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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What Are The Long-Term Effects Of Mounjaro vs Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription weight management injections available in the UK, but they work through different mechanisms and have different long-term safety profiles based on available clinical data. Understanding the differences between these dual GIP plus GLP-1 receptor agonist and single GLP-1 approaches can help you and your GP or prescriber decide which may be more suitable for your individual health needs. This guide compares the long-term effects, clinical trial evidence, and practical considerations for both medications.
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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 Long-Term Outcomes
Mounjaro was evaluated in the SURMOUNT-1 clinical trial, which followed participants for up to 72 weeks (approximately 17 months). At the maximum dose of 15 mg weekly, participants lost an average of 20.9% of their body weight. Wegovy was studied in the STEP-1 trial, which also ran for 68 weeks, showing average weight loss of 14.9% at the maximum dose of 2.4 mg weekly. Both trials included participants with a BMI of 27 or above with weight-related health conditions, or BMI 30 and above.
Long-term safety data beyond 72 weeks remains limited for both medications. Mounjaro's dual GIP and GLP-1 mechanism differs from Wegovy's single GLP-1 pathway, which may influence tolerability and individual response. The key difference: tirzepatide activates two hormone pathways, whilst semaglutide activates one. Both showed similar patterns of gastrointestinal side effects (nausea, diarrhoea, constipation), though frequency and severity varied between individuals. Weight regain can occur if treatment is discontinued without sustained lifestyle changes.
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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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Side Effect Profiles and Individual Suitability
Both medications share common long-term side effects including nausea, diarrhoea, constipation, and abdominal pain—particularly during dose escalation. Most gastrointestinal symptoms improve over time as your body adjusts. Hair loss has been reported with both medications, likely linked to gradual weight management rather than the medication itself. Serious side effects including pancreatitis, gallbladder issues, and thyroid concerns are rare but require immediate medical attention.
Long-term suitability depends on your individual health profile. A prescriber will assess whether the UK prescription for either medication is appropriate based on your medical history, other medications, and specific health conditions. Use our GLP-1 weight loss calculator to explore potential outcomes at different doses. Your GP or specialist can then discuss which mechanism—dual or single—may work best for your long-term weight management plan alongside lifestyle changes.
- 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.
Is Mounjaro safer long-term than Wegovy?
Both medications have similar long-term safety profiles based on available clinical trial data. Mounjaro's dual GIP and GLP-1 mechanism differs from Wegovy's single GLP-1 pathway, but neither is proven "safer"—individual suitability depends on your health profile. Your prescriber will assess which is appropriate for you.
How long can you safely take Mounjaro or Wegovy?
Clinical trials ran for 68–72 weeks. Long-term safety data beyond 18 months is still emerging. These are intended as ongoing treatments for weight management—discontinuing the medication typically results in weight regain without sustained lifestyle changes.
What happens if you stop Mounjaro after long-term use?
Weight loss is often reversed gradually after stopping. The medication does not permanently change appetite or metabolism. Sustainable weight management requires continued diet and physical activity changes alongside or after treatment ends.
Which is better for long-term weight loss, Mounjaro or Wegovy?
Clinical trials show Mounjaro achieved slightly higher average weight loss (20.9% vs 14.9%), but individual results vary significantly. "Better" depends on your personal response, tolerability, and how well you respond to the dual vs single hormone mechanism. Your prescriber will help decide.
Do long-term side effects improve on Mounjaro or Wegovy?
Most gastrointestinal side effects improve within weeks as your body adjusts. Hair loss may persist and is usually temporary. Regular check-ins with your prescriber monitor long-term safety. Report persistent or concerning symptoms immediately to your GP.
Ready to take control of your weight loss journey?
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