"How long does it take to work?" has at least four answers, because a GLP-1 medicine does several things on different timetables. It acts on the stomach from the first dose. It takes weeks to reach a steady level in the blood. It takes months to reach the full dose. And the weight it removes keeps coming off for well over a year.
This page sets out each of those clocks from the current US labels and the trials behind them, read on 26 September 2026. It reports what the documents say; how quickly one person responds is something only their prescriber can judge. How the drugs work in the body is on our how GLP-1 works page.
Clock 1: the first dose
Some effects begin immediately. The Zepbound label says tirzepatide's delay of gastric emptying — food leaving the stomach more slowly — "is largest after the first dose and diminishes over time." After a first 5 mg dose, the peak blood level of acetaminophen taken by mouth fell by 55% and arrived an hour later, which is how the label measures that slowing. The same property is why every GLP-1 label warns about absorption of other oral medicines, and why the tirzepatide and Foundayo labels ask users of oral contraceptives to add a backup method after starting and after each dose increase.
What the first dose does not do is produce the weight loss people are waiting for. The labels present it as a "starting dosage" from which the dose is stepped up, and the trials measured weight at maintenance doses.
Clock 2: a steady level in the blood
A medicine taken repeatedly builds up until the amount taken in balances the amount cleared. The labels state when that happens:
| Medicine | Half-life (label) | Steady level reached (label) |
|---|---|---|
| Tirzepatide (Zepbound, Mounjaro), weekly | about 5 days | after 4 weeks |
| Semaglutide tablets (Rybelsus / Ozempic tablets), daily | about 1 week | after 4 to 5 weeks |
| Semaglutide injection (Wegovy), weekly | about 1 week | not stated in the section read; present 5 to 7 weeks after the last dose |
| Orforglipron (Foundayo), daily | 29 to 49 hours | after about 1 week |
Because the dose keeps rising during escalation, each step starts this clock again at a higher level. Foundayo's short half-life is the outlier: it settles within about a week, and also clears within days of stopping. The half-life figures and what they mean for missed doses are on our missed dose page.
Clock 3: reaching the full dose
The labels escalate slowly, largely because stomach side effects concentrate during escalation (our nausea page has the timing). On the label schedules:
- Wegovy injection: 0.25 mg for weeks 1–4, then 0.5, 1 and 1.7 mg in four-week steps; the maintenance dose starts at week 17.
- Zepbound and Mounjaro: 2.5 mg to start, rising in 2.5 mg steps after at least four weeks at each; 15 mg cannot be reached before week 21.
- Saxenda: weekly steps from 0.6 mg to 3 mg daily, reaching the full dose at week 5.
- Foundayo and the Wegovy pill: dose steps roughly every 30 days.
These are minimums. The labels allow a prescriber to hold a lower dose for longer if side effects are a problem, and several trials let participants stay on a lower maintenance dose. All the schedules, reported from each label, are on the titration schedules page.
Clock 4: the weight
Weight starts to fall during escalation — the lead-in phases of two maintenance trials show how much:
| Trial | Lead-in | Who completed it | Weight change by end of lead-in |
|---|---|---|---|
| STEP 4 (semaglutide 2.4 mg) | 20 weeks, 16 of them escalation | 803 of 902 | 107.2 kg → about 96.1 kg, roughly −10% (our arithmetic) |
| SURMOUNT-4 (tirzepatide to max. tolerated dose) | 36 weeks, up to 20 of them escalation | 670 of 783 | −20.9% |
Two cautions. These figures describe people who completed the lead-in: in STEP 4, 99 of 902 (11%) stopped before week 20, 48 of them because of adverse reactions, and in SURMOUNT-4 14.4% stopped before week 36, 6.8% because of adverse events. And a lead-in is open-label — everyone knows they are on the drug.
After that, the curve flattens but keeps going. The weight-management trials measure their main result at 64 to 72 weeks, and in SELECT, a four-year heart trial of semaglutide 2.4 mg in 17,604 people, weight loss continued for about 65 weeks and was then held to four years (−10.2% at 208 weeks against −1.5% on placebo). SELECT had no structured diet programme, which is one reason its figure sits below the 15% of the dedicated weight trials. When and why the curve levels off is the subject of our GLP-1 plateau page.
