

The clinical milestones the calculator reports

• Moving to a different GLP-1. Nothing has to clear first. The new drug starts at the bottom of its own ladder, on the day the next injection was due anyway. Our switching guide sets out how the doses map across.
• Holding before an operation. A weekly GLP-1 is held for a week ahead of anything involving general anesthesia or deep sedation — that is the position of the 2023 ASA statement, carried forward by the 2024 multisociety update[6]. Our ASA guidance article walks through it in full.
• Planning a pregnancy. The Wegovy label asks for semaglutide to be stopped a full 2 months ahead of a planned pregnancy, on account of how long it lingers and how little human pregnancy data exists. Tirzepatide is usually given the same 2 months. Orforglipron, at 36 hours, needs far less. Our pregnancy and fertility guide covers the obstetric side.
• 5 half-lives, and 97% of it is gone. This is what a pharmacology textbook means by washed out, and it is the line many crossover trials draw.
• 10 half-lives, and 99.9% is gone. A stricter bar some trials insist on before counting someone drug-free enough to enroll on a different GLP-1.

Important caveats

• These are averages, not your figure. Body composition, kidney function and how long you had been at a steady dose each move a half-life, and the spread runs to about 30% either side. Kidneys working below par slow semaglutide and tirzepatide down.
• It assumes you had reached steady state. The arithmetic starts from a dose you had held long enough — five half-lives — for levels to settle. Stop partway up the ladder and you begin lower than that and finish sooner.
• The drug leaves before the effect does. Slowed gastric emptying, reduced appetite and the weight response itself can carry on for days or weeks after there is almost nothing left in the blood. Clearing the molecule and clearing what it did are two different clocks.
• There is no test for this. What counts as undetectable is a property of the assay, and no routine clinical lab measures semaglutide or tirzepatide in blood at all. The 0.1% mark is a pharmacokinetic idea, not a result you can go and have drawn.

What this is NOT

This is an educational calculator built on standard pharmacokinetic principles and FDA-label half-life values. It is not a substitute for clinical judgment. If you need to make a real decision about stopping a GLP-1 — for surgery, pregnancy, switching drugs, or any other reason — confirm the timing with your prescribing clinician and (for surgery) with your anesthesia team.

Related tools and research

• GLP-1 dose plotter — see the buildup curve during titration (the inverse of the washout curve)
• GLP-1 drug interaction checker — check anything else you take against the drug still in you or restart
• Stopping GLP-1s before surgery — the ASA hold guidance in full
• Switching between GLP-1 medications — how to move from one drug to another
• How to taper off a GLP-1 safely — coming off one for good
• What happens when you stop semaglutide — what the literature shows about regain afterward

Important disclaimer

Educational only, and not medical advice. What drives every figure here is an average half-life taken from FDA prescribing information, and real people sit a long way either side of an average. Stopping, holding or restarting a GLP-1 is a decision to make with the clinician who prescribed it.