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 Second Dose vs Wegovy: What's the Difference?
When considering GLP-1 weight loss medications in the UK, many people compare Mounjaro and Wegovy to understand which might be suitable. Both are prescription-only injections, but they work differently and follow different dosing schedules. Your second dose represents a key escalation point where you'll see how your body responds to the medication. Understanding the differences can help you have a more informed conversation with your GP or prescriber about which option may be right for your circumstances.
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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 Second Doses Compare: Clinical Trial Data
Mounjaro and Wegovy use different mechanisms of action, which affects their dosing schedules and potential outcomes. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways. Your second dose would be 5 mg after four weeks on the 2.5 mg starting dose. In the SURMOUNT-1 clinical trial, participants on the 5 mg dose showed an average weight loss of approximately 15% body weight at 72 weeks, alongside reduced-calorie diet and increased physical activity.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — activating a single hormone pathway. Your second dose escalates to 0.5 mg after four weeks on 0.25 mg. The STEP-1 clinical trial showed average weight loss of approximately 14.9% body weight at 68 weeks on the maximum 2.4 mg dose. The second dose in the Wegovy titration schedule is still a relatively low dose; several more escalations follow.
Individual results vary significantly, and these are clinical trial averages achieved alongside lifestyle changes. Your own prescriber can discuss which mechanism and dosing approach may suit your health profile.
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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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Which Medication Might Be Suitable for You?
Neither Mounjaro nor Wegovy is inherently "better" — suitability depends on your individual health, medical history, and response preferences. Factors your prescriber will consider include: your BMI and any weight-related health conditions (type 2 diabetes, prediabetes, high blood pressure, sleep apnoea, or cardiovascular disease), any other medications you take (especially diabetes or contraceptive medications), allergies, and whether you prefer a once-weekly injection with a shorter titration schedule (Mounjaro reaches higher doses more quickly) or a longer, more gradual escalation (Wegovy).
Both medications require a clinical assessment before prescription. You can access them through NHS specialist weight management services (tier 3) or private online prescribing with a UK-licensed prescriber. Your GP or prescriber will help determine which is appropriate for you based on clinical evidence and your personal circumstances. Use our free GLP-1 weight loss calculator to explore potential outcomes, and check your BMI using our free 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.
Is Mounjaro's second dose (5 mg) the same as Wegovy's second dose (0.5 mg)?
No. These are not equivalent doses despite both being "second doses." Mounjaro's 5 mg is roughly equivalent to Wegovy's 1.7 mg based on clinical practice. The medications have different potencies. Mounjaro reaches higher doses more quickly, while Wegovy escalates more gradually over many weeks.
Will I lose more weight on Mounjaro than Wegovy?
Clinical trials showed similar average weight loss (~15% body weight) with both medications when used alongside diet and exercise. Individual results vary considerably. Some people respond better to one mechanism than the other. Your prescriber cannot predict your personal outcome in advance.
Can I switch from Mounjaro to Wegovy after the second dose?
Switching between medications is possible but requires prescriber guidance. It's not as simple as substituting one for the other because doses are not directly equivalent. Any switch must be managed carefully by your prescriber to avoid gaps in treatment or dosing errors.
Do Mounjaro and Wegovy have different side effects?
Both share common side effects: nausea, diarrhoea, vomiting, and constipation — especially in the first weeks. Individual tolerability varies. Some people tolerate one medication better than the other. Your prescriber will monitor your response and can adjust your dose or suggest management strategies.
How do I know which medication to request from my GP?
Your GP or prescriber will recommend based on your clinical assessment, not personal preference alone. Both are NHS-approved for eligible adults. Mention if you have concerns about side effects, injection frequency, or dosing escalation speed, but the clinical decision rests with your prescriber following NICE guidance.
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