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 2.5 Side Effects vs Wegovy — What You Need to Know
Mounjaro 2.5 is the starting dose of tirzepatide, a prescription-only weight management injection that works differently from Wegovy (semaglutide). Both are licensed in the UK for weight loss, but they have distinct mechanisms and side effect profiles. Understanding how they compare can help you and your GP or prescriber make an informed decision. This guide explains the side effects specific to Mounjaro 2.5 and how they differ from Wegovy, based on clinical trial data and UK medical guidance.
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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.
How Mounjaro 2.5 and Wegovy Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two separate hormone pathways to regulate appetite and blood sugar. The 2.5 mg dose is the starting point in a titration schedule, with doses increasing every four weeks if tolerated. Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates a single pathway. This mechanism difference may explain why side effect profiles differ between the two injections, even though both reduce appetite and support feeling fuller after smaller meals.
- Mounjaro's dual mechanism may trigger slightly different gastric effects initially
- Wegovy's single mechanism has a longer clinical history in weight management
- Both slow how quickly food leaves the stomach (gastric emptying)
- Individual tolerance varies significantly — what affects one person may not affect another
Clinical trial data from the SURMOUNT studies (Mounjaro) and STEP trials (Wegovy) show both are well-tolerated overall, but side effect frequency and severity can differ at the starting 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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Mounjaro 2.5 vs Wegovy Side Effects — Direct Comparison
At the starting dose, Mounjaro 2.5 mg and Wegovy 0.25 mg (the first week dose) both commonly cause nausea, diarrhoea, and reduced appetite — though these typically reduce as your body adjusts. Clinical data suggests Mounjaro users may experience slightly more gastrointestinal side effects initially due to its dual mechanism, but serious adverse events are rare with both. Most side effects improve within the first 2–4 weeks as tolerance builds. Your GP or prescriber can discuss which injection suits your health profile and medical history. Use our free GLP-1 weight loss calculator above to explore how either medication might work with your personal circumstances, and track any side effects you experience with our progress tracker.
- 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.
What are the most common side effects of Mounjaro 2.5?
The most common side effects include nausea, diarrhoea, constipation, vomiting, and stomach pain. These affect more than 1 in 10 people in clinical trials but usually reduce significantly within the first few weeks. Smaller, more frequent meals and staying hydrated can help manage symptoms whilst your body adjusts to the injection.
Is Mounjaro 2.5 safer than Wegovy?
Both are safe when prescribed appropriately after clinical assessment. Safety profiles are similar overall — serious side effects are rare with both. The choice between them depends on mechanism suitability, your medical history, and prescriber recommendation rather than one being inherently "safer." Your GP will assess which is right for you.
Why does Mounjaro have more side effects than Wegovy at the same stage?
Mounjaro's dual GIP and GLP-1 mechanism can cause slightly more gastrointestinal effects initially compared to Wegovy's single GLP-1 action. However, this does not mean Mounjaro is worse — some people tolerate it better, and it may provide greater appetite suppression. Individual responses vary significantly, so clinical assessment is essential.
How long do Mounjaro 2.5 side effects last?
Most side effects improve significantly within 2–4 weeks as your body adapts. Nausea and stomach symptoms typically resolve first. Dose increases happen gradually (every 4 weeks), allowing adjustment time. If side effects persist or worsen, contact your prescriber — they may adjust your approach or consider an alternative.
Can I switch from Mounjaro 2.5 to Wegovy if I have side effects?
Potentially, yes — your prescriber can switch you to Wegovy if Mounjaro side effects are problematic. However, switching requires medical guidance and shouldn't be done without professional advice. It's worth discussing with your prescriber whether waiting 2–4 weeks for adjustment is reasonable, or whether switching is more appropriate for you.
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