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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Long Term Side Effects of Mounjaro vs Wegovy
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK, but how do their long-term side effect profiles compare? This guide breaks down what clinical trials revealed about sustained safety, differences between these GLP-1 and dual GIP/GLP-1 medications, and what to expect over months of use. Individual experiences vary significantly, and your GP or prescriber will assess which option suits your specific health needs.
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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.
Clinical Trial Data: SURMOUNT vs STEP
Mounjaro (tirzepatide) was tested in the SURMOUNT trials, which ran for 72 weeks and followed participants using the Mounjaro KwikPen at various doses up to 15mg weekly. Wegovy (semaglutide) was tested in the STEP trials, which also tracked safety over extended periods using once-weekly injections. Both trials documented side effects systematically, though the timeframes and participant populations differed slightly.
The most commonly reported side effects in both trials were gastrointestinal: nausea, vomiting, diarrhoea, and constipation. In SURMOUNT, these effects tended to be slightly more frequent with tirzepatide, particularly at higher doses during dose titration periods. STEP participants reported similar patterns with semaglutide, though the intensity varied. Importantly, both trials showed that side effects typically peak during the first 4–8 weeks and often decrease as the body adjusts, even while remaining on the same maintenance dose.
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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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Mechanism Differences and Long-Term Safety
Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways rather than one. This dual mechanism may account for slightly different long-term side effect patterns compared to Wegovy, which targets GLP-1 alone. Gastrointestinal side effects appear more pronounced with tirzepatide in the first 12 weeks but may stabilise more quickly during maintenance. Hair loss (alopecia) was reported in both trials, though it was slightly more common in Mounjaro participants — likely due to gradual weight management rather than the medication itself.
Long-term data (beyond 72 weeks) remains limited for both medications in the UK. Your prescriber will discuss individual suitability based on medical history, other medications, and any previous reactions to GLP-1 drugs. Use our free GLP-1 weight loss calculator to understand potential outcomes, and track any side effects with our free progress tracker during treatment.
- 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.
Which has worse long-term side effects, Mounjaro or Wegovy?
Clinical trials show both medications have similar long-term side effect profiles overall. Gastrointestinal issues appear slightly more frequent with Mounjaro during dose titration, but both tend to improve as your body adjusts. Individual responses vary significantly — what affects one person may not affect another. Your prescriber will assess which is most suitable based on your health history.
Do side effects from Mounjaro get worse over time?
Clinical trial data suggests side effects typically peak during the first 4–8 weeks and often improve during maintenance therapy, even at the same dose. However, if you experience severe or persistent side effects beyond 8–12 weeks, contact your GP or prescriber immediately. Do not stop the medication without medical guidance.
Is tirzepatide safer than semaglutide long-term?
Neither medication is inherently "safer" — they have different mechanisms (dual vs single hormone pathway) leading to slightly different side effect patterns. SURMOUNT and STEP trials both demonstrated acceptable safety profiles over 72 weeks. Suitability depends on your individual health factors, not superiority of one over the other.
How long does nausea last on Mounjaro or Wegovy?
Most participants experience peak nausea in weeks 1–4, with significant improvement by weeks 8–12 as the body adapts. However, nausea can persist in some individuals, particularly if doses are increased too quickly. Eating smaller, lower-fat meals often helps manage this side effect. Discuss persistent nausea with your prescriber.
What should I monitor for long-term with these medications?
Monitor gastrointestinal symptoms, appetite changes, injection site reactions, and general wellbeing. Track any side effects using our free progress tracker. Report persistent symptoms, changes in heart rate, or signs of pancreatitis (severe abdominal pain) to your GP immediately. Attend all follow-up appointments with your prescriber to adjust doses if needed.
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