What a population average means: reading the weight-loss projection

The projection tool takes your weight and applies a trial arm's average percent change to it, week by week. Used honestly it answers "what did the average participant in that trial experience, scaled to my starting point?" It does not answer "what will happen to me?", and the difference is the whole point of this page.

The two trials

STEP 1 randomised 1,961 adults with a BMI of 30 or more, or 27 or more with at least one weight-related condition, and without diabetes, to semaglutide 2.4 mg weekly or placebo for 68 weeks. Both arms got lifestyle counselling. Mean change in body weight was minus 14.9 percent with semaglutide and minus 2.4 percent with placebo, under the treatment-policy estimand, which counts everyone randomised regardless of whether they stayed on the drug.

SURMOUNT-1 randomised 2,539 adults with the same broad entry criteria to tirzepatide 5, 10 or 15 mg weekly or placebo for 72 weeks, again with lifestyle counselling. Mean changes were minus 15.0, minus 19.5 and minus 20.9 percent for the three doses and minus 3.1 percent for placebo, under the treatment-regimen estimand.

Those endpoint figures are the only measured numbers in the tool. Everything else is arithmetic on them.

What the curve is

Both papers show weight change over time as figures, not as tables of numbers, so the tool does not pretend to have the week-by-week data. Instead it draws a first-order approach curve: fast early loss that slows as it nears the endpoint, scaled so the curve lands exactly on the published figure at the trial's final week. The rate constant was chosen so about 60 percent of the total change is reached by week 20, which is consistent with the shape of the published figures, but it is an assumption and the formula panel says so. After the trial's final week the line stops; the papers report nothing beyond it.

If you want to know what happened at, say, week 20 in the actual trials, read the figures in the papers. The tool's week-20 point is a modelled value.

Why your result will differ

You are not the average. The mean hides a wide distribution. In STEP 1, 86.4 percent of the semaglutide group lost 5 percent or more and 32 percent lost 20 percent or more, which means some lost far less than 14.9 percent and some far more. The tool draws the mean because that is the number that was reported, not because it is what you should expect.

The trial included a lifestyle programme. Every arm received a reduced-calorie diet and physical activity counselling with regular contact. Part of the placebo arm's 2.4 or 3.1 percent is that programme. Your context may include more or less of it.

Adherence and dose. The treatment-policy estimand includes people who stopped the drug or never reached the full dose. Someone who tolerates and stays on the maintenance dose for the whole period is a different population from the whole trial arm, and the papers report that difference separately (the trial-product estimand) with larger numbers.

Using the tool well

• Enter your weight and pick the arm that matches the medication and dose your prescriber has chosen. Do not pick an arm to see a bigger number.
• Look at the endpoint first. That is the fact.
• Treat the intermediate points as a sketch of the general shape, not as targets for particular weeks.
• If you are on a compounded product, note that no trial of it exists. Compounded semaglutide and tirzepatide are not FDA approved.

For the broader picture of what tirzepatide is and how it is used, the FormBlends tirzepatide introduction covers the medication rather than the arithmetic.