How long does a GLP-1 stay in your system?

The math, explained

GLP-1 receptor agonists clear from the body via first-order elimination kinetics, which means a fixed fraction of the drug is eliminated per unit time rather than a fixed amount. The result is exponential decay: after one half-life, 50% of the peak concentration remains; after two half-lives, 25%; after three, 12.5%; and so on. The formula is:

C(t) = C_peak × 0.5^(t / t_half)

The half-life values used by this calculator come from the FDA prescribing information and the published clinical pharmacology literature [1][2][3][4][5]:

• Semaglutide — 7 days (168 hours). Sourced from the Wegovy and Ozempic prescribing information and the Hall 2018 PK review[4].
• Tirzepatide — 5 days (120 hours). Sourced from the Zepbound and Mounjaro prescribing information and Urva 2021[5].
• Orforglipron — ~36 hours. The newer non-peptide oral GLP-1 has a much shorter half-life than the injectable peptides.
• Retatrutide — ~6 days (144 hours), per the published phase 2 obesity trial.

The clinical milestones the calculator reports

• Switching to another GLP-1. No biological washout is required — start the new drug at its lowest titration step on the next scheduled injection day. See our switching guide for the dose mapping and protocol.
• Pre-surgery hold. The American Society of Anesthesiologists 2023 statement and the 2024 multi-society update[6] recommend holding weekly GLP-1s for 1 week before any procedure under general anesthesia or deep sedation. See our ASA guidance article for the full protocol.
• Pregnancy planning. The Wegovy prescribing information recommends discontinuing semaglutide at least 2 months before a planned pregnancy because of the long half-life and limited human pregnancy data. The same 2-month rule is generally applied to tirzepatide. Foundayo (orforglipron) has a shorter washout because of its 36-hour half-life. See our pregnancy and fertility guide for the full obstetric guidance.
• 5 half-lives ≈ 97% cleared. The standard pharmacology textbook definition of “washed out.” Used as the cutoff for many crossover trial designs.
• 10 half-lives ≈ 99.9% cleared. The research-grade washout used by some clinical trials to declare a patient drug-free for enrollment in a different GLP-1 study.

Important caveats

• Population means, not your half-life. Individual half-lives vary by ±30% based on body composition, kidney function, and the steady-state context. Patients with reduced kidney function may clear semaglutide and tirzepatide more slowly.
• Steady state vs partial titration. The calculator assumes you were on a stable maintenance dose for at least 5 half-lives before stopping. Patients who stopped during titration have lower starting concentrations and clear faster.
• Effect outlasts blood level. Gastric emptying delay, satiety, and weight-loss effects often persist for days or weeks after blood levels become negligible. Washout of the drug is NOT the same as washout of the clinical effect.
• No standardized assay. “Undetectable” depends entirely on which assay you use. There is no routine clinical lab for semaglutide or tirzepatide blood levels — the “0.1%” milestone is a research/PK concept, not a clinical lab result you can order.

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 — search any medication for interaction during washout or restart
• Stopping GLP-1s before surgery — the full ASA hold guidance
• Switching between GLP-1 medications — protocol for moving between drugs
• How to taper off a GLP-1 safely — discontinuation guide for permanent stops
• What happens when you stop semaglutide — the rebound and weight regain literature

Important disclaimer

This tool is for educational purposes only and does not constitute medical advice. The calculator uses population-mean half-lives from the FDA prescribing information, and individual values vary substantially. Always discuss any decision to stop, hold, or restart a GLP-1 with your prescribing clinician.