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
How Long Does Munjaro Take To Work vs Wegovy
Munjaro (tirzepatide) typically begins to show weight loss effects within 2–4 weeks, though individual timelines vary. Unlike Wegovy (semaglutide), which is a GLP-1 receptor agonist only, Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it works through two separate hormone pathways. Clinical trial data shows meaningful weight loss becomes noticeable between weeks 4–8 for most people. However, the full effects develop over several months as your dose increases. This guide compares how these weekly weight loss injections work and when you might expect changes.
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.
Timeline Differences Between Mounjaro and Wegovy
Mounjaro (tirzepatide) follows a dose escalation schedule over 16+ weeks. You start at 2.5 mg for 4 weeks, then increase every 4 weeks (5 mg, 7.5 mg, 10 mg, 12.5 mg, 15 mg maximum). Weight loss typically becomes noticeable within the first 4 weeks, with more significant results emerging by weeks 8–12. SURMOUNT clinical trial data showed average weight loss of 15% body weight at 5 mg, increasing to 20.9% at the maximum 15 mg dose over 72 weeks.
Wegovy (semaglutide) also uses a dose escalation (0.25 mg to 2.4 mg over 16+ weeks) and follows a similar timeline. STEP-1 trial data showed approximately 14.9% average body weight loss on the maximum 2.4 mg dose at 68 weeks. Both medications require consistent weekly injections; missing doses or extending the interval beyond 4 days may affect results. Use our GLP-1 weight loss calculator above to estimate potential outcomes at different doses.
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.
Mechanism Differences and What They Mean for Onset
The key difference is how each medication works. Mounjaro activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 receptors, creating a dual signalling pathway that may enhance appetite suppression and metabolic effects. Wegovy activates GLP-1 receptors only. This dual mechanism in Mounjaro may contribute to its slightly higher average weight loss in trials, though individual response varies significantly. Some people respond faster to one medication than the other—there is no way to predict personal response in advance. Your prescriber will assess which is more suitable during your clinical assessment, considering your health profile, medical history, and previous weight loss medication experience. Both require a UK prescription and are available through NHS specialist services or private prescribing following assessment.
- 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.
How quickly do you lose weight on Mounjaro?
Weight loss typically becomes noticeable within 2–4 weeks, with more significant changes by weeks 8–12. Clinical trials showed average loss of 15–20.9% body weight depending on dose, achieved over 72 weeks alongside reduced-calorie diet and increased activity. Individual results vary considerably—some people lose faster, others more gradually.
Does Mounjaro work faster than Wegovy?
Clinical trials suggest Mounjaro may produce slightly higher average weight loss (20.9% vs 14.9%), but both follow similar timelines. The difference lies in mechanism: Mounjaro is dual GIP+GLP-1, Wegovy is GLP-1 only. Onset timing is comparable; individual response varies more than the medication choice.
What week does Mounjaro start working?
Most people notice appetite reduction and early weight changes by weeks 2–4, though this varies. Meaningful measurable weight loss typically emerges by weeks 4–8. Full effects develop progressively as your dose increases over 16+ weeks. Track progress using our free progress tracker.
Is Mounjaro or Wegovy better for fast weight loss?
Neither assures rapid loss, and safety requires steady weight loss of 0.5–1 kg per week. Mounjaro showed slightly higher average weight loss in trials, but prescriber assessment determines suitability for you personally. Both are prescription-only and require clinical evaluation to determine eligibility.
Can I switch from Wegovy to Mounjaro if it's not working fast enough?
Any medication switch must be managed by your prescriber—never stop or change without clinical guidance. Your GP or prescriber can discuss whether switching is appropriate based on your progress, side effects, and health profile. Approximate dose equivalence exists, but individual assessment is essential.
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.