How much weight do GLP-1 drugs actually deliver?
Media coverage of Wegovy and Zepbound swings between miracle cure and dangerous shortcut. Almost nobody knows the actual trial numbers. The STEP trials showed an average 14.9% body weight loss on semaglutide over 68 weeks. The SURMOUNT trials showed tirzepatide at 20.9%. Enter your details to see where your projected result falls on the clinical trial distribution.
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Zepbound vs Wegovy: which produces more weight loss?
Tirzepatide (Zepbound) produces significantly more weight loss than semaglutide (Wegovy) at comparable doses, based on head-to-head trial data. The SURMOUNT-1 trial found that tirzepatide at its highest dose of 15 mg produced an average weight loss of 20.9% of body weight over 72 weeks. The STEP 1 trial for semaglutide at 2.4 mg produced an average loss of 14.9% over 68 weeks. The difference — approximately 6 percentage points — is clinically meaningful. For a person starting at 100 kg, tirzepatide produces approximately 21 kg of loss at the mean versus approximately 15 kg for semaglutide.
The mechanism explains the gap. Wegovy (semaglutide) is a GLP-1 receptor agonist: it mimics the GLP-1 hormone that signals satiety and slows gastric emptying. Zepbound (tirzepatide) is a dual GIP/GLP-1 receptor agonist — it activates both GLP-1 and GIP receptors simultaneously. GIP (glucose-dependent insulinotropic polypeptide) appears to amplify the appetite-suppressing effect, producing greater reductions in caloric intake and additional metabolic benefits beyond GLP-1 alone. This dual mechanism is why tirzepatide consistently outperforms semaglutide in direct comparisons.
The comparison between Ozempic and Mounjaro — the diabetes-approved versions of the same drugs — is frequently searched because both are commonly prescribed off-label for weight loss. The underlying pharmacology is identical to their weight management counterparts (Wegovy and Zepbound), but the trial populations and approved doses differ. For weight loss purposes, the SURMOUNT and STEP trial data from the dedicated weight management formulations is the most relevant benchmark. This calculator uses both trial datasets to project outcomes based on starting weight and medication choice.
How long does Mounjaro (tirzepatide) take to work?
Tirzepatide (Mounjaro/Zepbound) typically produces noticeable appetite suppression and reduced food intake within the first 1-2 weeks of starting treatment, even at the initial low dose of 2.5 mg. Measurable weight loss — typically 2-4 kg — is usually apparent by weeks 4-8. The dose escalation schedule (2.5 mg weekly, increasing by 2.5 mg every 4 weeks to a maximum of 15 mg) means the full therapeutic effect develops over approximately 20 weeks as the patient titrates to their optimal dose.
In the SURMOUNT-1 trial, the majority of weight loss occurred over the first 36 weeks, with the rate slowing as participants approached their new weight set point. By week 72, weight loss had largely plateaued for most participants at the highest dose. Patients typically report that appetite suppression is the dominant early effect: food feels less compelling, portion sizes reduce naturally, and the urge to snack decreases. Nausea, one of the most common side effects, typically peaks in the first 4-8 weeks and improves as the body adjusts or as dose escalation slows.
For semaglutide (Wegovy), the timeline is similar: noticeable appetite effects within 1-2 weeks, measurable weight loss by weeks 4-8, and peak loss achieved by weeks 52-68. How long does it take to lose weight on Wegovy? The STEP 1 trial showed 5% body weight loss was achieved by a majority of participants by approximately week 16-20. The critical difference from tirzepatide is the plateau level: semaglutide reaches its ceiling at approximately 14-16% loss for most patients, while tirzepatide continues to produce additional loss at higher doses for many patients.
How much weight can you realistically lose on Wegovy?
The STEP 1 trial reported a headline average of 14.9% body weight loss on semaglutide 2.4 mg (Wegovy) over 68 weeks. But averages obscure the full distribution, which is what this calculator is designed to show. In the trial, 86% of participants lost at least 5% of body weight, 69% lost at least 10%, approximately 50% lost 15% or more, and roughly 32% lost 20% or more. At the other end, approximately 14% of participants did not achieve 5% loss — meaning the drug does not produce clinically significant weight loss for everyone.
