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 Mounjaro Take to Lose Weight?
Weight loss with Mounjaro typically begins within the first 2–4 weeks as your body adjusts to the medication, though most significant results appear between weeks 8 and 12. Mounjaro and Wegovy are both once-weekly prescription weight loss injections licensed in the UK, but they work slightly differently and show different timelines in clinical trials. Understanding what to expect and how long results take can help you decide which option might suit your health goals. Use our free GLP-1 weight loss calculator to see potential outcomes at different doses, and talk to your GP or prescriber about which medication aligns with your situation.
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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.
Mounjaro vs Wegovy: Weight Loss Timeline Comparison
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly weight loss injections licensed in the UK, but clinical trial data shows different timelines:
- Mounjaro (SURMOUNT-1 trial): Average weight loss of 15% body weight at 72 weeks on the 5mg dose, reaching 20.9% at the maximum 15mg dose. Meaningful loss typically visible by week 12.
- Wegovy (STEP-1 trial): Average weight loss of 14.9% body weight at 68 weeks on 2.4mg (maximum dose). Results similarly emerge between weeks 8–16.
The key difference is mechanism: Mounjaro is a dual GIP + GLP-1 agonist, activating two hormone pathways simultaneously. Wegovy is a GLP-1 agonist only, activating one pathway. This dual action in Mounjaro may explain slightly faster initial appetite suppression for some people, though individual responses vary considerably. Both require a Mounjaro KwikPen (or Wegovy pen) used weekly, with doses escalating over 16–20 weeks to reach therapeutic levels.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Individual Timeline and Suitability Factors
How quickly you lose weight depends on many factors beyond the medication itself: starting weight, diet adherence, physical activity, metabolism, and individual biological response. Some people see results within 4 weeks; others take 12 weeks. Neither Mounjaro nor Wegovy assures specific outcomes — clinical trial averages mask wide individual variation.
Your GP or prescriber will assess whether you're eligible for a prescription weight loss injection based on your BMI, health conditions, and medical history. Both medications require clinical assessment before prescription. Track your progress using our free progress tracker to monitor changes beyond the scale, including energy levels and how your clothes fit, since weight loss is rarely linear.
- 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 does Mounjaro work compared to Wegovy?
Mounjaro may show appetite reduction within 1–2 weeks due to its dual GIP+GLP-1 mechanism, whereas Wegovy's single GLP-1 pathway often takes 2–4 weeks. Meaningful weight loss (1–2kg) typically appears by week 8–12 for both. Individual timelines vary significantly based on diet, exercise, starting weight, and personal metabolism.
Will I lose weight every week on Mounjaro?
No. Weight loss is rarely linear. You may lose weight steadily for several weeks, then plateau, then lose more. This is normal and expected. Plateaus typically last 2–4 weeks. Consistency with diet and activity is more important than weekly fluctuations. Use our free progress tracker to see patterns over months rather than weeks.
Is Mounjaro faster than diet and exercise alone?
Clinical trials show Mounjaro produces greater average weight loss than diet and exercise alone, but medication works best alongside both. The medication helps regulate appetite and fullness; lifestyle changes provide the calorie deficit needed for loss. Most people achieve best results combining all three approaches.
Can I switch from Wegovy to Mounjaro if results are too slow?
Yes, but only under prescriber guidance. A prescriber can switch you between medications if one isn't working well for you. There's no official conversion chart — your prescriber will assess your progress and decide the best approach based on your individual response and health.
How long should I take Mounjaro to see results?
Visible weight loss typically appears by week 8–12 if the medication is working well. Most people reach near-maximum results by 20–24 weeks (around 5–6 months of treatment). Weight loss continues slowly thereafter. If no meaningful loss occurs by week 16, discuss with your prescriber — you may need a dose adjustment or different medication.
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.