The clinical evidence behind each medicine
Every weight loss figure on this site comes from a published trial or the UK product information. This page lists each trial, the numbers and the source.
| Medicine and dose | Trial | Average loss | Placebo |
|---|---|---|---|
| Mounjaro 15 mg | SURMOUNT-1, 72 weeks | 20.9% | 3.1% |
| Wegovy 2.4 mg | STEP 1, 68 weeks | 14.9% | 2.4% |
| Wegovy pill 25 mg | OASIS 4, 64 weeks | 13.6% | 2.2% |
| Foundayo pill 17.2 mg | ATTAIN-1, 72 weeks | 11.2% | 2.1% |
Each bar counts every participant in the trial, including people who stopped treatment early. All trials ran alongside diet and activity support. These are four separate trials, so the bars give context and do not rank the medicines. In SURMOUNT-5, the one direct comparison, tirzepatide averaged 20.2% and semaglutide 13.7% at 72 weeks. See the evidence for each medicine.
How to read trial results #
Trials report results in two ways. The first counts everyone who started the trial, including people who stopped the medicine early. Researchers call this the treatment-regimen estimand. The second estimates the effect in people who stayed on the medicine. Researchers call this the efficacy estimand. The second figure is always higher.
We quote the first figure wherever a trial publishes one, because the first figure is closer to real life. Every trial below ran alongside diet and activity support. Your own result can be higher or lower than an average.
The four main trials were separate studies with different people. Comparing bars across trials gives context only. SURMOUNT-5 is the one trial here comparing two of these medicines directly.
Mounjaro (tirzepatide) #
Mounjaro is tirzepatide, a dual GIP and GLP-1 receptor agonist. Once weekly dosing.
| Detail | Information |
|---|---|
| People in the trial | 2,539 |
| Who took part | Adults with BMI 30 or more, or 27 or more with at least one weight-related complication, without diabetes. Mean baseline weight 104.8 kg, mean BMI 38.0. |
| Length | 72 weeks |
| Dose reported | 15 mg |
| Average weight loss, everyone counted | 20.9% |
| Average loss on placebo | 3.1% |
| Average loss in people who stayed on treatment | 22.5% |
| Lost at least 5% of body weight | 90.9% (placebo 34.5%) |
| Lost at least 10% of body weight | 83.5% (placebo 18.8%) |
| Lost at least 15% of body weight | 70.6% (placebo 8.8%) |
| Lost at least 20% of body weight | 56.7% (placebo 3.1%) |
Other trials
- SURMOUNT-4 (withdrawal). After a 36-week tirzepatide lead-in with 20.9% mean weight loss, 670 adults were randomised. Over the next 52 weeks weight changed by -5.5% with continued tirzepatide and +14.0% with placebo. Total loss from week 0 to 88 was 25.3% versus 9.9%. 89.5% versus 16.6% kept at least 80% of the lead-in loss. Source
- SURMOUNT-5 (head to head). In 751 adults with obesity without type 2 diabetes, mean weight change at 72 weeks was -20.2% with tirzepatide (10 or 15 mg) and -13.7% with semaglutide (1.7 or 2.4 mg). Waist change was -18.4 cm versus -13.0 cm. Open-label trial. Source
- SURMOUNT-OSA. In adults with moderate to severe obstructive sleep apnoea and obesity, AHI fell by 25.3 events per hour with tirzepatide versus 5.3 with placebo in people not using PAP, and by 29.3 versus 5.5 in people using PAP, at 52 weeks. Source
Common side effects in trials
| Side effect | On the medicine | On placebo |
|---|---|---|
| Nausea | 28% | 8.5% |
| Diarrhoea | 22.5% | 7.8% |
| Vomiting | 13% | 2% |
| Constipation | 11% | 5% |
| Abdominal pain | 10% | 5% |
Product information: Mounjaro KwikPen SmPC (emc, last updated 8 Sep 2026), Mounjaro KwikPen patient leaflet (emc)
Wegovy (semaglutide) #
Wegovy is semaglutide, a gLP-1 receptor agonist. Once weekly dosing.