How fast one person loses weight varies widely. In SURMOUNT-5, 44% of people on tirzepatide and 21% on semaglutide had lost at least 15% by week 24 — the "rapid responders" in a Lilly-authored post hoc analysis — which means most people had not.
Appetite
The clearest measurement comes from a 20-week study of semaglutide 2.4 mg in adults with obesity: at a test meal where people ate as much as they wanted, energy intake was 35% lower than on placebo (1,736 against 2,676 kJ), with less hunger, more fullness and fewer or weaker food cravings. The study measured this at week 20, so it shows the size of the effect at full dose rather than how soon it began.
The labels that set a deadline
This is the part of the timeline most pages miss. Three weight-loss labels tell prescribers when to decide that a drug is not working:
| Label | Rule (label wording, summarised) |
|---|---|
| Saxenda (liraglutide) | Evaluate at 16 weeks; discontinue if under 4% of baseline weight lost |
| Contrave (naltrexone/bupropion) | Evaluate after 12 weeks at the maintenance dose; discontinue if under 5% lost |
| Qsymia (phentermine/topiramate) | Evaluate after 12 weeks; if under 3% lost, escalate; discontinue if under 5% lost after 12 weeks at the top dose |
The current Wegovy, Zepbound and Foundayo labels contain no equivalent rule, as read on 26 September 2026. Each of the three older labels gives the same reason: a person who has not responded by then is unlikely to achieve and sustain clinically meaningful weight loss with continued treatment. The other approved medicines are compared on our GLP-1 alternatives page.
What this page cannot tell you
- When one person will see a result. The figures are averages, with wide spread around them.
- Whether a slow start means a drug will not work. None of the three GLP-1 weight labels sets a cut-off, and whether to continue, hold or change a dose is the prescriber's decision.
- Anything about compounded or research products, whose content and dosing were not what the trials tested.
Sources
- ZEPBOUND (tirzepatide) prescribing information, Eli Lilly, effective 2026-08-28 — DailyMed, sections 2, 5, 12.2 and 12.3.
- MOUNJARO (tirzepatide) prescribing information, Eli Lilly, effective 2026-08-27 — DailyMed, section 12.3.
- WEGOVY (semaglutide) injection and tablets prescribing information, Novo Nordisk, SPL effective 2026-06-18 — DailyMed, Table 1, section 12.3, section 14 and Table 9.
- OZEMPIC and RYBELSUS (semaglutide) tablets prescribing information, Novo Nordisk, effective 2026-01-30 — DailyMed, section 12.3.
- FOUNDAYO (orforglipron) prescribing information, Eli Lilly, effective 2026-07-29 — DailyMed, sections 7 and 12.3.
- SAXENDA (liraglutide) prescribing information, Novo Nordisk, effective 2026-02-25 — DailyMed, section 2.
- CONTRAVE (naltrexone/bupropion) prescribing information, effective 2025-11-10 — DailyMed, section 2.
- QSYMIA (phentermine/topiramate) prescribing information, Vivus, effective 2026-04-22 — DailyMed, section 2.
- Aronne LJ, et al. Continued treatment with tirzepatide for maintenance of weight reduction in adults with obesity (SURMOUNT-4). JAMA 2024 — PubMed 38078870.
- Ryan DH, et al. Long-term weight loss effects of semaglutide in obesity without diabetes in the SELECT trial. Nat Med 2024 — PubMed 38740993.
- Friedrichsen M, et al. The effect of semaglutide 2.4 mg once weekly on energy intake, appetite, control of eating, and gastric emptying in adults with obesity. Diabetes Obes Metab 2021 — PubMed 33269530.
- Aronne LJ, et al. Relationship of early rapid weight loss to efficacy and safety of tirzepatide and semaglutide for obesity: SURMOUNT-5 post hoc analysis. Am J Med 2026 — PubMed 41865857.