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 Jab vs Wegovy: How They Compare
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections available in the UK, but they work differently and have different dosing schedules. This guide compares the two medications side-by-side so you can understand the key differences. Both are licensed for weight management in UK adults with a BMI of 30 or above, or 27 or above with weight-related health conditions. Individual suitability is assessed by your prescriber during a clinical assessment.
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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 Work Differently
Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways in the body. This dual mechanism may help regulate appetite, reduce food cravings, and support feeling fuller after smaller meals. Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates a single pathway. Both slow gastric emptying and support weight management alongside a reduced-calorie diet and increased physical activity.
- Mounjaro (tirzepatide): Dual GIP + GLP-1 mechanism; maximum dose 15 mg once weekly
- Wegovy (semaglutide): GLP-1 only; maximum dose 2.4 mg once weekly
- Clinical trial results: Mounjaro showed average weight loss of approximately 15–21% body weight; Wegovy showed approximately clinically studied weight loss
- Dosing schedule: Both are once-weekly injections, but escalation timelines differ
Try our free GLP-1 weight loss calculator above to estimate potential results at different doses for either medication.
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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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Doses, Access, and How to Choose
Both medications require a UK prescription and clinical assessment — they are not available without prescriber approval. Mounjaro starts at 2.5 mg and escalates to a maximum of 15 mg. Wegovy starts at 0.25 mg and escalates to 2.4 mg. Both are available through NHS specialist weight management services (with regional variation and potential waiting lists) or through private online prescribing with a UK-licensed prescriber. Your GP or prescriber will assess which medication is more suitable based on your individual health, medical history, and response to treatment. Individual results vary significantly, and both work best alongside sustainable lifestyle changes.
Use our free BMI calculator above to check your personal BMI and understand whether you may be eligible for prescription weight loss treatment. Track your progress with our progress tracker once you begin treatment.
- 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 better than Wegovy for weight loss?
Both are effective, but they work differently. Mounjaro's dual GIP+GLP-1 mechanism showed slightly higher average weight loss in clinical trials (15–21% vs 14.9% for Wegovy). However, individual results vary considerably, and suitability depends on your medical history, other medications, and prescriber assessment. There is no universally "better" option — only what works best for each person.
What's the difference between Mounjaro and Wegovy injections?
Mounjaro (tirzepatide) activates two hormone pathways (GIP and GLP-1), while Wegovy (semaglutide) activates one (GLP-1 only). Mounjaro starts at 2.5 mg and goes up to 15 mg weekly. Wegovy starts at 0.25 mg and reaches 2.4 mg weekly. Both are once-weekly injections, but Mounjaro may offer a dual mechanism advantage for some patients.
Can I switch from Wegovy to Mounjaro?
Yes, you can switch between medications, but this must be managed by your prescriber. There is no official conversion chart — approximate equivalence exists but any switch requires medical guidance. Your prescriber will assess whether switching is appropriate based on your response to your current treatment and individual health factors.
Which weight loss injection is available on the NHS in the UK?
Both Mounjaro and Wegovy are available through NHS specialist weight management services (tier 3/4), though availability and waiting times vary significantly by region and integrated care board. Private access is also available through GPhC-registered online pharmacies following a clinical assessment with a UK-licensed prescriber.
What are the main side effects of Mounjaro and Wegovy?
Both commonly cause nausea, diarrhoea, vomiting, and stomach pain — especially during dose escalation. These often improve over time. Serious side effects are rare but include severe stomach pain (possible pancreatitis) and gallbladder problems. Always report persistent or concerning symptoms to your prescriber immediately.
Ready to take control of your weight loss journey?
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