Results were substantially lower in patients with type 2 diabetes. The STEP 2 trial, which enrolled participants with existing T2D, showed an average loss of 9.6% on the same 2.4 mg dose. This is a consistent finding across GLP-1 trials: insulin resistance appears to blunt the weight loss response, and the clinical community considers 5-10% loss a meaningful success in the diabetic population even when non-diabetic patients are achieving 15-20%.
Am I eligible for Wegovy? The FDA has approved Wegovy for adults with a BMI of 30 or higher, or a BMI of 27 or higher with at least one weight-related condition (type 2 diabetes, high blood pressure, high cholesterol, or obstructive sleep apnoea). In the UK, NICE guidance (TA875) specifies that semaglutide should be initiated through specialist weight management services. Approximately 42% of US adults have a BMI of 30 or above, meaning a substantial portion of the population meets the basic eligibility threshold. The eligibility check in this calculator applies published clinical criteria, but a clinician will assess your complete medical history before prescribing.
What do GLP-1 drugs actually do?
GLP-1 receptor agonists (glucagon-like peptide-1) work by mimicking a hormone that signals satiety to the brain and slows gastric emptying. Semaglutide (Wegovy) activates GLP-1 receptors. Tirzepatide (Zepbound) activates both GLP-1 and GIP (glucose-dependent insulinotropic polypeptide) receptors, which appears to produce greater weight loss.
The STEP 1 trial enrolled 1,961 adults (mean starting weight 105.3 kg) and found semaglutide 2.4 mg produced a mean loss of 14.9% over 68 weeks. The SURMOUNT-1 trial enrolled 2,539 adults and found tirzepatide 15 mg produced a mean loss of 20.9% over 72 weeks, with 57% of participants achieving 20%+ loss.
Results are lower in people with type 2 diabetes: STEP 2 showed 9.6% average loss on semaglutide; SURMOUNT-2 showed 14.7% on tirzepatide.
Frequently asked questions
The STEP 1 trial found an average loss of 14.9% of body weight over 68 weeks (semaglutide 2.4 mg). 86% of participants lost at least 5% and 50% lost at least 15%. About 14% did not achieve 5% loss. For someone starting at 100 kg, the average would be approximately 15 kg over the trial period.
The SURMOUNT-1 trial found tirzepatide 15 mg produced an average loss of 20.9% over 72 weeks, meaningfully more than semaglutide. 57% achieved 20%+ loss. Tirzepatide acts on both GLP-1 and GIP receptors. The 10 mg dose averaged 19.5% and the 5 mg dose averaged 15.0%, comparable to Wegovy.
The FDA has approved both Wegovy and Zepbound for adults with BMI ≥30, or BMI ≥27 with at least one weight-related condition (type 2 diabetes, hypertension, high cholesterol, or sleep apnoea). NHS prescribing follows similar criteria with additional specialist referral requirements. A clinician will assess your full medical history; BMI alone does not guarantee a prescription.
The STEP 4 trial found that switching from semaglutide to placebo resulted in regaining approximately two-thirds of weight lost over the following 48 weeks. GLP-1 medications suppress appetite while being taken but do not permanently change the body's weight set point. For most people, this means long-term or indefinite treatment rather than a short course.
"Ozempic face" refers to facial volume loss and sagging that can accompany rapid significant weight loss on GLP-1 medications. It is a consequence of losing subcutaneous facial fat, the same thing that happens with any substantial weight loss, not a specific drug side effect. It is more pronounced in older patients due to lower skin elasticity. Not everyone experiences noticeable facial changes.
Weight loss on semaglutide (Wegovy) follows a predictable timeline based on the STEP trials. Most patients see meaningful weight loss within the first 12 weeks as the dose is titrated upward. The maximum dose of 2.4 mg is typically reached at week 16-20. Peak results occur around weeks 52-68, with the STEP 1 trial showing an average 14.9% body weight loss at 68 weeks. Weight loss tends to plateau after about a year on the full dose as the body reaches a new equilibrium. Approximately 69% of patients achieve at least 10% body weight loss. Source: Wilding JPC et al., NEJM 2021 (STEP 1).
