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 Munjaro Take to Work?
Mounjaro (tirzepatide) is a prescription-only weight loss injection licensed in the UK that works gradually over several weeks. Most people start noticing changes in appetite and food cravings within the first 2–4 weeks, though significant weight loss typically becomes visible after 8–12 weeks of consistent use. Results vary between individuals depending on dose, diet, activity level, and metabolism. Understanding the realistic timeline helps set appropriate expectations when starting this GLP-1 receptor agonist medication through NHS specialist services or private prescribing in the UK.
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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 Typically Works
Weeks 1–4 (Starting Phase): At the starting dose of 2.5 mg, most people report a noticeable reduction in appetite and fewer cravings. Some experience early nausea, which usually settles within a few days. Weight loss may be minimal at this stage — often 0.5–1 kg.
Weeks 5–12 (Escalation Phase): Your prescriber will increase the dose every 4 weeks (moving to 5 mg, then 7.5 mg if appropriate). This is when appetite suppression strengthens and more visible weight loss typically occurs. Many people lose 2–4 kg during this period. The GIP receptor component of tirzepatide works alongside GLP-1 signalling to enhance satiety.
Weeks 13–20+ (Maintenance Phase): At your target dose (often 10–15 mg), effects stabilise and consistent weekly weight loss of 0.5–1 kg becomes typical. Clinical trials showed average weight loss of 15–21% of body weight over 72 weeks, though individual results vary considerably.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 vs Private Access and Realistic Expectations
If accessing Mounjaro through NHS specialist weight management services (tier 3), you'll receive clinical support throughout — your GP can refer you, though waiting lists vary significantly by region. If using private prescribing via an online consultation, a UK-licensed prescriber will assess your eligibility based on BMI and health context before issuing a prescription.
Remember: Mounjaro works best alongside a reduced-calorie diet and increased physical activity. Weight loss is not assured, and plateaus are normal. Our free GLP-1 weight loss calculator can help you understand potential outcomes at different doses. Weight loss injections support your efforts — they don't replace the need for lifestyle changes.
- 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 see weight loss in the first week on Mounjaro?
Unlikely. The first week focuses on your body adjusting to the medication at the starting 2.5 mg dose. Most appetite suppression and noticeable weight changes begin in weeks 2–4. Very rapid initial loss is usually water weight, not fat loss.
What if Mounjaro doesn't work for me after 3 months?
If you've reached your target dose (usually 10+ mg) and seen minimal weight loss after 12 weeks, discuss this with your prescriber. They may adjust your dose, review your diet and activity, or consider whether this medication is right for you. Not everyone responds equally.
How does the Mounjaro dose schedule affect how quickly it works?
Your prescriber increases your dose every 4 weeks to allow your body to adjust and minimise side effects. Higher doses typically produce more appetite suppression and weight loss, but there's an individual ceiling — the maximum is 15 mg weekly.
Is Mounjaro weight loss permanent after I stop taking it?
Weight loss achieved with Mounjaro can be maintained with ongoing lifestyle changes (diet and exercise). However, if you stop the medication and return to previous eating habits, weight regain is common. Long-term use or continued healthy habits help sustain results.
Why does weight loss slow down after a few months on Mounjaro?
Weight loss plateaus are normal as your body adapts and metabolism changes. This is not a sign the medication has stopped working — it's a natural biological response. Your prescriber can review your dose or lifestyle approach if this happens.
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.