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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Mounjaro vs Wegovy: Long Term Effects Compared
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections used in the UK, but they work differently and have different long term effect profiles. While both are once-weekly injections that may support weight loss alongside diet and exercise, Mounjaro uses a dual GIP and GLP-1 mechanism, whereas Wegovy is a GLP-1 agonist only. Understanding the differences in how they work and what clinical trials showed can help you and your prescriber decide which might be suitable for your individual needs. Use our free GLP-1 weight loss calculator to explore potential outcomes with either medication.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
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.
Clinical Trial Data: SURMOUNT vs STEP
The SURMOUNT-1 trial evaluated Mounjaro (tirzepatide) over 72 weeks in adults with obesity. At the maximum 15 mg dose, participants lost an average of 20.9% of their body weight alongside reduced-calorie diet and increased physical activity. The STEP-1 trial evaluated Wegovy (semaglutide) over 68 weeks, with participants at the 2.4 mg dose losing an average of approximately 14.9% of body weight under the same lifestyle conditions.
These averages represent clinical trial populations—individual results vary significantly. Both trials demonstrated that weight loss was sustained over the study period, though some regain occurred in follow-up phases when medication was discontinued. The dual-mechanism action of Mounjaro (activating both GIP and GLP-1 receptors) may explain the slightly higher average loss in SURMOUNT, but clinical equivalence has not been formally established, and individual suitability depends on personal health factors, not trial averages.
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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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How They Work Differently: Mechanism Matters
Mounjaro activates two hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action may enhance appetite suppression and support greater weight loss for some individuals. Wegovy activates GLP-1 only, a single but well-established pathway for reducing appetite and supporting feeling fuller after meals.
The long term difference is that Mounjaro's dual mechanism may maintain its effectiveness over time more consistently, though both medications can lose some effect if the body adapts. Gastric emptying (how quickly food leaves the stomach) may be affected differently—Mounjaro may maintain this effect longer for some users. Your prescriber will assess which mechanism aligns better with your individual health profile, metabolic needs, and response to similar medications.
- 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.
Does Mounjaro have fewer long term side effects than Wegovy?
Both medications have similar common side effects (nausea, diarrhoea, constipation, stomach pain). Long term data shows these typically improve over weeks as the body adjusts. Individual tolerance varies. Clinical trials did not show one significantly safer than the other long term. Your prescriber will assess your personal risk factors.
Which lasts longer in your system—Mounjaro or Wegovy?
Both are once-weekly injections with similar pharmacokinetics. Mounjaro (tirzepatide) and Wegovy (semaglutide) are designed for weekly dosing, not extended duration. If you stop either medication, weight regain typically begins within weeks. Neither is designed as a permanent long term solution.
Can you stay on Mounjaro or Wegovy long term?
Both are approved for ongoing use in the UK if clinically appropriate and effective. Your prescriber will review this periodically. Long term safety data continues to accumulate. If you stop either medication, your prescriber should discuss weight management strategy going forward, as weight regain is common.
How long do you take Mounjaro or Wegovy for weight loss?
There is no fixed duration. Treatment continues as long as it is effective and tolerated, and your prescriber agrees it remains clinically appropriate. Many people take these medications for months or years. Your GP or prescriber will monitor this regularly.
Does tirzepatide (Mounjaro) work better long term than semaglutide (Wegovy)?
Clinical trials suggest Mounjaro may produce slightly higher average weight loss, but individual results vary greatly. "Better" depends on your personal tolerance, side effects, and response. Both require a UK prescription and clinical assessment. Your prescriber will help determine suitability based on your health, not trial statistics.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.