| Detail | Information |
|---|---|
| People in the trial | 1,961 |
| Who took part | Adults with BMI 30 or more, or 27 or more with at least one weight-related condition, without diabetes |
| Length | 68 weeks |
| Dose reported | 2.4 mg |
| Average weight loss, everyone counted | 14.9% |
| Average loss on placebo | 2.4% |
| Lost at least 5% of body weight | 86.4% (placebo 31.5%) |
| Lost at least 10% of body weight | 69.1% (placebo 12%) |
| Lost at least 15% of body weight | 50.5% (placebo 4.9%) |
Other trials
- STEP UP (7.2 mg). 1,407 adults with BMI 30 or more without diabetes, 72 weeks. Mean weight change was -18.7% with 7.2 mg, -15.6% with 2.4 mg and -3.9% with placebo. 47.7%, 33.3% and 2.9% lost at least 20%. 31.2%, 15.3% and 0% lost at least 25% (UK SmPC table). Source
- SELECT (cardiovascular outcomes). 17,604 adults aged 45 or over with established cardiovascular disease and BMI 27 or more, without diabetes, mean follow-up 39.8 months. Cardiovascular death, non-fatal heart attack or non-fatal stroke occurred in 6.5% with semaglutide 2.4 mg and 8.0% with placebo (hazard ratio 0.80, 95% CI 0.72 to 0.90). 16.6% versus 8.2% stopped because of adverse events. Source
- STEP 4 (withdrawal). After a 20-week run-in with 10.6% mean weight loss, 803 adults were randomised. From week 20 to 68 weight changed by -7.9% with continued semaglutide and +6.9% with placebo, a difference of 14.8 percentage points. People who stayed on semaglutide lost 17.4% in total from week 0 to 68 (SmPC). Source
- STEP 1 extension (after stopping). 327 participants were followed for a year after stopping. The semaglutide group had lost 17.3% by week 68, regained 11.6 percentage points by week 120 and kept a net loss of 5.6%. Source
Common side effects in trials
| Side effect | On the medicine | On placebo |
|---|---|---|
| Nausea | 43.9% | 16.1% |
| Diarrhoea | 29.7% | 15.9% |
| Vomiting | 24.5% | 6.3% |
| Constipation | 24.2% | 11.1% |
| Nausea | 38.9% | 12.6% |
Product information: Wegovy FlexTouch SmPC (emc, last updated 22 Sep 2026), Wegovy patient leaflet (emc)
Wegovy pill (oral semaglutide) #
Wegovy pill is oral semaglutide, a gLP-1 receptor agonist. Once daily dosing.
| Detail | Information |
|---|---|
| People in the trial | 307 |
| Who took part | Adults with BMI 30 or more, or 27 or more with at least one obesity-related complication, without diabetes |
| Length | 64 weeks |
| Dose reported | 25 mg |
| Average weight loss, everyone counted | 13.6% |
| Average loss on placebo | 2.2% |
| Lost at least 5% of body weight | 76.3% (placebo 30.5%) |
| Lost at least 10% of body weight | 59.8% (placebo 14.1%) |
| Lost at least 15% of body weight | 47% (placebo 5.4%) |
| Lost at least 20% of body weight | 27.5% (placebo 3%) |
Other trials
- OASIS 1 (50 mg, not a UK licensed dose). 667 adults with overweight or obesity without type 2 diabetes, 68 weeks. Mean weight change was -15.1% with oral semaglutide 50 mg and -2.4% with placebo. 85% versus 26% lost at least 5%, 69% versus 12% at least 10%, 54% versus 6% at least 15% and 34% versus 3% at least 20%. Gastrointestinal adverse events occurred in 80% versus 46%. Source
Common side effects in trials
| Side effect | On the medicine | On placebo |
|---|---|---|
| Nausea | 46.6% | 18.6% |
| Vomiting | 30.9% | 5.9% |
| Constipation | 20.1% | 9.8% |
| Diarrhoea | 17.6% | 8.8% |
| Hair loss | 6.4% | 2% |
Product information: Wegovy 1.5 mg tablets SmPC (emc, first authorised 11 June 2026, text revised 06/2026), OASIS 4, NEJM 2025
Foundayo pill (orforglipron) #
Foundayo pill is orforglipron, a oral non-peptide (small molecule) GLP-1 receptor agonist. Once daily, at any time of day, with or without food, no water restriction dosing.
