Semaglutide vs Tirzepatide: What the Clinical Trial Evidence Actually Shows
Semaglutide (Wegovy) and tirzepatide (Mounjaro) are currently the two most effective licensed weight management medications available in the UK. Patients researching their options frequently want a direct comparison, and the clinical trial data provides a clear and evidence-based answer. This guide explains the trial evidence for each medication, the mechanistic difference between them, the relevant safety considerations, and the clinical context for choosing between them.
Semaglutide: The STEP Trial Programme
Semaglutide 2.4 mg (Wegovy) was evaluated in the STEP (Semaglutide Treatment Effect in People with obesity) phase 3 clinical trial programme, consisting of four pivotal trials. STEP 1, published in the New England Journal of Medicine in 2021, enrolled 1,961 adults with a BMI of 30 kg/m2 or above (or 27 kg/m2 with at least one weight-related comorbidity) without type 2 diabetes. Participants received once-weekly subcutaneous semaglutide 2.4 mg or placebo for 68 weeks alongside lifestyle intervention. Mean weight loss in the semaglutide group was 14.9 percent of body weight, compared to 2.4 percent in the placebo group. Approximately 86 percent of semaglutide-treated participants achieved at least five percent weight loss.
STEP 2 enrolled adults with obesity and type 2 diabetes, demonstrating 9.6 percent weight loss with semaglutide 2.4 mg versus 3.4 percent with placebo, along with meaningful HbA1c reduction. STEP 3 and STEP 4 provided evidence for the role of intensive behavioural therapy and the consequences of treatment discontinuation (significant weight regain within one year of stopping medication).
Tirzepatide: The SURMOUNT Trial Programme
Tirzepatide (Mounjaro) was evaluated in the SURMOUNT (Studies of Tirzepatide for Obesity Research) phase 3 programme. SURMOUNT-1, published in the New England Journal of Medicine in 2022, enrolled 2,539 adults without type 2 diabetes with a BMI at or above 30 kg/m2 (or at or above 27 kg/m2 with comorbidities). Mean weight loss at 72 weeks was 20.9 percent with the 10 mg dose and 22.5 percent with the 15 mg dose, compared to 2.4 percent with placebo. At the 15 mg dose, 57 percent of participants achieved at least 20 percent body weight loss, an outcome that was not achievable with semaglutide in corresponding trials.
Clinical Evidence: A 2023 network meta-analysis published in the Lancet Diabetes and Endocrinology, comparing GLP-1 and GIP/GLP-1 agonists in over 45,000 participants across 143 trials, found that tirzepatide 15 mg produced the greatest mean percentage weight loss of any licensed agent, followed by semaglutide 2.4 mg. The absolute difference between the two agents in terms of mean weight loss was approximately seven to eight percentage points in favour of tirzepatide.
Why Tirzepatide Produces Greater Weight Loss: The Dual Mechanism
Semaglutide is a GLP-1 receptor agonist only. Tirzepatide is a dual GIP and GLP-1 receptor agonist, activating both the GLP-1 receptor and the GIP (glucose-dependent insulinotropic polypeptide) receptor simultaneously. The additional GIP receptor activation appears to produce synergistic effects on appetite suppression and fat metabolism beyond what GLP-1 agonism alone achieves, though the precise neurobiological mechanisms of this synergy are still being characterised in ongoing research.
| SEMAGLUTIDE 2.4 MG (WEGOVY) | TIRZEPATIDE (MOUNJARO) |
| GLP-1 receptor agonist only. Mean weight loss 14.9% at 68 weeks (STEP 1). Dose range: 0.25 mg weekly, titrated over 16 weeks to 2.4 mg. MHRA licensed for weight management. Cardiovascular outcome data (SELECT trial) demonstrates 20% reduction in major cardiovascular events. | Dual GIP and GLP-1 receptor agonist. Mean weight loss 22.5% at 72 weeks (SURMOUNT-1 at 15 mg). Dose range: 2.5 mg weekly, titrated over 20 weeks to 5, 10, or 15 mg. MHRA licensed for weight management. Cardiovascular outcome data from SURPASS-CVOT programme ongoing. |
Choosing Between Semaglutide and Tirzepatide
Important: The choice between semaglutide and tirzepatide should be made in consultation with a prescriber following a comprehensive clinical assessment. Both medications are prescription-only and require structured clinical review including assessment of contraindications, current medications, medical history, and individual treatment goals. At Curelo Care, all prescriptions are issued following a thorough online clinical assessment reviewed by a qualified prescriber. Neither medication is appropriate for use in pregnancy, or in patients with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2.
Clinical Disclaimer: This guide is for general informational purposes only and does not constitute personalised medical advice. Curelo Care is regulated by the General Pharmaceutical Council (GPhC). All weight management medication supplied by Curelo Care is provided following a structured online clinical assessment and prescriber review. If you have any questions about your treatment, please contact our clinical team.
