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 It Take To Lose Weight With Mounjaro and Wegovy?
Weight loss with Mounjaro and Wegovy doesn't happen overnight — both are prescription medications that work gradually as your body adjusts and doses increase. Most people see measurable weight loss within 4–8 weeks, with more significant results becoming visible after 12–16 weeks of treatment. The timeline depends on your starting dose, how quickly doses are increased, your individual metabolism, and how consistently you combine the medication with a reduced-calorie diet and increased physical activity. Understanding what to expect helps set realistic goals and maintain motivation throughout your treatment journey.
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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.
Clinical Trial Data: SURMOUNT vs STEP Trial Results
Mounjaro (tirzepatide) and Wegovy (semaglutide) were tested in separate clinical trials with different timelines and dose escalation schedules:
- Mounjaro (SURMOUNT-1 trial): Participants on the maximum dose (15 mg) lost an average of 20.9% of body weight over 72 weeks (approximately 17 months). At the 12-week mark, average loss was around 6–8% of starting weight.
- Wegovy (STEP-1 trial): Participants on the maximum dose (2.4 mg) lost an average of 14.9% of body weight over 68 weeks (approximately 16 months). At the 12-week mark, average loss was around 4–6% of starting weight.
Important context: These are average results from clinical trials — individual responses vary significantly. Both trials involved participants following a reduced-calorie diet and increasing physical activity. Results achieved faster initially, then plateau somewhat as treatment continues.
Clinically BackedWeight Loss
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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.
Timeline, Dose Titration, and Individual Factors
Both medications use a gradual dose escalation schedule lasting 16–20 weeks before reaching maximum dose. Mounjaro starts at 2.5 mg weekly and increases every 4 weeks; Wegovy starts at 0.25 mg and increases every 2–4 weeks. Weight loss typically begins within the first 2–4 weeks but accelerates as doses increase. Your prescriber may pause dose increases if side effects occur, extending the timeline. Individual results depend on your starting weight, how well you tolerate each dose, your dietary adherence, exercise level, age, and metabolism. Use our free GLP-1 weight loss calculator to estimate potential outcomes at different doses, and track progress with our free progress tracker throughout your treatment journey.
- 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.
When should I see weight loss results with Mounjaro or Wegovy?
Most people notice the first changes within 2–4 weeks, though early changes may be modest (1–2 kg). More visible results typically appear by weeks 8–12. Maximum results are usually reached after 12–16 weeks of treatment at or near your target dose. Clinical trials measured results over 16–17 months, so patience is important.
Will I lose weight at the same rate as clinical trial participants?
No — clinical trial results are averages from large groups. Your individual weight loss depends on your starting weight, how well you stick to a reduced-calorie diet, exercise consistency, metabolism, and how your body responds to the medication. Some people lose faster, others slower. Individual results always vary.
Does weight loss continue throughout treatment or does it plateau?
Weight loss is typically fastest during the first 8–16 weeks, then the rate of loss often slows as you approach your personal weight loss plateau. This is normal. Continuing the medication usually maintains your weight loss, preventing regain — though stopping medication may result in gradual weight regain over time.
Which works faster — Mounjaro or Wegovy?
Mounjaro showed slightly greater average weight loss in clinical trials (20.9% vs 14.9%), but the difference is modest and individual variation is large. Some people respond better to one than the other. Your prescriber will assess which is more suitable for your health, not which is "faster." Both require a prescription and clinical assessment.
What if I'm not seeing weight loss after 8 weeks?
Discuss this with your prescriber — you may still be on a low dose, may benefit from dose acceleration, or the medication may not be optimal for you. Some people respond better to one medication over another. Your prescriber can adjust your treatment plan based on your individual progress and tolerance.
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.