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
When Does Mounjaro Start Working?
Mounjaro (tirzepatide) typically begins working within the first few weeks of treatment, though the timeline varies between individuals. Most people notice changes in appetite and food cravings within 2–4 weeks of their first injection, with weight loss effects becoming more noticeable from week 4 onwards. However, significant weight loss usually takes 8–12 weeks to become apparent. It's important to remember that Mounjaro is designed to work alongside a reduced-calorie diet and increased physical activity — results depend on your individual metabolism, lifestyle, and adherence to the treatment plan.
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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 Timeline Comparison
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly GLP-1 receptor agonist injections, though Mounjaro uses a dual GIP+GLP-1 mechanism. Timeline expectations differ slightly:
- Mounjaro: Appetite suppression often felt within 1–2 weeks; noticeable weight loss typically from week 4–8 onwards
- Wegovy: Similar appetite reduction within 1–4 weeks; weight loss usually visible from week 6–8
The SURMOUNT clinical trial for Mounjaro showed average weight loss of approximately 15–21% of body weight over 72 weeks, whilst the STEP trials for Wegovy demonstrated around 14.9% average loss over 68 weeks. Individual results vary significantly based on dose, diet adherence, exercise, and metabolism. Both medications reach their full effect only after dose escalation is complete — Mounjaro escalates over 20 weeks, Wegovy over 16 weeks.
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.
Factors Affecting When You'll See Results
Several factors influence how quickly you'll notice Mounjaro working. Key considerations: your starting weight (larger weight losses may be more noticeable early on), the dose you're on (higher doses typically produce stronger appetite suppression), your diet quality (calorie deficit is essential), physical activity level, and individual metabolism. Some people also notice appetite suppression before any weight on the scale changes — this is a normal part of how the medication works and indicates the drug is active in your system. Starting dose is 2.5 mg, escalating every 4 weeks to a maximum of 15 mg; most people feel effects strengthen with each dose increase. If you're not noticing any changes after 8 weeks on a stable dose, discuss this with your prescriber — they may adjust your approach or investigate other factors affecting your weight loss.
- 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 I lose weight in the first week of Mounjaro?
Most people do not experience significant weight loss in the first week. The initial 2–4 weeks focus on appetite suppression and food cravings reduction. Weight loss on the scale usually becomes noticeable from week 4 onwards, though individual timelines vary. The first week is about your body adjusting to the medication.
Is Mounjaro faster than Wegovy for weight loss?
Clinical trials show Mounjaro and Wegovy produce similar average weight loss outcomes — approximately 15–21% and 14.9% respectively. Both are once-weekly injections. Mounjaro uses a dual GIP+GLP-1 mechanism whilst Wegovy uses GLP-1 only. Individual response varies, and prescriber assessment determines which is suitable for you.
What should I do if Mounjaro doesn't seem to be working after 4 weeks?
It's normal for the full effects to take 8–12 weeks, especially if you're still on a lower starting dose. Ensure you're maintaining a reduced-calorie diet and increasing physical activity — Mounjaro works best alongside lifestyle changes. Discuss any concerns with your prescriber, who may check your dose, adherence, or other factors affecting results.
How does Mounjaro's dose titration affect when it starts working?
Mounjaro starts at 2.5 mg and escalates every 4 weeks to a maximum of 15 mg. Each dose increase typically strengthens appetite suppression and weight loss effects. Most people feel the medication working more noticeably at higher doses. Full effects are usually seen once you reach a stable maintenance dose around week 16–20.
Can I use a weight loss calculator to estimate my Mounjaro results?
Yes — our free GLP-1 weight loss calculator can help you estimate potential weight loss at different Mounjaro doses, combined with your baseline weight and lifestyle factors. Use this alongside our BMI calculator to track your starting point and monitor progress over time with our free progress tracker tool.
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.