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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What to Expect When Starting Mounjaro in the UK
Starting Mounjaro is a significant step in your weight management journey, and understanding what to expect — from your NHS tier 3 assessment through dose escalation and results — helps you feel prepared and confident. This guide covers the entire Mounjaro process in a UK context, including eligibility with NICE guidance, how the medication works, typical side effects, and realistic outcomes based on clinical trial data. Whether you're referred through the NHS or considering private prescription, knowing what's ahead makes the experience less uncertain.
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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.
NHS Eligibility, Assessment, and the Tier 3 Pathway
In the UK, Mounjaro (tirzepatide) is accessed through NHS tier 3 specialist weight management services following NICE approval, or through private prescription. NHS eligibility requires a BMI of 30 kg/m² or above, or 27–29.9 kg/m² with weight-related health conditions such as type 2 diabetes, hypertension, or cardiovascular disease. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower thresholds apply — eligibility may begin at BMI 27.5.
The NHS pathway starts with your GP, who may refer you to tier 3 specialist services in your region. Assessment includes a clinical review, discussion of lifestyle changes, and confirmation that Mounjaro is appropriate for you. Regional availability varies significantly — some areas have waiting lists of several months, whilst others have limited access. Private prescription routes bypass NHS waiting lists but require an online clinical assessment with a UK-licensed prescriber.
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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.
Dose Titration, Side Effects, and What the First Weeks Feel Like
Mounjaro starts at 2.5 mg once weekly, with escalation every 4 weeks: 5 mg, 7.5 mg, 10 mg, 12.5 mg, and maximum 15 mg. Most people notice appetite reduction within the first 1–2 weeks. Common side effects include nausea, constipation, and reduced appetite — these often ease as your body adjusts. Track your progress using our free progress tracker to monitor how you feel alongside weight changes.
The KwikPen pre-filled device is straightforward to use; each pen contains 4 weeks of doses. Store unopened pens in a refrigerator at 2–8°C, or at room temperature below 30°C after first use for up to 30 days. Most people inject into the stomach, thigh, or upper arm, rotating sites weekly to minimise irritation.
- 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 does it take to see weight loss results with Mounjaro on the NHS?
Clinical trials (SURMOUNT-1) showed average weight loss of 15–21% body weight over 72 weeks at maximum doses. Individual results vary significantly — some see results within 4–8 weeks, others take longer. Weight loss typically accelerates as you reach higher maintenance doses. Your specialist will monitor progress at regular appointments.
What's the difference between NHS Mounjaro and private Mounjaro?
The medication itself is identical — tirzepatide is the active ingredient either way. The main differences are access speed (NHS has waiting lists; private is quicker), cost (NHS is free; private costs vary), and ongoing monitoring (NHS includes regular specialist reviews; private varies by provider). Eligibility criteria are the same.
Can I switch from Wegovy to Mounjaro on the NHS?
In theory yes, but in practice this depends entirely on your NHS tier 3 service's protocols and whether they have Mounjaro available. Mounjaro is dual GIP/GLP-1 (vs Wegovy's GLP-1 only), so a switch would require prescriber reassessment. Discuss this directly with your specialist if you're considering a change.
Will my GP prescribe Mounjaro, or does it have to be tier 3?
Mounjaro for weight management is typically prescribed only through NHS tier 3 specialist weight management services, not by GPs. Your GP can refer you and support your journey, but the prescription usually comes from the specialist team. Private GPs and prescribers can prescribe it independently following a clinical assessment.
What happens if I stop taking Mounjaro?
Weight typically returns gradually over weeks to months — there is no "cliff edge" weight gain. Some people continue at a lower dose for maintenance. SURMOUNT trial data suggests weight regain occurs if the medication is stopped without ongoing lifestyle support. Discuss long-term plans with your prescriber before stopping.
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.