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 For Mounjaro To Work?
Mounjaro (tirzepatide) typically begins to show effects within the first 2–4 weeks, with more noticeable results by week 4–8. However, weight loss is a gradual process, and individual timelines vary significantly. Unlike some weight loss approaches, Mounjaro works by activating your body's natural appetite and appetite-related hormones — the GIP receptor and GLP-1 receptor — which takes time to regulate. Clinical trials showed meaningful results accumulating over 12–16 weeks, with maximum benefit often seen by 6 months. Our GLP-1 weight loss calculator can help you understand potential outcomes based on different doses and timelines.
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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: Onset Times and Trial Data
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly weight loss injections requiring a UK prescription and clinical assessment. The key difference in onset relates to their mechanisms: Mounjaro is a dual GIP receptor and GLP-1 receptor agonist, while Wegovy activates GLP-1 only.
- Mounjaro (SURMOUNT trial): Weight loss typically becomes apparent by weeks 4–8; average 15–clinically studied weight loss at 72 weeks
- Wegovy (STEP trial): Weight loss typically becomes apparent by weeks 4–8; average clinically studied weight loss at 68 weeks
- Onset similarity: Both show early appetite changes within 2 weeks; cumulative weight loss follows similar timelines
- Dose escalation: Mounjaro escalates over 16–20 weeks to 15mg maximum; Wegovy escalates over 16 weeks to 2.4mg maximum
The dual-action mechanism of tirzepatide may contribute slightly faster metabolic effects for some individuals, but individual variation is significant. Your prescriber will monitor progress and adjust doses based on your personal response.
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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.
What To Expect: Timeline and Individual Variation
Weight loss is rarely linear. Most people report initial appetite suppression within days to 1 week, making it easier to eat smaller portions. By week 4, some see scale changes; others may take 8–12 weeks. This variation depends on starting weight, metabolic factors, diet, activity level, and genetics — not the injection itself.
Both Mounjaro and Wegovy require lifestyle support — reduced-calorie diet and increased physical activity — to achieve results. A weekly injection alone does not clinically studied weight loss. Your GP or prescriber should discuss realistic timelines for your individual situation. Many people continue to lose weight beyond the initial 3-month period, with plateau periods alternating with new loss. Use our progress tracker to log weekly changes and identify your personal pattern.
- 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.
Does Mounjaro work faster than Wegovy?
Clinical data shows similar onset — both typically produce noticeable appetite changes within 2 weeks and weight loss by 4–8 weeks. Mounjaro's dual GIP+GLP-1 mechanism may offer slightly different metabolic effects for some individuals, but individual results vary significantly. Prescriber choice depends on personal suitability, not speed alone.
How much weight will I lose in the first month on Mounjaro?
First-month weight loss varies widely — some people lose 2–4kg, others see minimal scale change initially. Early changes often reflect water loss and reduced food intake rather than fat loss. Meaningful fat loss typically accumulates from weeks 4–12 onwards. Individual variation is substantial and depends on many factors beyond the medication.
What if Mounjaro doesn't work after 8 weeks?
If minimal appetite suppression or weight changes occur by week 8, your prescriber may increase your dose or, rarely, consider switching medications. However, some individuals need longer adjustment periods. Discuss expectations with your GP or prescriber — they can assess whether the dose is optimal for you.
Can I speed up weight loss on Mounjaro by dieting more aggressively?
The NHS recommends losing no more than 0.5–1kg per week for sustainable results. Very rapid dieting (under 800 kcal/day) without medical supervision risks muscle loss and nutritional deficiency. Mounjaro works best alongside moderate, sustainable calorie reduction — not extreme restriction.
How do I know if Mounjaro is working for me?
Early signs include reduced appetite, earlier fullness, fewer food cravings, and easier portion control — often within 2 weeks. Weight loss on scales typically follows 4–8 weeks later. Track hunger levels, energy, and food intake alongside weight. Our free BMI calculator and progress tracker help you monitor overall health changes objectively.
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.