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 Mounjaro Take To Start Working?
Mounjaro (tirzepatide) typically begins showing weight loss effects within the first 2–4 weeks of treatment, though the timeline varies between individuals. Most people notice gradual changes across the first 8–12 weeks as their dose is adjusted upwards. This guide explains what to expect during the Mounjaro dose titration schedule, how the NHS prescribes tirzepatide in the UK, and the realistic timeframe for seeing meaningful results alongside a reduced-calorie diet and increased physical activity.
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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.
Timeline: When Mounjaro Effects Appear
Mounjaro works differently for everyone, but clinical experience and the SURMOUNT trials suggest a general timeline:
- Weeks 1–4 (2.5 mg starting dose): Many people report reduced appetite and early appetite suppression within days. Nausea is common initially and often improves as the body adjusts.
- Weeks 5–8 (5 mg dose): Weight loss becomes more noticeable for most people. Effects compound as the dose increases.
- Weeks 9–12 (7.5 mg dose): Meaningful weight loss typically becomes evident. The appetite suppression effect stabilises.
- Weeks 13+ (10 mg, 12.5 mg, 15 mg maintenance): Maximum weight loss effect is usually reached by week 16–20 at your final maintenance dose.
It's important to remember that weight loss continues gradually throughout treatment. The dose titration schedule (increasing every 4 weeks) allows your body to adjust and maximises tolerability.
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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.
NHS Prescribing and What Affects Timeline
The NHS prescribes tirzepatide through specialist tier 3 weight management services following NICE guidance. Your personalised timeline depends on several factors: your starting weight, metabolic rate, whether you're following a reduced-calorie diet, physical activity level, and how your body responds at each dose stage.
If you're accessing Mounjaro via a UK-licenced prescriber, the standard KwikPen dose schedule remains the same. Some people see results at lower doses (5–10 mg), whilst others require the maximum 15 mg weekly injection to experience significant weight loss. Your prescriber will monitor your progress and can adjust your treatment plan if needed based on your individual response.
- 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 see weight loss in the first week of Mounjaro?
Most people do not see significant weight loss in week one. However, many report reduced appetite and food cravings within the first few days, which can feel like the medication is "working" immediately. Actual measurable weight loss typically begins in weeks 2–4 as appetite suppression takes full effect. Use our GLP-1 weight loss calculator to project realistic timelines for your dose.
What if I don't see results after 8 weeks on Mounjaro?
If you're not noticing changes by week 8, discuss this with your prescriber before your next dose increase. Ensure you're following a reduced-calorie diet and increasing physical activity—Mounjaro works best alongside lifestyle changes. Some people need higher doses (10+ mg) to see meaningful results. Your prescriber may adjust your plan based on your response.
How does Mounjaro timeline compare to Wegovy for weight loss?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) follow similar dose titration schedules and show weight loss within 2–4 weeks. Mounjaro uses a dual GIP+GLP-1 mechanism, whilst Wegovy is a GLP-1 agonist only. Clinical trials suggest Mounjaro may achieve slightly greater average weight loss (up to 20.9% body weight vs 14.9% with Wegovy), but individual results vary significantly.
Can I speed up Mounjaro's effect by increasing my dose faster?
No—the standard 4-week intervals between dose increases are based on safety and tolerability. Increasing your dose faster than recommended can increase side effects like nausea and gastric distress without improving weight loss results. Your prescriber will follow the established titration schedule to help your body adjust properly to each dose level.
When should I expect maximum weight loss results?
Maximum weight loss typically occurs between weeks 16–24 after starting Mounjaro, once you've reached your final maintenance dose (usually 10–15 mg weekly). From that point, weight loss plateaus but remains stable with continued treatment. Calculate your BMI now using our free tool to set a realistic healthy weight goal for your height and frame.
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.