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 Pen vs Wegovy in the UK
If you're considering prescription weight loss treatment in the UK, you may have heard about both Mounjaro and Wegovy. Both are once-weekly injectable medications licensed for weight management in UK adults, but they work differently and have different dose ranges. This guide compares the two side-by-side so you understand the key differences. Remember that suitability depends on your individual health, and only a prescriber can assess which — if either — is right for you following a clinical assessment.
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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 and Wegovy Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. This means it activates two separate hormone pathways in the body: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism may help reduce appetite more effectively than a single pathway approach. In the SURMOUNT-1 clinical trial, participants on Mounjaro 15 mg lost an average of approximately 20.9% of their body weight over 72 weeks.
Wegovy (semaglutide) is a GLP-1-only receptor agonist. It activates the GLP-1 pathway alone. In the STEP-1 clinical trial, participants on Wegovy 2.4 mg lost an average of approximately 14.9% of their body weight over 68 weeks. Both injections slow how food leaves your stomach and help you feel fuller sooner, but they use different biological mechanisms.
- Mounjaro: dual mechanism (GIP + GLP-1)
- Wegovy: single mechanism (GLP-1 only)
- Both: once-weekly injections, prescription-only, require clinical assessment
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Doses, Side Effects, and NHS Access in the UK
Mounjaro starts at 2.5 mg and increases to a maximum of 15 mg weekly. Wegovy starts at 0.25 mg and increases to a maximum of 2.4 mg weekly. Both require a minimum of 4 weeks at each dose before titrating up. Side effects for both are similar and common: nausea, diarrhoea, vomiting, and constipation. In the UK, both medications are available through NHS specialist weight management services (though availability varies by region and waiting lists can be long) or through private online prescribing following a clinical assessment. Your GP or prescriber will assess eligibility based on your BMI and any weight-related health conditions. Use our free GLP-1 weight loss calculator to explore potential outcomes, and calculate your personal BMI using our BMI calculator to understand your starting point.
- 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.
Which is more effective — Mounjaro or Wegovy?
Clinical trial averages suggest Mounjaro may produce greater average weight loss, but individual results vary significantly. SURMOUNT-1 showed Mounjaro 15 mg averaged ~21% loss; STEP-1 showed Wegovy 2.4 mg averaged ~15% loss. However, effectiveness depends on your personal response, lifestyle changes, and adherence. Only a prescriber can advise on suitability for you.
What's the difference between Mounjaro KwikPen and Wegovy pen design?
Both are pre-filled pens for once-weekly injection, but Mounjaro uses the KwikPen device while Wegovy uses an auto-injection pen. Both are straightforward to use. Storage and handling differ slightly — ask your prescriber or pharmacist for specific guidance on your medication.
Can I switch from Wegovy to Mounjaro or vice versa in the UK?
Switching between medications must be managed by your prescriber. There is no official conversion chart — approximate equivalences exist, but any switch requires medical supervision and assessment. Never stop one medication and start another without guidance.
Are Mounjaro and Wegovy available on the NHS in the UK?
Both are available through NHS specialist tier 3 weight management services, but access varies significantly by region. NICE has approved both for NHS use. Private access is available through UK-licensed prescribers via online consultation. Waiting lists for NHS services can be several months to years in some areas.
What side effects should I expect with either medication?
Both commonly cause nausea, diarrhoea, vomiting, and constipation — especially when starting or increasing dose. Most side effects decrease over time. Serious side effects are rare but include severe abdominal pain (possible pancreatitis) and allergic reactions. Always report any concerning symptoms to your prescriber immediately.
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