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
Free tracker included with every download this week
Will Wegovy Work If Mounjaro Didn't?
If Mounjaro hasn't delivered the weight loss results you hoped for, switching to Wegovy may be an option — but there's no assures. Wegovy (semaglutide) works differently to Mounjaro (tirzepatide) as a single GLP-1 receptor agonist rather than a dual GIP and GLP-1 mechanism. Some people respond better to one medication than the other. However, individual responses vary significantly, and suitability depends on your personal health circumstances, NHS tier 3 eligibility, and clinical assessment. This guide explains the NHS context, how the medications differ, and what switching between them involves in UK practice.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
How Mounjaro and Wegovy Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two separate hormone pathways to regulate appetite, slow gastric emptying, and support satiety. Clinical trials showed average weight loss of approximately 15–21% of body weight at the maximum 15 mg dose.
Wegovy (semaglutide) is a single GLP-1 receptor agonist — it activates only the GLP-1 pathway. STEP trial data showed average weight loss of around 14.9% at the maximum 2.4 mg dose.
The key difference: Mounjaro's dual mechanism may provide stronger appetite suppression for some people, but others respond equally well or better to Wegovy's single GLP-1 action. If Mounjaro didn't work for you, it doesn't mean Wegovy won't — they have different modes of action, and individual biology varies. Your GLP-1 weight loss calculator can help estimate potential outcomes at different dose levels for comparison.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
NHS Access, Eligibility, and the Switching Process
In the UK, both Mounjaro and Wegovy are available through NHS tier 3 specialist weight management services following NICE guidance approval. If you've tried Mounjaro through the NHS without adequate response, your specialist team may discuss switching to Wegovy as an alternative. This requires a new clinical assessment — your GP or specialist will review whether Wegovy is appropriate for your circumstances, any contraindications, and whether the change is likely to be more effective.
Private access offers a faster route: both medications are available through GPhC-registered online prescribing services following an online clinical consultation. If you wish to switch from private Mounjaro to private Wegovy, you can request this through your prescriber without NHS referral. Always inform your prescriber about your previous Mounjaro use and why you're considering a change.
- 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.
Why might Wegovy work if Mounjaro didn't?
Individual biology varies significantly. Mounjaro's dual GIP and GLP-1 mechanism works better for some people, whilst others respond more effectively to Wegovy's single GLP-1 pathway. Switching medications can sometimes unlock better results — but there's no assures. Your specialist will assess whether Wegovy is worth trying based on your individual response to Mounjaro.
How do you switch between Mounjaro and Wegovy on the NHS?
Contact your tier 3 weight management service and discuss your response to Mounjaro with your specialist. If switching is appropriate, they'll arrange a new clinical assessment. You'll stop Mounjaro and begin Wegovy at the starting dose (0.25 mg), with escalation over weeks. The process requires specialist oversight and prescription.
Can you switch between Mounjaro and Wegovy privately?
Yes. If you're using private prescribing, you can request a switch to your prescriber following an online consultation. They'll assess your suitability for Wegovy, review your Mounjaro experience, and issue a Wegovy prescription if appropriate. This typically happens faster than NHS switching but costs apply.
What's the difference between Mounjaro's tirzepatide and Wegovy's semaglutide?
Tirzepatide (Mounjaro) activates both GIP and GLP-1 receptors — a dual mechanism. Semaglutide (Wegovy) activates GLP-1 only. The dual action of tirzepatide may provide stronger appetite suppression for some people, but Wegovy's single mechanism works effectively for others. They're fundamentally different drugs with different dose ranges.
How long should you give Wegovy before deciding if it's working?
Weight loss with Wegovy typically becomes noticeable within 4–6 weeks, with more significant results visible by 12 weeks. However, you need at least 3–4 months of consistent use at therapeutic doses to fairly assess whether Wegovy is working for you. Your specialist will monitor your progress and decide whether to continue or explore other options.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.