The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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How Long Has Mounjaro Been Tested Compared to Wegovy?
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both prescription-only weight management injections available in the UK, but they have different testing histories. Mounjaro is newer to the weight loss market, with its SURMOUNT clinical trial programme beginning after Wegovy's STEP trials. Understanding how long each medication has been tested can help you and your prescriber make an informed decision about which option might be right for you.
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What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
SURMOUNT vs STEP: Clinical Trial Timelines and Data
Wegovy (semaglutide) completed its primary weight loss trial programme earlier. The STEP-1 trial (conducted 2015–2017) followed adults for 68 weeks on semaglutide 2.4 mg, showing average weight loss of approximately 14.9% of body weight. This trial data led to Wegovy's approval in the US (2021) and UK (2022).
Mounjaro (tirzepatide) entered clinical testing later, with the SURMOUNT-1 trial (conducted 2019–2021) running for 72 weeks. At the maximum dose of 15 mg, participants lost approximately 20.9% of body weight on average. SURMOUNT-2 and SURMOUNT-4 trials extended this data, including participants with type 2 diabetes. Mounjaro received UK approval in 2023, approximately one year after Wegovy.
Both medication classes work via the GLP-1 pathway, but Mounjaro adds a second mechanism: GIP receptor agonism. This dual action may contribute to greater average weight loss in trials, though individual results vary significantly and prescriber assessment is essential.
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
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Real-World Experience and Individual Suitability
Whilst Wegovy has slightly longer real-world use in the UK market, the difference in testing duration is relatively modest — both have now been in clinical use for approximately 2–3 years. Real-world evidence from both medications continues to accumulate. Your GP or prescriber will assess your individual health profile, medical history, and current medications when deciding which is appropriate for you.
Neither medication should be viewed as "more tested" or "safer" than the other based solely on timeline. Both require a clinical assessment before prescription. Using our free GLP-1 weight loss calculator can help you explore potential outcomes at different doses and compare realistic expectations — but your prescriber's recommendation, based on your personal circumstances, is what matters most.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
How long was the SURMOUNT trial for Mounjaro?
The SURMOUNT-1 trial ran for 72 weeks (approximately 17 months), with participants receiving escalating doses of tirzepatide from 2.5 mg up to 15 mg weekly. Follow-up data was collected for up to one year after the trial ended.
How long was the STEP trial for Wegovy?
The STEP-1 trial ran for 68 weeks (approximately 16 months) on semaglutide 2.4 mg once weekly. Multiple STEP trials (STEP-2, STEP-3, STEP-4) provided additional safety and efficacy data in different populations.
Which medication has been tested longer overall?
Wegovy has been tested slightly longer in humans — semaglutide research began earlier for diabetes management, with weight loss trials starting in 2015. Mounjaro's weight loss trials began later, in 2019. However, both now have robust clinical data spanning several years.
Does longer testing mean Mounjaro or Wegovy is safer?
Not necessarily. Both medications have undergone rigorous clinical trials and regulatory approval. Longer testing history doesn't automatically mean safer — it simply means more real-world experience. Your prescriber will assess which is suitable based on your health profile and medical history, not testing duration alone.
When did Mounjaro and Wegovy become available on the NHS in the UK?
Wegovy was approved for NHS use in 2022 through specialist tier 3 weight management services. Mounjaro received UK approval in 2023 and has since been approved by NICE for NHS weight management pathways. Availability varies by region and waiting lists can be lengthy in many areas.
Ready to take control of your weight loss journey?
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