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 2nd Dose vs Wegovy — How They Compare
When considering Mounjaro or Wegovy for weight management in the UK, many people wonder how the two injections compare—especially at the 2nd dose stage. Both are prescription-only GLP-1 medications available through NHS specialist services or private prescribing, but they work slightly differently and show different results in clinical trials. Understanding these differences helps you have an informed conversation with your GP or prescriber about which might be more suitable for your individual health needs.
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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.
Clinical Trial Data and Mechanism Differences
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways rather than one. At the 2nd dose (5 mg weekly), the SURMOUNT-1 trial showed average weight loss of around 15% of body weight over 72 weeks alongside a reduced-calorie diet. The dual mechanism may help regulate appetite and fat utilisation more comprehensively than single-pathway medications.
Wegovy (semaglutide) is a GLP-1-only receptor agonist. At the 2nd dose (0.5 mg weekly), the STEP-1 trial showed average weight loss of approximately 6–7% of body weight at that dose level, with the maximum 2.4 mg dose reaching around 15% average loss. Wegovy has a longer clinical history in weight management—trials have been running for several years with extensive safety data.
- Both require a reduced-calorie diet and physical activity to achieve results
- Both are once-weekly injections
- Individual results vary significantly based on adherence, diet, and metabolism
- Neither is a substitute for lifestyle changes
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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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Choosing Between Them — Prescriber Assessment
Your GP or licensed prescriber will assess which medication is more appropriate based on your medical history, current health conditions, and personal circumstances. Both require a clinical assessment before prescription. Some prescribers may choose Mounjaro's dual mechanism if you have specific metabolic concerns, while others may recommend Wegovy due to its extensive trial data or lower starting dose. A UK prescriber cannot predict your individual response—what matters is starting safely, monitoring your progress, and adjusting doses gradually. Use our free BMI calculator to establish your baseline, and our GLP-1 weight loss calculator can help you understand potential dose pathways. Always discuss side effect tolerability and any existing medications with your prescriber, as both can interact with oral contraceptives and some diabetes medications.
- 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 is the 2nd dose of Mounjaro?
Mounjaro's 2nd dose is 5 mg once weekly, usually taken in weeks 5–8 of treatment. At this dose, average weight loss in clinical trials was around 15% of body weight over 72 weeks, though individual results vary. Your prescriber will assess whether to continue, maintain, or escalate based on your response and side effects.
How does Mounjaro's 2nd dose compare to Wegovy's 2nd dose?
Mounjaro 5 mg (2nd dose) is a dual GIP+GLP-1 agonist showing ~15% average weight loss. Wegovy's 2nd dose is 0.5 mg (GLP-1 only) showing ~6–7% average loss at that stage. Wegovy reaches ~15% at its maximum 2.4 mg dose. Both require lifestyle changes and produce individual results.
Is one safer than the other at the 2nd dose?
Both have robust safety data from clinical trials in UK-licensed studies. Common side effects are similar (nausea, diarrhoea, constipation). Serious side effects are rare. Safety depends on individual health factors—your prescriber will assess contraindications. Always report side effects to your healthcare provider.
Can I switch from Mounjaro 2nd dose to Wegovy or vice versa?
Switching between medications must be managed by your prescriber. There is no official conversion chart, though approximate equivalences exist (Mounjaro 5 mg ≈ Wegovy 0.5 mg in terms of mechanism, but doses are different). Your prescriber will advise timing and how to transition safely.
Where can I get Mounjaro or Wegovy in the UK?
Both are available through NHS tier 3 specialist weight management services (via GP referral) and through private online prescribing following a clinical assessment with a UK-licensed prescriber. Availability on the NHS varies by region. Private access is faster but involves costs; NHS access is free but may involve waiting lists.
Ready to take control of your weight loss journey?
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