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
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How Long Does Mounjaro Take To Work — UK Comparison
Most people notice initial effects from Mounjaro (tirzepatide) within 1–2 weeks, though meaningful weight loss typically develops over 4–12 weeks of consistent use. The timeline depends on your starting dose, individual metabolism, diet, and physical activity. Compared to Wegovy (semaglutide), Mounjaro's dual GIP and GLP-1 mechanism may produce results slightly faster in clinical trials, but individual timelines vary considerably. This guide explains what to expect from the SURMOUNT clinical trial data and how Mounjaro compares to other weight loss treatments available through UK prescription.
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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.
Timeline For Mounjaro Effects — SURMOUNT Trial Data
Weeks 1–2: Most people report reduced appetite and early appetite suppression as the most noticeable effect. Some experience nausea initially as the body adjusts. This is normal and typically improves within days to weeks.
Weeks 4–8: At 5 mg dose (first maintenance level), noticeable changes in hunger levels and portion control become clearer. Weight loss typically begins during this window, averaging 1–2 kg per week alongside a reduced-calorie diet and increased physical activity.
Weeks 8–12: As dose titration continues (stepping up to 7.5 mg then 10 mg), weight loss often accelerates. Most people see cumulative results by 12 weeks—the SURMOUNT clinical trial measured weight loss at this point.
Months 4–6: Full effects typically emerge after months of consistent use at maintenance doses. The Mounjaro KwikPen device makes weekly dosing straightforward. By week 24, SURMOUNT trial participants averaged clinically studied weight loss at 5 mg dose.
Months 6–12: Maximum weight loss typically occurs around 52 weeks. At 15 mg dose, SURMOUNT data showed approximately 20.9% average body weight loss alongside lifestyle changes.
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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.
Mounjaro Vs Wegovy — Speed Of Effect Comparison
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) show similar initial appetite suppression within 1–2 weeks. However, Mounjaro's dual GIP and GLP-1 mechanism activates two hormone pathways simultaneously, which some evidence suggests produces slightly faster cumulative weight loss. SURMOUNT trial data showed Mounjaro at 15 mg achieved ~clinically studied weight loss versus Wegovy's ~14.9% in comparable trials—though individual results vary significantly.
Wegovy requires slower dose titration (starting 0.25 mg) versus Mounjaro's 2.5 mg start, which may delay reaching effective doses. Both require a UK prescription following clinical assessment. Your GP or prescriber will recommend which suits your individual health profile. Neither medication produces assured outcomes—lifestyle diet and exercise changes are essential alongside either 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.
Can I expect weight loss in the first week of Mounjaro?
Most people notice appetite reduction within days to 1–2 weeks, but measurable weight loss typically begins after 2–4 weeks as the body adjusts. Initial appetite suppression does not always translate immediately to weight loss—diet adherence matters significantly. Nausea in week 1–2 is common and usually resolves.
How does Mounjaro dose titration affect how quickly it works?
Mounjaro starts at 2.5 mg, increasing every 4 weeks (5 mg, 7.5 mg, 10 mg, 12.5 mg, 15 mg). The tirzepatide dose titration schedule means most people don't reach higher effective doses until week 12–16. This slower build-up reduces nausea risk but also delays maximum effectiveness. Your prescriber determines your individual schedule.
Is Mounjaro faster than Wegovy for weight loss?
SURMOUNT clinical trial data suggests Mounjaro produces slightly faster cumulative weight loss due to dual GIP+GLP-1 action versus Wegovy's GLP-1-only mechanism. However, individual results vary. Both require prescription assessment. Speed of effect depends on your personal response, diet adherence, activity level, and starting dose.
What if I don't see weight loss after 8 weeks of Mounjaro?
If meaningful appetite suppression occurs but weight loss is absent, review calorie intake and physical activity with your GP. Mounjaro supports—not assures—weight loss. Without a reduced-calorie diet and activity increase, results will be limited. Your prescriber may adjust your dose or review suitability.
Can I track Mounjaro progress using a weight loss calculator?
Yes. MyWeightCheck's free GLP-1 weight loss calculator can help project potential outcomes at different Mounjaro doses based on SURMOUNT trial averages. However, this shows population averages—your individual timeline depends on your starting weight, health factors, diet, and exercise. Use our progress tracker to log weekly weight and monitor personal trends.
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.