What the trial numbers describe
The weight figures quoted online come from long, placebo-controlled trials, and each belongs to its own study. In STEP 1, 1,961 adults without diabetes took semaglutide 2.4 mg or placebo once a week for 68 weeks, alongside a lifestyle intervention. The average change in body weight was −14.9% with semaglutide and −2.4% with placebo [1]. The US Wegovy® label describes the same trial as its Study 2 [4].
In SURMOUNT-1, 2,539 adults without diabetes took tirzepatide or placebo for 72 weeks, including a 20-week dose-escalation period. The average change was −15.0%, −19.5% and −20.9% with the 5 mg, 10 mg and 15 mg doses, compared with −3.1% on placebo [2]. The Zepbound® label lists it as Study 1 [5].
The two trials enrolled different people with different designs, so their numbers are not a ranking of the medicines. Both main analyses also counted people whether or not they stopped treatment along the way [1][2]. They are averages across large groups, measured over more than a year: not a weekly target, and not a forecast for one person.
Why the placebo groups lost weight too
Every participant, including those on placebo, was taught a reduced-calorie diet and counseled to be more active; the Zepbound® label adds counseling on ways to stick with both [4][5]. The placebo result shows what that programme achieved on its own, and the gap between the two arms is the part credited to the medicine. NIDDK puts it from the other side: weight-management medicines work best with a lifestyle programme and do not replace healthy eating or physical activity [3].
MedlinePlus adds a point for the long view: most people regain some or all of the weight after stopping these medicines unless they have made lasting lifestyle changes [8]. A weight log that carries on through and after treatment is one way to notice that pattern early and talk about it.
Checkpoints named in official sources
How progress is judged is a clinical decision, but a few documents name a point in time when it is reviewed. The table gathers them with their source. They are for your prescriber to apply, not a schedule to run on your own.
| Source | When | What it says |
|---|---|---|
| NIDDK [3] | 12 weeks on the full dose | If you are not losing weight, ask your health care professional whether to stop; with less than 5% lost by then, stopping is the likely advice |
| Saxenda® label [6] | 16 weeks after starting | Weight change is evaluated, and Saxenda® is stopped if at least 4% of starting weight has not been lost |
| Zepbound® label [5] | No fixed week | The maintenance dose is chosen with treatment response and tolerability in mind (section 2.1) |
| Wegovy® label [4] | No fixed week | The dosing section sets no weight-based stopping point |
A calmer way to weigh
NIDDK’s guide to changing habits treats tracking as a way to see your strengths and where you could improve, and suggests noting how you felt alongside what you did. It is also clear that a slip-up is not the same as failing; the useful step is getting back on track [7].
- Pick one time of day and similar conditions, for example in the morning before breakfast, so readings are comparable.
- Choose a rhythm you can live with. Daily readings give a smoother trend; a weekly reading is also enough to see direction.
- Look at the trend across several weeks rather than the change since yesterday. A single reading can move for many reasons.
- Think in percentages of your starting weight, the way trials report results: change divided by starting weight, times 100 [1][2].
- Jot down anything unusual next to a reading, such as a late meal, a hot day or a stomach upset, so an odd number has an explanation later.
- If weighing starts to feel stressful, weigh less often, and mention it to your clinician.
When a fast drop is a warning, not progress
Nausea, vomiting and diarrhea are among the most common side effects in these labels, and in the main Zepbound® trials most of those events happened while the dose was being raised [4][5]. A week of vomiting or diarrhea can pull the scale down quickly because the body is losing fluid, not fat. The Wegovy® Medication Guide connects that kind of fluid loss with kidney problems, asks you to drink fluids to lower the chance of dehydration, and to let your healthcare provider know promptly when the stomach symptoms keep going [4]. The Zepbound® label gives prescribers the same message about kidney injury from dehydration [5].
Severe stomach pain that will not settle, with or without vomiting, is a separate warning in the same guide: it can point to pancreatitis, and the guide says to stop the medicine and call your provider [4]. These signs belong in a call, not in a weight chart; they are listed in the box below.
What the scale cannot separate
The Wegovy® and Zepbound® labels both say the medicine lowers body weight with more fat mass lost than lean mass [4][5]. A bathroom scale counts both together, so it cannot show what changed underneath, whether the week was flat or fast. Waist size, which the trials also measured, is covered in our body composition article.
Other things move the number too. A week with little food because of nausea, a week of poor sleep or constipation, which is also on the common side effect lists [4][5], can all put a reading in a different light. A few words next to the number keep that context.
Bringing weight to an appointment
Weight makes more sense next to your doses. When you can line up readings with the dates of your shots, you can see whether a change followed a missed dose, a new dose your clinician prescribed or a difficult week.
A short summary is easy to read across a desk: starting weight and its date, current weight, the change as a percentage of where you started, and anything that interrupted treatment. Bring the trend, not just the latest number, and say if weighing has become stressful; that is part of the picture too.
Wegovy® and Saxenda® are registered trademarks of Novo Nordisk A/S. Zepbound® is a registered trademark of Eli Lilly and Company. PepLabo is not affiliated with or endorsed by Novo Nordisk or Eli Lilly and Company.
When to get help
| Sign | Who to contact | How fast | Source |
|---|---|---|---|
| Severe stomach pain that does not go away, with or without vomiting | Stop the medicine and call your healthcare provider | Emergency | Wegovy® Medication Guide [4] |
| Nausea, vomiting or diarrhea that keeps going, especially with fast weight loss or little to drink | Your healthcare provider | Today | Wegovy® Medication Guide [4]; Zepbound® label section 17 [5] |
| Shakiness, sweating, confusion or other signs of low blood sugar while also taking insulin or a sulfonylurea | Your healthcare provider, about how to recognise and treat it | Today | Wegovy® Medication Guide [4] |
| No weight loss after 12 weeks on the full dose | Your health care professional | At your next appointment | NIDDK [3] |
| Less than 4% weight loss 16 weeks after starting Saxenda® | Your prescriber | At your next appointment | Saxenda® label section 2.2 [6] |
