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 7.5mg Side Effects vs Wegovy: What's the Difference?
When considering weight loss injections in the UK, many people compare Mounjaro (tirzepatide) with Wegovy (semaglutide) to understand which side effect profile might suit them better. Both are prescription-only GLP-1 or dual-action weight management medications, but they work differently in your body and may cause different side effects at equivalent doses. This guide compares the side effects of Mounjaro 7.5mg with Wegovy, explains the clinical trial data, and helps you understand why individual experience varies so much.
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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.
Mechanism Differences: Why Mounjaro and Wegovy Aren't Identical
The key difference between these medications affects which side effects you might experience. Mounjaro is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously. Wegovy is a GLP-1 only receptor agonist — it works on a single pathway. This difference matters for side effects: Mounjaro's dual mechanism may cause slightly different gastrointestinal effects because it influences appetite regulation and glucose control through two separate signals, whereas Wegovy's single mechanism works more narrowly. In the SURMOUNT-1 trial, Mounjaro 7.5mg showed nausea, diarrhoea, and vomiting as the most common side effects, while Wegovy's STEP-1 trial showed a similar pattern but with some variation in frequency. Neither medication is universally "better" or "worse" — it depends entirely on your individual body's response, your dose titration schedule, and your personal health context.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Side Effect Profiles: What the Clinical Trials Show
In the SURMOUNT-1 trial (Mounjaro at 7.5mg), the most commonly reported side effects were nausea (25.3%), diarrhoea (23.1%), and vomiting (11.2%). In the STEP-1 trial (Wegovy at maximum 2.4mg), nausea affected 25.2%, diarrhoea 20.7%, and vomiting 8.9%. Both medications share the same top three side effects — gastrointestinal symptoms — because both activate GLP-1 pathways. However, Mounjaro's additional GIP action may influence how quickly your stomach empties food, potentially affecting bloating or indigestion differently than Wegovy. Important: clinical trial percentages reflect averages across all participants. Your experience depends on your dose, how quickly you increase dose (titration speed), your diet, and your individual metabolism. Many people experience minimal side effects; others find gastrointestinal symptoms manage within 4–6 weeks as their body adapts.
- 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 7.5mg more likely to cause side effects than Wegovy?
Not necessarily. Both medications show similar side effect frequencies in clinical trials. Mounjaro 7.5mg and Wegovy at comparable doses cause nausea and diarrhoea in roughly 20–25% of users. Individual experience varies based on personal physiology, dose titration speed, and diet. Your prescriber will monitor your response and adjust your approach if needed.
Why do Mounjaro and Wegovy have similar side effects if they work differently?
Both activate the GLP-1 hormone pathway, which is responsible for appetite regulation and gastrointestinal effects. Mounjaro's additional GIP component influences glucose metabolism and may affect satiety slightly differently, but both primarily cause nausea, diarrhoea, and stomach discomfort through the same GLP-1 mechanism.
How long do Mounjaro 7.5mg side effects last?
Most side effects peak during dose escalation and typically improve within 4–6 weeks as your body adjusts. Nausea and stomach symptoms are most common in weeks 1–4 after a dose increase. By week 8, many people report significant improvement. However, individual timelines vary considerably.
Should I choose Mounjaro or Wegovy based on side effects?
No single medication is universally better. Your prescriber will recommend based on your BMI, weight-related health conditions, medical history, and medications you take. Neither medication is suitable for everyone. A clinical assessment will determine which is appropriate for you. Side effect tolerability is monitored after you start treatment.
Can I use our GLP-1 weight loss calculator to compare Mounjaro and Wegovy results?
Yes. Our free GLP-1 weight loss calculator helps you understand potential weight loss outcomes at different doses for both medications based on clinical trial data. Results vary individually, but the calculator provides realistic projections based on SURMOUNT and STEP trials. It's useful for discussing expectations with your prescriber.
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