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 Soon Does Mounjaro Start Working in the UK?
Mounjaro (tirzepatide) typically begins working within days of your first injection, though noticeable weight loss effects usually appear within 2–4 weeks as your body adjusts to the medication. However, the timeline varies significantly between individuals depending on starting dose, dose titration schedule, and how your body responds. This guide covers when you can expect results, how NHS and private access work in the UK, what Mounjaro costs on prescription, and the online consultation process for getting started.
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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.
When Does Mounjaro Start Having an Effect?
Mounjaro begins influencing appetite and food intake almost immediately after your first injection, but you may not notice significant weight loss for several weeks. Most people report reduced appetite and cravings within 3–7 days, though this varies. Actual weight loss typically becomes measurable within 2–4 weeks of starting treatment. The dose titration schedule (starting at 2.5 mg and increasing every 4 weeks up to a maximum 15 mg) means your body gradually adapts, which can delay visible results initially.
- Weeks 1–2: Appetite suppression may begin; possible nausea or digestive changes as your system adjusts
- Weeks 2–4: Some weight loss may become visible on the scale, alongside continued side effects
- Weeks 4–8: As dose increases occur, results typically become more consistent and noticeable
- Weeks 12–16: By this point, most people see meaningful progress if lifestyle changes support the medication
Clinical trials show that sustained weight loss accumulates over time—the SURMOUNT-1 trial measured results at 72 weeks, demonstrating that patience and consistency matter significantly.
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.
How to Access Mounjaro in the UK: NHS vs Private Routes
NHS access: NICE has approved tirzepatide (Mounjaro) for specialist weight management services, but availability is limited and varies by region. You'll need a GP referral to a tier 3 specialist weight management service, which can involve waiting lists of several months. Private access: You can obtain a UK prescription through an online clinical assessment with a licensed prescriber, typically receiving your prescription within 24–48 hours. Private prescriptions cost approximately £129–£320 per month depending on your dose and provider. Both routes require a clinical assessment confirming you meet eligibility criteria (BMI 30+ or 27–29.9+ with weight-related health conditions). Our GLP-1 weight loss calculator can help you explore potential outcomes at different doses before discussing options with your prescriber.
- 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.
How long until I see weight loss results from Mounjaro?
Most people notice reduced appetite within days, but measurable weight loss typically appears within 2–4 weeks. The gradual dose titration (starting at 2.5 mg) means full effects develop over 12–16 weeks. Individual timelines vary based on starting dose, lifestyle changes, and how your body responds to tirzepatide.
Does Mounjaro work faster than Wegovy?
Mounjaro (dual GIP/GLP-1 agonist) and Wegovy (GLP-1 agonist only) have similar onset timelines. SURMOUNT trials showed Mounjaro achieved higher average weight loss percentages at maximum dose, but individual results vary considerably. Your prescriber can discuss which suits your circumstances.
What's the difference between NHS and private Mounjaro in the UK?
NHS Mounjaro is available through specialist tier 3 weight management services (requires GP referral, potential waiting lists). Private prescriptions come from online clinical assessments and are dispensed by GPhC-registered pharmacies within 24–48 hours, costing £129–£320/month depending on dose.
Will Mounjaro stop working over time?
Long-term effectiveness data shows sustained benefit in clinical trials, but weight loss plateaus are possible. If this occurs, your prescriber may adjust your dose, review lifestyle factors, or explore alternative options. Consistency with diet and physical activity strongly influences ongoing results.
Can I get Mounjaro on the NHS quickly?
NHS waiting times for specialist weight management services vary significantly by region (ICB). Some areas have waits of 6–12+ months. If waiting time is a concern, private prescription via online consultation offers faster access, though there is a cost involved that the NHS route doesn't have.
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.