Why read the original papers
Tools that estimate "how long until my goal" usually draw a trial's average curve and rescale it to your weight. The idea is easy to misread. This guide explains how the curves are made, so you can read the originals yourself. The figures here come from each paper's summary.
Where to look
Two well-known places to start are the main papers for the large trials of tirzepatide and semaglutide in people with obesity. Find them by name:
Each has a table or figure of weight change by week and by dose group. Open the full text and look for the figure that plots the mean percentage change in body weight over time.
How the figures are built
Percentage of starting weight
The usual unit is percent change from baseline. Baseline is the person’s weight at the start. For one person, change in % = (weight now − baseline) ÷ baseline × 100. The paper then averages across everyone in the group.
Percentages let people of different sizes be averaged together. They also mean that a given percentage is a different number of pounds for different people.
Time points
Papers report at scheduled visits, for example every few weeks. The curve between visits is a line joining the dots. It is not a measurement.
| Trial and week | Group | Mean change | Placebo |
|---|
| STEP 1, week 68 | Semaglutide 2.4 mg | −14.9% | −2.4% |
| SURMOUNT-1, week 72 | Tirzepatide 5 mg | −15.0% | −3.1% |
| SURMOUNT-1, week 72 | Tirzepatide 10 mg | −19.5% | −3.1% |
| SURMOUNT-1, week 72 | Tirzepatide 15 mg | −20.9% | −3.1% |
Which people are in the average
Trial reports often give two versions of the same result:
- Treatment-regimen (intention-to-treat) estimand. Everyone who was randomised counts, including people who stopped the medicine or changed dose.
- Efficacy (on-treatment) estimand. Only people still taking the medicine as planned count, or the result is modelled as if all did.
The second average is usually larger. Neither is wrong. They answer different questions. A tool that does not say which it uses can mislead. The figures in the table above count everyone randomised, including people who stopped treatment.
Dose groups
Trials often have several dose arms, including a placebo arm. A curve for one arm applies to people who took that dose. Placebo arms also lose or gain weight on average, because trial participants usually receive lifestyle support.
Average versus spread
Averages hide a range. Papers show this in a few ways: the share of people who reached given thresholds of weight loss, confidence intervals, or a spread of individual responses. In STEP 1, 86.4% on semaglutide lost at least 5% and 50.5% lost at least 15%, against 31.5% and 4.9% on placebo. In SURMOUNT-1, 85% to 91% on tirzepatide lost at least 5%, against 35% on placebo. A person at the low end of the spread and a person at the high end took the same medicine.
What a curve does not tell you
- How you will do. It describes a group.
- What happens after stopping. Look for a separate report or extension study. In the STEP 1 extension, people regained about two-thirds of the weight they had lost in the year after stopping semaglutide.
- Safety or tolerability. Those are reported in other sections of the paper.
- Whether a goal is right for you. That is for you and your clinician.
A worked example of the arithmetic
Example. These numbers are made up to show the steps and do not come from any trial.
A person starts at 200 lb. A made-up curve reads −10% at some week.
- Change = 200 × 0.10 = 20 lb.
- Weight at that week = 200 − 20 = 180 lb.
Running it in reverse: for a goal of 170 lb from 200 lb, the needed change is (200 − 170) ÷ 200 = 15%. A tool would look for the week where the curve reaches −15%. If the curve never reaches it, there is no answer. That is a fact about the average, not about you.
Frequently asked questions
In short
- A trial curve is an average of many people. It is not a prediction for one person.
- Papers report change as a percentage of starting weight, at set weeks.
- Two kinds of analysis can give different averages from the same trial.
- Spread between people is as important as the average, and is often shown separately.
- Every figure on this page comes from the cited paper.
Can I use a trial curve to set a date?
It can give a rough shape. It cannot give a date. Individual results spread widely around the average.
Why do two sources quote different averages?
They may use different estimands, different dose groups or different weeks.
Why is the placebo group in the table?
Because it shows what happened without the medicine, under the same support.
Where do I find the curve?
In the main results paper, usually a figure of mean percent change in body weight by week.
Does Reko predict my weight?
No. Reko records your weights and doses and draws your own line.
Related pages
Medical disclaimer. This page is for information only and is not medical advice. Talk to your prescriber before changing how you take your medicine. Reko does not suggest doses and takes no responsibility for dosing errors.
How Reko does this in the app
Reko does not forecast. It draws your own weight over time, in lb, st or kg, next to your logged doses, side effects, and the calendar. You set the units. The record is yours to show a clinician. Doses export as a spreadsheet and everything as a file.
Reko records what you and your prescriber decide. It never suggests a dose, and this page is not medical advice.
Download Reko
Your record, in one private place
Open every screen with an example record before you pay.