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 Complications vs Wegovy — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK, but they work through different mechanisms and may carry different risk profiles. Understanding the complications and side effects of each can help you and your prescriber decide which medication might be more suitable for your individual circumstances. This guide compares the complications of both medications based on clinical trial data and UK prescribing information.
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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 — Side Effect Profiles Compared
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, which means it activates two hormone pathways simultaneously. In the SURMOUNT-1 clinical trial, the most common side effects at the 15 mg maintenance dose were nausea (25%), diarrhoea (22%), and vomiting (11%). Gastrointestinal side effects were the primary concern, though most were mild to moderate and often improved over time as the body adjusted.
Wegovy (semaglutide) is a single GLP-1 receptor agonist. In the STEP-1 clinical trial, the most common side effects at the 2.4 mg maintenance dose were nausea (26%), diarrhoea (20%), and vomiting (9%). The side effect profile is very similar to Mounjaro, though the mechanism of action differs. Both medications slow stomach emptying, which explains why gastrointestinal effects are common to both.
- Mounjaro: dual mechanism (GIP + GLP-1) may offer stronger appetite suppression
- Wegovy: single mechanism (GLP-1 only) may be suitable if you've previously tolerated similar medications
- Both: nausea typically peaks in early weeks and often resolves with continued use
- Both: serious complications are rare but require medical attention if they occur
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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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Serious Complications — When to Seek Medical Help
Whilst most side effects are mild to moderate, both medications carry a small risk of serious complications that require immediate medical attention. These include severe pancreatitis (inflammation of the pancreas), gallbladder disease, and diabetic retinopathy complications in people with existing diabetes. Changes in heart rate, severe allergic reactions, and persistent severe stomach pain are also serious warning signs.
Neither medication is suitable during pregnancy or whilst breastfeeding. If you're taking oral contraceptives, a prescriber may recommend switching to a barrier method or non-oral contraceptive for 4 weeks after starting either medication and after each dose increase, as both may reduce oral medication absorption. Your prescriber will assess which medication carries the lowest risk for your specific health profile — this is a personalised decision based on your medical history, existing conditions, and other medications you take.
Your GP or prescriber will conduct a full clinical assessment before recommending either medication. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and our BMI calculator to understand your current health status. Both tools can help inform your conversation with your healthcare provider about which medication might suit you best.
- 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 causes more nausea — Mounjaro or Wegovy?
Clinical trials show very similar nausea rates: Mounjaro 25% and Wegovy 26% at maintenance doses. Individual experience varies significantly. Nausea typically peaks in the first few weeks and often improves as your body adjusts. Your prescriber can help manage this through dose titration and lifestyle adjustments.
Is Mounjaro safer than Wegovy in the UK?
Both are safe when used under clinical supervision in eligible adults. Safety profiles are similar. The choice between them depends on your individual health circumstances, medical history, and response to similar medications. Neither is universally "safer" — suitability is personalised.
Can I switch from Wegovy to Mounjaro if I get side effects?
Possibly, but this requires your prescriber's guidance. Some patients tolerate one medication better than the other, even though side effect profiles are similar. Do not switch without medical supervision. Your prescriber will advise on timing and any dose adjustments needed.
What are the rare but serious complications of both?
Both carry small risks of pancreatitis (severe upper stomach pain), gallbladder problems, and serious allergic reactions. Diabetic retinopathy complications can occur in people with existing diabetes. Seek immediate medical attention if you experience severe persistent pain, difficulty breathing, or swelling of face or throat.
Do I need different monitoring for Mounjaro vs Wegovy?
Both require baseline clinical assessment and ongoing monitoring by your prescriber. If you have diabetes, you may need more frequent blood sugar monitoring as both can affect glucose levels. Your prescriber will advise on the monitoring schedule appropriate for whichever medication you're prescribed.
Ready to take control of your weight loss journey?
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