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 2.5mg Side Effects vs Wegovy: What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections available in the UK, but they work differently and may produce different side effect profiles. Mounjaro 2.5mg is the starting dose and uses a dual GIP and GLP-1 mechanism, whilst Wegovy uses GLP-1 only. Understanding the side effect differences can help you and your prescriber decide which medication may be more suitable for you. Both require a clinical assessment before prescription, and individual experiences vary significantly.
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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 Differ in Mechanism and Side Effects
Mounjaro (tirzepatide) activates two hormone pathways simultaneously—GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action is the key difference and may explain why some people experience different side effects. Mounjaro 2.5mg is the starter dose used during the first four weeks of treatment.
Wegovy (semaglutide) activates GLP-1 only, a single hormone pathway. Wegovy starts at 0.25mg and escalates more gradually than Mounjaro.
Both cause similar common side effects—nausea, diarrhoea, constipation, and stomach pain—but the dual mechanism of Mounjaro may intensify gastrointestinal effects in some people, particularly during the initial dose titration phase. Clinical trial data from SURMOUNT (Mounjaro) and STEP (Wegovy) showed both caused nausea in over 40% of users, but the timing and severity varied by individual.
Start with our free GLP-1 weight loss calculator to explore how both medications compare in projected outcomes at different doses, which may help inform your conversation with your prescriber.
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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 at Starting Dose and Individual Suitability
At the 2.5mg starter dose, Mounjaro users most commonly report nausea, diarrhoea, and reduced appetite—effects that typically improve as the body adjusts over the first few weeks. Wegovy's lower 0.25mg start may cause fewer initial side effects, but this advantage reduces as the dose increases toward 2.4mg.
Which medication produces fewer side effects depends entirely on your individual biology. Some people tolerate the dual mechanism of Mounjaro better; others find Wegovy's single-pathway approach suits them. Your prescriber will assess your medical history, current medications, and health conditions to recommend the most suitable option. Both require a UK prescription following clinical assessment—neither can be prescribed based on personal preference alone. If side effects emerge with one medication, your prescriber may suggest switching to the other, which requires a managed transition period.
- 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.
Does Mounjaro 2.5mg cause more side effects than Wegovy 0.25mg?
Mounjaro's dual GIP+GLP-1 mechanism may intensify gastrointestinal side effects in some people compared to Wegovy's single GLP-1 pathway. However, individual responses vary considerably. Clinical trials showed similar nausea rates (40%+) for both. Your prescriber will assess which suits you best based on your health profile.
How long do Mounjaro 2.5mg side effects last?
Most side effects—nausea, diarrhoea, stomach discomfort—typically improve within 2–4 weeks as your body adapts. By week 4, many people transition to the next dose (5mg) as gastrointestinal symptoms settle. If side effects persist, contact your prescriber.
Can I switch from Mounjaro 2.5 to Wegovy if I experience side effects?
Yes, but any switch between medications must be managed by your prescriber. A transition period may be needed rather than stopping one and starting the other immediately. Your prescriber will advise on the safest approach based on your situation.
Is the dual GIP+GLP-1 mechanism of Mounjaro safer than Wegovy's GLP-1 only?
Neither is inherently "safer"—both are licensed for weight management in the UK and have similar safety profiles. The dual mechanism is different, not better or worse. Suitability depends on individual factors your prescriber will assess during your clinical evaluation.
Should I use the BMI calculator to help decide between Mounjaro and Wegovy?
Yes, calculating your BMI using our free BMI calculator can help establish your eligibility for either medication. Both require a BMI of 30+ (or 27+ with weight-related health conditions). This starting point will help your prescriber assess which option aligns with your health goals.
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.