Most of the weight lost on GLP-1 medications is regained after stopping. The STEP 4 trial (Rubino et al., 2021) found that patients who discontinued semaglutide after 20 weeks regained two-thirds of their lost weight within 1 year, while those who continued maintained their loss. This is consistent with the understanding that GLP-1 medications suppress appetite and cravings while being taken, but do not produce lasting physiological changes after cessation. For most patients, these medications represent a long-term treatment, not a short-term course. Source: Rubino D et al., JAMA 2021 (STEP 4).
The long-term safety profile of semaglutide and tirzepatide is still being established, though both have now been used in large populations. The SELECT trial demonstrated that semaglutide reduced cardiovascular events (heart attack and stroke) by 20% in adults with cardiovascular disease, providing important long-term safety and efficacy data. Known side effects include nausea, vomiting, diarrhoea, and constipation, particularly during dose escalation. Rare but serious risks include pancreatitis, gallbladder disease, and a theoretical (observed in rodents but not humans) risk of thyroid tumours. The medications are contraindicated in patients with a personal or family history of medullary thyroid cancer. Source: Lincoff AM et al., NEJM 2023 (SELECT trial); FDA prescribing information.
Ozempic face is an informal term describing the facial appearance changes that some people experience with significant GLP-1-related weight loss — specifically, increased facial hollowing, sagging skin, and a gaunt or aged appearance. It is not a direct drug side effect but a consequence of rapid, substantial fat loss anywhere on the body. The face naturally loses volume with significant weight loss, and fat stores in the cheeks, temples, and perioral area can reduce noticeably when 15-20% of total body weight is lost. Dermatologists note that the faster the weight loss, the less time skin has to adapt and contract. The phenomenon is not unique to GLP-1 medications — it was described with bariatric surgery decades ago — but the speed and magnitude of GLP-1-related loss has brought it into wider public awareness. Treatments include dermal fillers and skin-tightening procedures, though most clinicians advise completing and stabilising weight loss before pursuing cosmetic interventions.
The STEP 4 trial directly studied this question. Participants who had been taking semaglutide for 20 weeks and then switched to placebo regained approximately two-thirds of the weight they had lost over the following 48 weeks. Most also saw reversal of improvements in cardiovascular risk factors and blood sugar levels. Similar rebound patterns have been observed in observational data for tirzepatide. This reflects the biological reality that GLP-1 medications suppress appetite and food intake while being taken but do not permanently alter the body's weight set point. Once medication stops, appetite returns to baseline and weight typically follows. For most patients, GLP-1 medication is therefore a long-term or indefinite treatment rather than a time-limited course. This is a critical consideration in the cost-benefit analysis and is worth discussing with a prescribing clinician before starting.
The STEP 1 trial data shows that most meaningful weight loss on semaglutide 2.4 mg (Wegovy) occurs within the first 36-52 weeks of treatment. In the first 4 weeks, loss is typically modest (1-2% of body weight) as the dose is still being titrated upward. By weeks 16-20, approximately 50-60% of participants have achieved 5% body weight loss. By weeks 36-52, participants approaching the highest dose are typically nearing their plateau. The final weeks of the 68-week trial show relatively slow additional loss as most participants have reached their new weight equilibrium on medication. For practical planning: most patients see their most significant results between weeks 12 and 52. Results after 52 weeks are typically maintenance rather than continued loss. Consistent use and dose titration as prescribed are the strongest predictors of reaching the upper end of the expected range.
- Wilding JPH et al. (2021). Once-Weekly Semaglutide in Adults with Overweight or Obesity. NEJM, 384(11):989–1002. STEP 1, N=1,961.
- Jastreboff AM et al. (2022). Tirzepatide Once Weekly for the Treatment of Obesity. NEJM, 387(3):205–216. SURMOUNT-1, N=2,539.
- Davies M et al. (2021). Semaglutide 2.4 mg once a week in adults with overweight or obesity, and type 2 diabetes (STEP 2). Lancet. N=1,210.
- Garvey WT et al. (2023). Tirzepatide once weekly for the treatment of obesity in people with type 2 diabetes (SURMOUNT-2). Lancet. N=938.
- FDA Prescribing Information: Wegovy (2021). FDA Prescribing Information: Zepbound (2023).