How fast do semaglutide and tirzepatide work?

How fast do they start working?

In the trials recorded here, weight loss started within the first months of treatment. It then continued for more than a year in the longer trials.

• Tirzepatide: in a SURMOUNT-1 analysis of 1,545 people taking tirzepatide, 82% had lost 5% or more of their body weight by week 12.
• Semaglutide: the European Medicines Agency says weight loss with semaglutide 2.4 mg in STEP 1 started early and continued throughout the 68-week trial.

These are results for groups of trial participants. They are not a prediction for any one person. More about each medicine is on the semaglutide and tirzepatide pages.

Why the first weeks are a starting phase

The first weeks are spent on a low starting dose that is raised in steps. The US labels set this schedule to reduce the risk of digestive side effects.

• US Wegovy label (Revised: 6/2026): the dose is raised every 4 weeks, and the maintenance dose starts from week 17.
• US Zepbound label (Revised: 8/2026): the starting dose is used for 4 weeks to start treatment and is not approved as a maintenance dose. After that, the dose may be raised after at least 4 weeks on each dose.

Your doctor sets your dose. In the Zepbound weight trials, weight was assessed after 72 weeks of treatment, with at least 52 weeks at the maintenance dose.

Week-by-week results from the trials

The table puts the trial results in order of the week they were measured. Each trial had different participants, doses and programs, so the rows are not directly comparable.

Doses are the doses tested in each trial, not a guide to treatment.

What if weight loss is slower?

Some people in the trials took longer to reach 5% weight loss. Most of them got there later.

In the SURMOUNT-1 analysis, 18% of people on tirzepatide had not lost 5% by week 12. Of these later responders:

• 70% reached 5% or more by week 24.
• 90% reached 5% or more by week 72.
• Their mean time to reach 5% was 24.8 weeks.

This analysis included people who took at least 75% of their assigned doses and had their weight measured at weeks 0, 12, 24 and 72. Talk to your doctor about your own progress.

When does weight loss level off?

For semaglutide, one trial recorded a plateau after week 68. For tirzepatide, no plateau week was found in the sources.

• STEP 5 (semaglutide): mean body weight went down from the start until week 68, then reached a plateau. With placebo, weight went down less and levelled off after about 20 weeks.
• Tirzepatide: the US Zepbound label shows the week 0 to 72 time course only as figures, without a stated plateau week. In SURMOUNT-4, people who stayed on tirzepatide were still losing weight between week 36 and week 88.
• Over three years: in SURMOUNT-1 participants with obesity and prediabetes, the authors describe "substantial and sustained weight reduction" at 176 weeks.

Why results vary

Trial results are group averages. The trials recorded here show how results differed between trials and doses.

• Diabetes: in STEP 2, adults with type 2 diabetes had a mean weight change of -9.6% with semaglutide 2.4 mg over 68 weeks. STEP 1, in adults without diabetes, reported -14.9%. These were separate trials, so the results are not directly comparable. In SURMOUNT-2, it was -12.8% and -14.7% with tirzepatide 10 mg and 15 mg over 72 weeks.
• Diet and activity: in STEP 1 to STEP 5, participants also followed a lifestyle or behavioral program, and the US Zepbound label says tirzepatide is used with a reduced-calorie diet and increased physical activity. In STEP 3, which added intensive behavioral therapy over 68 weeks and a low-calorie diet for the first 8 weeks, the mean change was -16.0% with semaglutide versus -5.7% with placebo.
• Dose: SURMOUNT-1 compared three doses. Mean weight change at 72 weeks was -15.0% on 5 mg, -19.5% on 10 mg and -20.9% on 15 mg. These are the doses tested in the trial, not a guide to treatment. Your doctor sets your dose.
• Late responders: some people took longer than 12 weeks to reach 5% weight loss, as shown above.

These figures describe groups. They are not a prediction for any one person, and your own results may be different.

When do side effects happen?

Side effects tend to come early, while the dose is being raised.

• US Zepbound label: most nausea, vomiting and diarrhea happened during dose escalation and decreased over time.
• SURMOUNT-1, three-year analysis: digestive side effects "occurred primarily during the dose-escalation period in the first 20 weeks of the trial."
• STEP 1: nausea and diarrhea with semaglutide were typically short-lived and mild to moderate.

What happens if treatment stops?

Weight came back in trials where treatment stopped. In STEP 4, people switched from semaglutide to placebo gained 6.9% over 48 weeks, and in SURMOUNT-4, people switched from tirzepatide to placebo gained 14.0% over 52 weeks.