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
Free tracker included with every download this week
Which Is Safer: Wegovy Or Mounjaro?
Both Wegovy and Mounjaro are prescription-only weight loss injections licensed and available in the UK, but they work differently and carry different safety profiles. There is no single "safer" option—safety depends on your individual health circumstances, medical history, and how your body responds. Your GP or prescriber will assess which is more suitable for you following a clinical evaluation. Both require prescription and ongoing medical monitoring.
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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 Wegovy And Mounjaro Differ In Mechanism And Clinical Evidence
Wegovy (semaglutide) is a GLP-1 only receptor agonist, based on clinical evidence from the STEP trials showing an average weight loss of approximately 15% body weight at the maximum 2.4 mg dose. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, with SURMOUNT trial data showing average weight loss of 15–21% depending on dose, suggesting potentially stronger effects through its dual mechanism.
- Wegovy uses a single hormone pathway (GLP-1) with a longer track record in UK clinical practice
- Mounjaro's dual mechanism may affect appetite and blood sugar differently, potentially offering greater weight loss but with a different side effect profile
- Both slow gastric emptying and reduce hunger signals, but through slightly different biological pathways
- Clinical trial populations were different, making direct comparisons complex—individual results vary significantly
Neither medication is objectively "safer" in absolute terms; safety depends on your personal health status, medication interactions, and individual tolerance to gastrointestinal effects.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Side Effects, Eligibility, And How Your Prescriber Decides
Both medications share common side effects including nausea, diarrhoea, constipation, and stomach pain—particularly during dose titration phases. Mounjaro's dual mechanism may cause slightly different patterns of gastrointestinal symptoms, though clinical data suggests similar tolerability overall. Eligibility criteria are identical: UK adults with a BMI of 30 or above, or BMI of 27–29.9 with weight-related health conditions including type 2 diabetes, hypertension, or sleep apnoea.
Your prescriber will assess your medical history, current medications, and personal preferences. If you've had an adverse reaction to one GLP-1 medication, this doesn't automatically exclude you from the other—your clinical team can guide this decision. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses for either medication, helping you understand what realistic results might look like based on clinical trial data.
Both Wegovy and Mounjaro require a prescription and clinical assessment—you cannot choose between them based on safety alone without prescriber guidance. The NHS KwikPen format for Mounjaro vs auto-pen for Wegovy reflects manufacturer design, not safety differences. Access in the UK is through NHS specialist weight management services (tier 3) or private online prescribers following clinical evaluation. Individual tolerability is often the deciding factor: some people manage Wegovy's GLP-1 mechanism better; others find Mounjaro's dual action suits them. Once your prescriber identifies which medication is appropriate based on your health profile, your personal response during the first few weeks of treatment often determines whether you continue or explore alternatives.
- 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?
Neither is inherently safer. Both are prescription medications with similar safety profiles. Safety depends on your individual health, medical history, and how your body responds. Your GP or prescriber will assess which suits you based on clinical evaluation, not general safety comparisons.
What are the main side effect differences between Wegovy and Mounjaro?
Both share common side effects: nausea, diarrhoea, constipation, and stomach pain. Mounjaro's dual GIP+GLP-1 mechanism may cause slightly different symptom patterns during dose escalation. Individual tolerance varies considerably—what causes problems for one person may not for another.
Which medication causes more nausea—Wegovy or Mounjaro?
Clinical trials show both cause nausea in a significant proportion of users, especially during initial dose titration. Individual experiences vary. Nausea typically reduces over time as your body adapts. Your prescriber can advise management strategies or consider switching if nausea is severe.
Can I switch from Wegovy to Mounjaro if I'm not tolerating Wegovy?
Yes, switches are possible and managed by your prescriber. There is no official conversion chart, but approximate equivalences exist (e.g. 2.4 mg Wegovy ≈ 10–15 mg Mounjaro). Any switch requires clinical assessment and should be planned with your healthcare team.
Which weight loss injection is available on the NHS in the UK?
Both Mounjaro and Wegovy are available through NHS specialist tier 3 weight management services, though regional availability varies significantly. Private access to both is available through GPhC-registered online prescribers following clinical assessment. NHS waiting lists can be lengthy in many areas.
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.