| Detail | Information |
|---|---|
| People in the trial | 3,127 |
| Who took part | Adults with BMI 30 or more, or 27 to under 30 with at least one weight-related complication, without diabetes. Mean age 45, 64.2% women, mean baseline weight 103.2 kg. |
| Length | 72 weeks |
| Dose reported | 36 mg capsule (equivalent to the 17.2 mg tablet) |
| Average weight loss, everyone counted | 11.2% |
| Average loss on placebo | 2.1% |
| Average loss in people who stayed on treatment | 12.4% |
| Lost at least 5% of body weight | 71.8% (placebo 26.8%) |
| Lost at least 10% of body weight | 54.6% (placebo 12.9%) |
| Lost at least 15% of body weight | 36% (placebo 5.9%) |
| Lost at least 20% of body weight | 18.4% (placebo 2.8%) |
Other trials
- ATTAIN-2 (obesity with type 2 diabetes). 72 weeks. Mean weight change (efficacy estimand, UK SmPC) was -5.5% with 6 mg, -7.8% with 12 mg and -10.5% with 36 mg versus -2.2% with placebo. 72.8% versus 24.4% lost at least 5% on the top dose, 50.1% versus 7.0% at least 10%. HbA1c fell by 1.3, 1.6 and 1.8 percentage points versus 0.1. Source
- ATTAIN-MAINTAIN (maintenance after injections). 376 adults who finished 72 weeks of tirzepatide or semaglutide in SURMOUNT-5 were randomised to orforglipron or placebo for 52 weeks. After tirzepatide, orforglipron kept 74.7% of the earlier weight loss versus 49.2% with placebo. After semaglutide, 79.3% versus 37.6%. Average regain on orforglipron was about 5 kg after tirzepatide and about 1 kg after semaglutide. Source
Common side effects in trials
| Side effect | On the medicine | On placebo |
|---|---|---|
| Nausea | 33.7% | 10.4% |
| Constipation | 25.4% | 9.3% |
| Vomiting | 24% | 3.5% |
| Diarrhoea | 23.1% | 9.6% |
| Abdominal pain | 14% | 7% |
Product information: Foundayo 0.8 mg film-coated tablets SmPC (emc, updated 14 Aug 2026), Foundayo patient leaflet (emc)
Direct comparisons #
- SURMOUNT-5. Tirzepatide (10 or 15 mg) versus semaglutide (1.7 or 2.4 mg), 751 adults with obesity without type 2 diabetes, 72 weeks, open label. Mean weight change -20.2% versus -13.7%. Waist change -18.4 cm versus -13.0 cm. Aronne LJ et al, N Engl J Med 2025;393(1):26-36. Source
- STEP UP. Semaglutide 7.2 mg versus 2.4 mg versus placebo, 1,407 adults with BMI 30 or more, 72 weeks. Mean weight change -18.7% versus -15.6% versus -3.9% (UK SmPC). Source
What happens after stopping #
Four trials followed people after the medicine stopped. Each one found weight returning without ongoing treatment or a maintenance plan.
- SURMOUNT-4. After 36 weeks on tirzepatide (20.9% mean loss), people switched to placebo regained 14.0% of body weight over 52 weeks while people who continued lost a further 5.5%. 16.6% of the placebo group kept at least 80% of the earlier loss versus 89.5% on tirzepatide. Source
- STEP 4. After 20 weeks on semaglutide (10.6% mean loss), people switched to placebo regained 6.9% over 48 weeks while people who continued lost a further 7.9%. Source
- STEP 1 extension. One year after stopping semaglutide 2.4 mg, participants had regained 11.6 percentage points of a 17.3% loss, about two-thirds, leaving a net loss of 5.6% at week 120. Source
- ATTAIN-MAINTAIN. After stopping weekly injections, placebo groups kept 49.2% (after tirzepatide) and 37.6% (after semaglutide) of the earlier loss at 52 weeks. People switched to orforglipron kept 74.7% and 79.3%. Source
Read weight loss maintenance for the plan after your goal.
How we check these figures #
Each number on this page was read from the source named beside the number on 15 October 2026. Sources are the UK summary of product characteristics on the electronic medicines compendium, the journal abstract, or the regulator's label. We recheck the figures at every page review and when product information changes.
Read our editorial policy for the full sourcing rules.
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Page information
- Summary
- The trial results behind each medicine at Aero Clinic. Average weight loss, placebo results, trial size and length, with a link to every source.
- Publisher
- Aero Clinic, operated by AERO HEALTHCARE LTD.
- Editorial update
- Clinical review
- Pending. An editorial update is not a clinical sign-off.
Cite as: The clinical evidence behind each medicine, Aero Clinic (https://www.cagri.co.uk), AERO HEALTHCARE LTD. https://www.cagri.co.uk/clinical-evidence. Editorial update: 4 November 2026. Clinical review pending.