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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Wegovy vs Mounjaro: Side Effects Compared
Both Wegovy and Mounjaro are prescription-only weekly weight loss injections used by UK adults, but they work differently and may cause different side effect profiles. Wegovy is a GLP-1 receptor agonist, whilst Mounjaro is a dual GIP and GLP-1 agonist — this means Mounjaro activates two hormone pathways instead of one. Understanding how side effects compare can help you discuss which medication might suit you best when speaking with your GP or a licensed prescriber following a clinical assessment.
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Calculate Your BMI Instantly
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.
Side Effects: Wegovy vs Mounjaro — Clinical Comparison
Common side effects are remarkably similar between both medications. Nausea, diarrhoea, constipation, vomiting, and stomach pain are reported frequently with both Wegovy and Mounjaro — these occur because both slow how quickly food leaves your stomach. The main difference is mechanism: Wegovy acts on GLP-1 pathways alone, whilst Mounjaro's dual GIP and GLP-1 action may theoretically cause different tolerability profiles, though clinical trial data shows comparable side effect rates.
Very common side effects (affecting more than 1 in 10 people on either medication) include nausea and gastrointestinal symptoms, particularly during dose escalation. These typically improve over time as your body adjusts. Injection site reactions — redness, bruising, or mild pain — are also common with both weekly weight loss injections. Serious side effects such as pancreatitis (severe stomach pain) or gallbladder problems are rare but require immediate medical attention with either medication.
- Nausea and vomiting — common, usually improves after 4–8 weeks
- Diarrhoea or constipation — depends on individual response
- Stomach pain or indigestion — very common during initial weeks
- Injection site reactions — minor and typically short-lived
- Hair loss — reported with both, temporary in most cases
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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.
Which Medication Is Right for You? NHS and Prescriber Guidance
Neither Wegovy nor Mounjaro is automatically "better" — suitability depends entirely on your individual health profile, medical history, and how your body responds. Your GP or licensed prescriber will assess which medication is appropriate following a clinical assessment. Both require a BMI of 30 or above, or 27–29.9 with weight-related health conditions. Use our free BMI calculator above to check where you stand, and our GLP-1 weight loss calculator to estimate potential outcomes at different doses with either medication.
If you experience significant side effects on one medication, your prescriber may consider switching to the other — though this is a clinical decision made case-by-case. Both are prescription-only and require ongoing monitoring. Individual results vary considerably; some people tolerate one better than the other. Your GP will discuss which aligns best with your medical history, other medications you take, and your weight loss goals.
- 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's the main difference between Wegovy and Mounjaro side effects?
Both cause similar common side effects (nausea, diarrhoea, stomach pain) because they both slow gastric emptying. The key difference is mechanism: Wegovy targets GLP-1 only, whilst Mounjaro activates both GIP and GLP-1 pathways. Clinical trials show comparable side effect rates, but individual responses vary. Your prescriber will assess which suits you best.
Are Wegovy side effects worse than Mounjaro?
No — clinical trial data (STEP for Wegovy, SURMOUNT for Mounjaro) show comparable side effect profiles. Both commonly cause nausea and gastrointestinal symptoms during dose escalation, which typically improve within weeks. Neither has significantly worse side effects than the other; tolerance depends on individual factors. Your prescriber will discuss this during assessment.
How long do Wegovy or Mounjaro side effects last?
Most gastrointestinal side effects improve within 4–8 weeks as your body adjusts to the medication. Nausea during dose increases usually settles within days to weeks. Serious side effects like pancreatitis are rare but require immediate medical attention. If side effects persist or worsen, contact your GP or prescriber immediately.
Can I switch from Wegovy to Mounjaro if side effects are bad?
Possibly — but this is a clinical decision made by your prescriber. Switching medications is not automatic; your GP will discuss whether another GLP-1 or GIP/GLP-1 medication might suit you better. Some people tolerate one better than the other. Always contact your prescriber before stopping or switching any prescription medication.
Which weekly weight loss injection is prescribed more in the NHS?
Both Wegovy (semaglutide) and Mounjaro (tirzepatide) are licensed in the UK for weight management, but availability varies by region and NHS tier 3 services. Private prescribing offers access to both without NHS referral waiting lists. Your prescriber will offer whichever is clinically appropriate and available. Check NHS NICE guidance for current regional prescribing recommendations.
Ready to take control of your weight loss journey?
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