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
Mounjaro Didn't Work For Me — Is Wegovy a Better Option?
If Mounjaro hasn't delivered the results you hoped for, you're not alone — individual responses to weight loss medications vary significantly. Some people respond better to Wegovy (semaglutide), a different GLP-1 injection, than to Mounjaro's dual GIP and GLP-1 mechanism. This guide explains the key differences between the two prescription medications, the clinical trial evidence behind each, and how your prescriber decides which option might suit you better. Both are licensed for weight management in the UK and require a clinical assessment before prescribing.
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. This means it activates two hormone pathways simultaneously — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism may explain why some people lose more weight on Mounjaro in clinical trials, but it doesn't work equally well for everyone.
Wegovy (semaglutide) is a GLP-1-only receptor agonist — it activates just the GLP-1 pathway. Despite the single mechanism, clinical trial data (STEP-1) showed an average weight loss of approximately 14.9% body weight, which is comparable to Mounjaro for many people.
- SURMOUNT trial: Mounjaro at 15 mg showed ~20.9% average weight loss over 72 weeks
- STEP-1 trial: Wegovy at 2.4 mg showed ~14.9% average weight loss over 68 weeks
- Results vary widely between individuals — some respond better to one mechanism than the other
Your prescriber may suggest switching if the first medication hasn't produced meaningful results after adequate dose titration and time on treatment.
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.
Why Your Individual Response Matters
Weight loss medications work differently depending on your individual genetics, gut hormone sensitivity, and how your body processes each drug. Not everyone's system responds equally to the dual GIP+GLP-1 mechanism of Mounjaro — some people find the GLP-1-only approach of Wegovy more effective, with fewer side effects or better appetite suppression.
Before switching medications, ensure you've given Mounjaro adequate time — at least 4 weeks at your target maintenance dose (typically 10–15 mg weekly). Your prescriber will also check that gastrointestinal side effects have settled and that you're following a reduced-calorie diet and physical activity plan alongside the injection. If after this period you've had no significant weight loss (typically expecting at least a few kilograms over 3–4 months), a switch to Wegovy or dose adjustment may be considered. Use our free GLP-1 weight loss calculator to understand expected outcomes at different doses.
- 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 Wegovy better than Mounjaro if Mounjaro didn't work?
Not necessarily "better" — just different. Wegovy uses a single GLP-1 mechanism versus Mounjaro's dual GIP+GLP-1 approach. Some people respond better to one than the other due to individual differences in hormone sensitivity and metabolism. Your prescriber will assess which suits you based on your response and side effect profile.
How long should I stay on Mounjaro before switching to Wegovy?
Most prescribers recommend at least 12–16 weeks at your target maintenance dose (usually 10–15 mg weekly) before considering a switch. This allows time for your body to adjust and for weight loss to develop. If you've experienced significant side effects preventing dose escalation, a switch may be considered sooner — discuss with your prescriber.
Can I switch directly from Mounjaro to Wegovy?
Yes, switching is possible, but must be managed by your prescriber. There's no standard conversion chart between Mounjaro and Wegovy doses, so your prescriber may restart you on a low Wegovy dose (0.25 mg) and escalate gradually, even though you've previously titrated higher on Mounjaro. The timing of the switch depends on your last Mounjaro injection.
What if Wegovy doesn't work either?
If neither GLP-1 medication produces meaningful results after adequate trial, your prescriber may explore other options including dose optimisation, lifestyle programme intensification, or referral to specialist tier 3 weight management services on the NHS. Some people benefit from combination approaches or bariatric surgery assessment for severe obesity.
Will my GP prescribe Wegovy if Mounjaro didn't work?
Standard NHS GPs typically do not prescribe weight loss medications — these are initiated through tier 3 specialist weight management services following referral. If you're accessing Mounjaro or Wegovy privately through a registered online prescriber, they can assess suitability for switching based on your previous response and medical history.
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.