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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Problems With Mounjaro — What You Need to Know
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, but they work differently and suit different people. Understanding the potential issues with Mounjaro — and how it compares to Wegovy — helps you and your prescriber decide which medication might be most appropriate for your individual health needs. This guide covers the key differences in how they work, their side effects, and what clinical trials actually showed about their effectiveness.
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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.
How Mounjaro and Wegovy Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways in your body. This dual mechanism may help regulate appetite and support feeling fuller after meals more effectively than single-pathway medications.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it targets one hormone pathway. Both work to slow gastric emptying and reduce appetite, but the dual approach of Mounjaro is what distinguishes it mechanically.
In clinical trials, Mounjaro showed average weight loss of around 15–21% of body weight (SURMOUNT-1 trial), whilst Wegovy showed approximately 15% average weight loss (STEP-1 trial). However, individual results vary significantly, and these are averages across trial participants — your personal outcome depends on many factors including dose, adherence, diet, and activity level. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses.
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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 Effects and Tolerability — Which Is Better?
Both medications share similar common side effects: nausea, diarrhoea, constipation, stomach pain, and vomiting. These typically occur early in treatment and often settle as your body adjusts. Some people tolerate Mounjaro better than Wegovy, and vice versa — there is no universally "better" option.
Problems reported with Mounjaro specifically include severe nausea or vomiting in some users, particularly during dose escalation. Wegovy users report similar tolerability issues. Injection site reactions (redness, bruising) occur with both. Rarer but serious concerns include pancreatitis symptoms (severe abdominal pain) and gallbladder problems — seek urgent medical attention if these occur.
Your GP or prescriber will assess which medication suits your health history. If you experience significant problems on one, switching to the other is sometimes possible, though this requires medical guidance.
- 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 than Wegovy?
Both are prescription-only medications with similar safety profiles. Neither is universally "safer" — tolerability and suitability vary between individuals. Your prescriber will assess which is more appropriate based on your medical history, other medications, and health conditions. Both require careful monitoring.
Why does Mounjaro cause worse nausea than Wegovy for some people?
Nausea is common with both medications but varies individually. Mounjaro's dual mechanism may trigger different side effect patterns in some users. Dose escalation speed, food intake, and individual stomach sensitivity all affect nausea severity. Slowing dose escalation can help, but discuss with your prescriber.
Can I switch from Mounjaro to Wegovy if I'm having problems?
Yes, switching between medications is sometimes possible, but requires prescriber guidance. There is no official conversion chart — your prescriber will assess whether switching is appropriate and determine a safe starting dose. Never self-switch medications.
Which is more effective — Mounjaro or Wegovy?
Clinical trial data shows Mounjaro slightly higher average weight loss (15–21% vs ~15%), but individual results vary enormously. Effectiveness depends on adherence, lifestyle changes, dose tolerance, and personal metabolism. Your prescriber considers your specific health situation, not just trial averages.
How long until I notice problems with these injections?
Most side effects appear in the first few weeks, particularly during dose increases. Many settle within 4–8 weeks as your body adjusts. Serious issues (pancreatitis symptoms, severe allergic reactions) need immediate medical attention. Keep your prescriber informed of any ongoing problems.
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.