Half-life is the number that explains most of what the first months on a GLP-1 medicine feel like: why the dose steps up monthly, why side effects cluster after each step, and why a missed dose is rarely an emergency.
The numbers, from the labels
| Agent | Half-life | Time to steady state | Absolute bioavailability | Source |
|---|---|---|---|---|
| Liraglutide (Saxenda) | about 13 hours | about 3 days, daily dosing | about 55% | Saxenda label |
| Semaglutide injection (Ozempic, Wegovy) | about 1 week | 4 to 5 weeks | 89% | Ozempic label |
| Semaglutide tablets (Rybelsus) | about 1 week | 4 to 5 weeks | about 0.4 to 1% | Rybelsus label |
| Tirzepatide (Mounjaro, Zepbound) | about 5 days | 4 weeks | 80% | Zepbound label |
The pattern: a long half-life allows weekly dosing, and reaching steady state takes four to five half-lives regardless of the drug. Those two facts set the monthly escalation schedule.
How a peptide is made to last a week
Native GLP-1 lasts one to two minutes. Lau and colleagues describe the three changes that turned it into semaglutide: substituting the amino acid DPP-4 attacks (position 8) so the enzyme cannot cut it; replacing a lysine at position 34 so the fatty chain attaches at one defined site; and attaching a C18 fatty diacid through a spacer that binds tightly to albumin (PubMed 26308095). Albumin-bound drug is protected from enzymes and too large to filter through the kidney, so it circulates for days and slowly releases. Tirzepatide uses the same strategy with a C20 diacid. Liraglutide uses a shorter C16 fatty acid with weaker albumin binding, which gives it a 13-hour half-life and daily dosing.
Elimination for all of them is by general proteolysis and beta-oxidation of the fatty chain, with fragments leaving in urine and faeces. There is no single organ that clears the drug, which is why the labels do not require dose adjustment for kidney or liver impairment.
What the numbers imply
Escalation timing. Each dose level takes about a month to settle. That is why every label steps up at four-week intervals and permits holding a step longer if side effects need time. Stepping faster stacks an unfinished rise on top of a new one.
Side effects cluster after steps. Peak concentration within a week comes one to three days after the injection for semaglutide (Ozempic label) and 8 to 72 hours for tirzepatide (Zepbound label). Nausea that tracks the weekly peak and the monthly step-up is the pharmacokinetics showing through; the nausea page has the rates.
Missed doses. With a week-long half-life, exposure one week after a missed dose is about half of steady state, not zero. The labels give practical windows: Wegovy, take a missed dose within 5 days, otherwise skip; Zepbound, within 4 days, otherwise skip. The labels also give instructions for changing the injection day, which require a minimum gap between doses. Follow the label for your product and confirm with your prescriber.
Washout. After stopping, exposure falls by half each half-life. For semaglutide the labels note about five weeks to clear, which is why the label advises stopping at least two months before a planned pregnancy.
Oral semaglutide
Peptides are digested. Rybelsus gets semaglutide through the stomach wall by co-formulating it with SNAC, an absorption enhancer that raises local pH and lets the peptide cross gastric cells before it is broken down (PubMed 30429357). Even so, absolute bioavailability is about 0.4 to 1% (Rybelsus label), which is why the tablet doses are in milligrams where the injection doses are fractions of a milligram, and why the label requires an empty stomach, no more than 4 ounces of water and a 30-minute wait before eating. The oral GLP-1 page covers the trials.
Compounded products
Compounded semaglutide and tirzepatide are not FDA approved and were not the products studied in the label pharmacokinetics. Vial concentration varies by pharmacy, and the arithmetic for converting a prescribed milligram dose to a volume belongs to a calculator, not to memory.
Where to go next
- Dose and unit conversions for any vial concentration: FormBlends Calculators.
- Label dosing schedules by product: Tirzepatide Hub and Semaglutide Hub.
- FormBlends' compounded tirzepatide program, not FDA approved and not interchangeable with Zepbound: formblends.com/products/tirzepatide.
Questions people ask
Why does the dose go up every four weeks?
Because that is roughly how long each dose takes to reach steady state. Four to five half-lives is the usual rule; for a one-week half-life that is about a month. Stepping up before the previous level has settled stacks the rise in exposure on top of an unfinished one, which the labels avoid.
If I miss a dose, does the drug vanish?
No. With a one-week half-life, half the steady-state amount is still present a week later. That is why the labels give windows rather than panic instructions: the Wegovy label says to take a missed dose within 5 days, and the Zepbound label within 4 days; after that, skip it and resume the schedule. Follow your label and prescriber, not this page.
Sources
- Ozempic (semaglutide) prescribing information. Drugs@FDA, NDA 209637 Accessed September 4, 2026.
- Wegovy (semaglutide) prescribing information. Drugs@FDA, NDA 215256 Accessed September 4, 2026.
- Zepbound (tirzepatide) prescribing information. Drugs@FDA, NDA 217806 Accessed September 4, 2026.
- Saxenda (liraglutide) prescribing information. Drugs@FDA, NDA 206321 Accessed September 4, 2026.
- Rybelsus (semaglutide tablets) prescribing information. Drugs@FDA, NDA 213051 Accessed September 4, 2026.
- Lau J, Bloch P, Schaffer L, et al. Discovery of the once-weekly glucagon-like peptide-1 (GLP-1) analogue semaglutide. J Med Chem 2015. PubMed 26308095 Accessed September 4, 2026.
- Buckley ST, Bækdal TA, Vegge A, et al. Transcellular stomach absorption of a derivatized glucagon-like peptide-1 receptor agonist. Sci Transl Med 2018. PubMed 30429357 Accessed September 4, 2026.
Canonical URL: https://formblendsscience.com/mechanisms/pharmacokinetics. Written by the FormBlends editorial team. This page is educational and is not medical advice; see the medical disclaimer.