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 Quickly Will Mounjaro Work Compared to Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only GLP-1 weight management injections licensed in the UK, but they work at different speeds. Mounjaro typically shows measurable results within 4–8 weeks, while Wegovy often takes 8–12 weeks. However, individual timelines vary significantly based on dose, diet, activity level, and metabolism. Neither medication assures specific weight loss, and results depend entirely on clinical assessment and sustained lifestyle changes alongside medication use.
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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 Evidence: Speed of Weight Loss
Mounjaro (SURMOUNT trials): Participants began seeing weight changes within the first 4 weeks of starting treatment. By week 8 (after the first dose escalation), many experienced noticeable appetite reduction. Average weight loss ranged from 15–21% of body weight over 72 weeks, depending on the final dose (5 mg, 10 mg, or 15 mg weekly).
Wegovy (STEP trials): Weight loss typically became apparent around weeks 8–12 as participants reached maintenance doses (1 mg to 2.4 mg weekly). Average weight loss was approximately 15% of body weight at 68 weeks, achieved alongside reduced-calorie diet and increased physical activity.
The key difference: Mounjaro's dual GIP and GLP-1 mechanism may produce faster appetite suppression, while Wegovy's single GLP-1 pathway takes slightly longer to reach peak effect. Both require consistent dose titration over 4–5 months before maximum benefit is achieved.
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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 Factors and Suitability
How quickly either medication works depends on multiple personal factors: your starting weight, metabolism, how well you tolerate each dose, dietary adherence, and physical activity level. Some people feel appetite changes within days; others take 2–3 weeks. A prescriber will assess your individual eligibility using NHS NICE guidelines and may recommend one medication over the other based on your health profile and UK eligibility criteria (BMI 30+, or BMI 27–29.9 with weight-related conditions like type 2 diabetes or hypertension).
Using our free GLP-1 weight loss calculator can help you explore projected timelines at different doses, though actual results will vary. Track your progress with our free progress tracker once you begin treatment.
- 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.
Will Mounjaro work faster than Wegovy for me personally?
Neither medication has assured speed. Clinical trials show Mounjaro's dual mechanism may produce earlier appetite changes (4–6 weeks), but individual response varies widely. Wegovy's single GLP-1 pathway takes slightly longer (8–12 weeks) to reach full effect. Your prescriber will recommend the medication best suited to your health profile and eligibility.
How long before I see weight loss results on Mounjaro?
Many people notice reduced appetite within 4 weeks of starting. Measurable weight changes often appear by week 8, though some take 12 weeks. Results depend on dose, diet consistency, activity level, and individual metabolism. Never expect clinically studied weight loss — clinical trials show average results only.
Is Mounjaro faster than diet and exercise alone?
Clinical trials show Mounjaro achieved 15–21% average weight loss over 72 weeks alongside diet and exercise. Without lifestyle changes, medication alone is less effective. Mounjaro is a tool to support behaviour change, not a replacement for it.
Can I speed up Mounjaro results by increasing my dose faster?
No. The titration schedule (starting at 2.5 mg, increasing every 4 weeks) is designed for safety and tolerability. Faster escalation increases nausea and side effects without improving results. Your prescriber will manage dose timing according to clinical guidelines.
What if Mounjaro or Wegovy doesn't work quickly enough for me?
If you're not seeing results after 3–4 months at a stable dose, discuss this with your prescriber. They may adjust your dose, review your diet and exercise approach, or consider whether a different medication might suit you better. Weight loss medication works best alongside sustained lifestyle changes.
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.